3' End (3-prime) The end of a DNA or RNA strand that has a free hydroxyl group at the 3' carbon of the sugar (deoxyribose or ribose).

5' End (5-prime) The end of a DNA or RNA strand that has a free phosphate group at the 5'-carbon of the sugar (deoxyribose or ribose).



A site see Aminoacyl (A) site

Adenine A nitrogen containing base (a purine) found in DNA, RNA, ATP, NAD, etc.

Adenosine 5'-diphosphate The nucleotide formed by adding a pyrophosphate group to the 5'-OH group of adenosine.

Adenosine 5'-triphosphate A compound containing adenine, ribose, and three phosphate groups. When it is formed it contains two high-energy phosphoanhydride bonds. Hydrolysis (to ADP or AMP) is accompanied by a large free-energy change which is used to drive endergonic reactions. ATP is an energy storage compound and is also a precursor for RNA synthesis.

Adenosine A purine nucleoside found in DNA, RNA, and many cofactors.

A-DNA A right-handed DNA double helix which has 11 base pairs per turn. The base pairs are tilted about 70° with respect to the helix axis. DNA exists in this form in a crystal under low water conditions.

ADP see Adenosine 5' diphosphate

Agarose gel electrophoresis see Gel electrophoresis

Allele frequency The proportion of an allele relative to all alleles at a locus in a population.

Allele One of a pair, or series, of alternative forms of a gene at a given locus in a chromosome.

Alpha helix A right-handed spiral in a polypeptide.

Alu repeats see Highly repetitive DNA

Amino acid Any one of a class of organic compounds containing an amino (NH2) and a carboxyl (COOH) group linked to a central carbon known as the a carbon. The different properties of amino acids are provided by a group on this a carbon which varies between amino acids. These amino acids are the building blocks of proteins.

Aminoacyl (A) site A binding site on a ribosome that accepts the incoming aminoacyl-tRNA.

Aminoacyl-tRNA synthetases Enzymes that join an amino acids with a tRNA via a high energy bond.

Aminoacyl-tRNA A molecule which consists of an amino acid joined to the 3'hydroxyl group of a tRNA molecule via a high energy ester bond.

Anaphase The stage in nuclear division at which the first separation of sister chromatids (or, in the first meiotic division, of paired homologues) occurs.
Animal cloning The process by which an entire organism is reproduced from a single cell taken from the parent organism and in a genetically identical manner. This means the cloned animal is an exact duplicate in every way of its parent; it has the same exact DNA.

Annealing curve see Cot curve

Annealing The process by which two complementary single stranded nucleic acids (can be DNA or RNA or a mixture) reform a double stranded molecule.

Antibodies Proteins that are found in blood or other bodily fluids of vertebrates, and are used by the immune system to identify and neutralize foreign objects, such as bacteria and viruses.

Antibiotic Resistance Factor A plasmid that contains one or more genes that encode resistance to antibiotics and has the ability to transfer from one bacterium to another.

Anticodon A "triplet" of three nucleotides in a transfer RNA (tRNA) that is complementary to a codon in a messenger RNA (mRNA). During protein synthesis, base pairing between a codon and anticodon aligns the incoming tRNA carrying the new amino acid with the tRNA carrying the growing peptide chain.

Antisense DNA see Antisense nucleic acid

Antisense gene A gene which is transcribed to give a mRNA which is complementary to the mRNA or pre-mRNA of a normal gene (usually made by inverting the coding region relative to promoter).

Antisense nucleic acid A single-stranded nucleic acid which is complementary to and so can bind to a mRNA transcribed from a gene whose product is not required.

Antisense RNA see Antisense nucleic acid

Antisense therapy A form of treatment for genetic disorders or infections. When the genetic sequence of a particular gene is known to be causative of a particular disease, it is possible to synthesize a strand of nucleic acid that will bind to the messenger RNA (mRNA) produced by that gene and inactivate it, effectively turning that gene "off".

Apolar see Hydrophobic
ATP see Adenosine 5'-triphosphate

Autonomously Replicating Sequence (ARS) A segment of a DNA molecule necessary for the initiation of its replication; generally a site recognized and bound by the proteins of the replication system.

Autonomous specification Determination of cell fate by cytoplasm acquired during cleavage, independent of interactions with neighbouring cells.

Autosome Any chromosome (in humans, 1 to 22) that is not a sex chromosome (X or Y).



B DNA The naturally occurring form of double stranded DNA. It consists of a right-handed helix which contains 10.5 base pairs per turn of the helix axis.

B form see B DNA

BAC see Bacterial Artificial Chromosome

Bacterial Artificial Chromosome Cloning vectors constructed from bacterial fertility (F) factors. They can carry large inserts of 200 to 500 kb.

Bacteriophage Any virus that infects bacteria. Some bacteriophage (notably bacteriophage l) are used as cloning vectors.

Balanced polymorphism A system of genes in which two alleles are maintained in stable equilibrium because the heterozygote is more fit than either of the homozygotes.

Base analog A purine or pyrimidine base that differs slightly in structure from the normal bases but can still be incorporated into nucleic acids. They are often mutagenic.

Base composition Frequency of occurrence of adenine (A), guanosine (G), cytosine (C), thymine (T), or uracil (U) in a nucleic acid.

Base pair Association of complementary nucleotides, stabilised by hydrogen bonding, in a nucleic acid. Adenine pairs with thymine or uracil (A-T, A-U) and guanine pairs with cytosine (G-C).

Base Pairing see Complementary base pairing

Base Excision Repair (BER) A cellular mechanism that repairs damaged DNA throughout the cell cycle. It is primarily responsible for removing small, non-helix distorting base lesions from the genome.

Base stacking The close packing of the planes of base pairs, commonly found in DNA and RNA structures.

Base substitution An instance when a single base change has occurred in a DNA molecule.

Base A chemical containing carbon, hydrogen and nitrogen in a ring structure. It is attached to each of the sugars in the backbone of DNA and RNA or An aromatic nitrogen-containing molecule with basic properties. Such bases include purines and pyrimidines which are found in DNA and RNA.

Bioinformatics The application of statistics and computer science to the field of molecular biology.

Biotechnology The use of living cells or organisms to produce medicines, foods and other products of economic importance.

Blotting (blot analysis) A widely used technique in biochemistry for detecting specific macromolecules (proteins, mRNA, or DNA sequences). A sample of the mixture is first resolved into individual components using agarose or polyacrylamide gel electrophoresis. The components are transferred (blotted) from the gel to a filter (usually nitrocellulose). The filter is then reacted with a labelled probe (which might be radioactive) that specifically binds to the DNA/RNA/protein of interest. The probe is then detected using a suitable mechanism. There are three types of blotting namely: Northern blotting (detect mRNAs with a cDNA probe); Southern blotting (detect DNA fragments with cDNA or oligonucleotide sequences) and Western blotting (detect proteins with specific antibodies).

Bottleneck effect The reduction of a population’s gene pool and the accompanying changes in gene frequency produced when a few members survive the widespread elimination of a species.


Cap see Capping

CAP see Catabolite gene Activator Protein

Capping Covalent modification of mRNA at the 5' end where a modified guanidine is covalently attached in a 5' - 5' linkage.

Carbohydrates Organic compounds with the general formula CnH2mOm. Examples are sugars and sugar polymers such as starch, and cellulose.

Carcinogen A substance which can cause changes in DNA sequence which induces cancer in an organism.

Catabolite repression Reduction in transcription of prokaryotic genes coding for sugar-metabolising enzymes caused by glucose or a glucose metabolite. The reduction is mediated by camp and CAP (catabolite activator protein).

cDNA library see Complementary DNA library

cDNA see Complementary DNA

Cell cycle Ordered sequence of events during which a cell replicates its chromosomes and then divides. Most eukaryotic cell cycles can be divided into four phases: G1 prior to DNA synthesis; S DNA replication occurs at this time; G2 after DNA synthesis takes place; and M cell division occurs to yield two daughter cells. In some circumstances, cells exit from the cell cycle during G1 and transfer to the G0 state as nondividing (quiescent) cells.

Cell determination The process by which embryonic cells, previously undifferentiated, take on a specific developmental character.

Cell differentiation Changing of cells into different types to do specific jobs in the body.

Cell division The reproduction of a single cell to produce two new, daughter, cells. In eukaryotes, this involves nuclear division (mitosis).

Centromere Region of a chromosome that attaches to the spindle in cell division and where sister chromatids join.

Centrosome An organelle in the cytoplasm of the cell that is involved in the formation of the mitotic spindle, the assembly of microtubules and the regulation of the cell cycle.

Chain-terminating mutation see Nonsense mutation

Chimaeric DNA A recombinant DNA, usually constructed in vitro, whose components are derived from two or more sources.

Chip See DNA chip

Chloroplasts Organelles found in cells of plants and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy through a complex set of processes called photosynthesis.

Chromatid In mitosis or meiosis, one of the two identical double stranded DNA molecules, with associated proteins, resulting from replication of a chromosome.

Chromatin The nucleic acid-protein complex found in eukaryotic chromosomes.

Chromosome The DNA molecule that contains the genetic information of a bacterial cell or virus. In eukaryotes, the genetic material consisting of a single, linear double-stranded molecule of DNA which is complexed with proteins.

Cis-acting sequence A nucleotide sequence that regulates genes located immediately adjacent, that is cis, to itself.

Cistron A genetic unit that encodes a single polypeptide chain.

Clone A population of cells descended from a single progenitor which are genetically identical.
Cloning vector An autonomously replicating DNA molecule which can accept foreign DNA which will then be amplified in host cells. The most commonly used vectors are bacterial plasmids, modified baceriophage, yeast and bacterial artificial chromosomes (YACs and BACs).

Coding region Those sequences in DNA that code for the amino acid sequence of all or part of a protein.

Coding sequence see Exon

Codon Sequence of three nucleotides in DNA or mRNA that either specifies the incorporation of an amino acid into a polypeptide chain or signals the end of polypeptide synthesis.

Comparative genomics The study of the relationship of genome structure and function across different biological species or strains.

Complementary base pairing The pairing of the bases A with T (or A with U), and G with C in double-stranded DNA.

Complementary DNA library A collection of bacterial clones each containing a cDNA derived from the RNAs isolated from a specific tissue or cell and inserted into a suitable cloning vector.

Complementary sequence Two single stranded nucleic acid sequences that can form a perfect base-paired double helix with each other according to normal base pairing rules.

Complementary see Complementary sequence

Complex and multifactorial inheritance The inheritance of traits or characteristics that display continuous variation, that cannot be easily divided into clear categories. In many of these cases, the final phenotype is the result of an interaction between genetic factors and environmental influences.

Conditional specification Cells are differentiated by interactions with neighbouring cells. In some cases, cells, or groups of cells, in the developing embryo release proteins, including transcription factors. These act on the control genes of other cells, starting cascades of gene activation and inactivation. In some cases, the morphogens set up concentration gradients as they diffuse through the embryo, and the differentiation of cells is dependent on their position along the gradient.

Consensus sequence The nucleotides or amino acids most commonly found at each position, the "average" sequence, of related DNAs, RNAs, or proteins.

Contig A set of overlapping DNA sequences that provide a longer sequence.

Continuous variation Variation in phenotypic traits in which a series of types are distributed on a continuum rather than grouped into discrete categories.

Cosmid A plasmid DNA containing cos ends from l bacteriophage that can be packaged into virus particles in vitro for infection purposes. The hybrid DNA molecules use plasmid origins of DNA replication.

Crossing over Exchange of genetic material between nonsister chromatids of homologous chromosomes during meiosis.

Cytidine A pyrimidine nucleoside found in DNA and RNA.

Cytokinesis The process by which a cell divides its cytoplasm to produce two daughter cells.

Cytoplasm The part of a cell that is enclosed within the cell membrane. In eukaryotic cells, the contents of the cell nucleus are not part of the cytoplasm and are instead called the nucleoplasm.

Cytosine A pyrimidine base found in DNA and RNA.


Degeneracy There are 64 possible codons but only 20 amino acids therefore a single amino acid may be coded for by two or more codons in messenger RNA.

Deletion A mutation resulting from the loss of part of a gene(s).

Denaturation Nucleic acids are denatured by chemicals or heat which disrupt the hydrogen bonds between nucleotides. The double-stranded molecule is thus converted into a single-stranded molecule.

Density-gradient centrifugation see Buoyant density centrifugation

Deoxyribose Five-carbon sugar component of DNA.

Deoxyribonuclease Any enzyme that breaks down (hydrolyses) DNA.

Deoxyribonucleic Acid A long linear polymeric molecule which is the genetic material which is made up of genes. DNA is composed of four different bases each liked to the sugar deoxyribose. The sugar-bases are linked to each other by phosphodiester bonds. DNA is a double helix composed of two antiparallel strands in its naturally occurring state.

Derepression Turning on expression of a gene or set of genes whose expression.

Diakinesis Fifth of five phases within prophase I, homologous chromosomes continue to separate, and chiasmata move to the ends of the chromosomes.

Dideoxy (Sanger) sequencing The classical chain-termination sequencing method requiring a single-stranded DNA template, a DNA primer, a DNA polymerase, radioactively or fluorescently labelled nucleotides, and modified nucleotides that terminate DNA strand elongation.

Differentiation Process whereby similar cells follow different developmental pathways. These cells express different genes and perform different functions.

Diploid Having a chromosome complement consisting of two copies (homologues) of each chromosome, one derived from each parent.

Diplotene Fourth of five phases within prophase I, homologous chromosomes start to separate but remain attached by chiasmata.

DNA see Deoxyribonucleic Acid

DNA "chip" A small glass or plastic plate with many short single stranded DNA sequences attached, used for nucleic acid hybridisation experiments including for example analysis of single nucleotide polymorphisms.

DNA cloning see Gene cloning

DNA fingerprint DNA isolated from an individual is cleaved with a restriction enzyme(s) and then hybridised with an appropriate nucleic acid probe(s). The banding pattern obtained is known as a DNA fingerprint.

DNA fingerprinting see Genetic fingerprinting.

DNA footprinting A technique used for identification of regions of DNA that bind protein. A DNA molecule, radiolabeled on one strand, is digested with DNase in the presence or absence of a DNA-binding protein and then electrophoresed on a gel. Regions of DNA to which the protein has bound are protected from nuclease digestion therefore the digestion pattern between the two samples will differ. This will allow identification of the protein-binding regions.

DNA gyrase An enzyme in bacteria that forms negative supercoils in DNA.

DNA helicase An enzyme that unwinds the strands of a DNA double helix.

DNA hybridisation The process by which two single stranded nucleic acids from different source reform a double stranded molecule.

DNA library A collection of cloned DNA molecules inserted into a suitable cloning vector which, together either, contain the entire genome (genomic library) or cDNA copies of all the mRNAs found in a cell type (cDNA library).

DNA ligase An enzyme that catalyses the covalent linking of adjacent 3' OH and 5' phosphate groups in a gapped double stranded DNA. The enzyme joins newly synthesised fragments in the lagging strand and can also be used to join recombinant DNA fragments in vitro.

DNA methylation Addition of methyl groups to DNA bases. Methylation bases can be found in non-expressed regions of the genome of eukaryotes and can be used to protect a bacterium's DNA against its own restriction enzymes.

DNA microarray A series of DNA sequences attached to a glass plate or nylon membrane, used in nucleic acid hybridisation experirments.

DNA polymerase An enzyme that catalyses the formation of 5'-3' phosphodiester bonds between deoxyribonucleotide triphosphates. DNA polymerase uses base pairing rules to ensure the correct nucleotide triphosphate is added one base at-a-time.

DNA primase Catalyses the synthesis of short stands of RNA required for the initiation of DNA synthesis.

DNA profile Encrypted sets of numbers produced by Genetic fingerprinting, that reflect a person's DNA makeup, which can also be used as the person's identifier.

DNA profiling see Genetic fingerprinting

DNA repair Enzymes catalyse the repair of damaged DNA.

DNA sequencing see Dideoxy (Sanger) sequencing

DNA testing see Genetic fingerprinting.

DNA topoisomerase Catalyses the positive or negative supercoiling of DNA.

DNase see Deoxyribonuclease

Dominant An allele whose phenotype is expressed regardless of whether the organism is homozygous or heterozygous for that allele.

Double helix The molecular structure of DNA. Two complementary single stranded polynucleotide chains are wound round each other in an antiparallel direction with hydrogen bonding and base stacking between the bases.

Double-stranded helix See Double helix

Downstream The direction of movement of RNA polymerase during transcription and ribosome movement during translation ie towards the 3' hydroxyl containing end of a nucleic acid.

Drift see genetic drift

Duplex DNA see Double helix

Duplicated gene see Gene duplication

Depurination Loss of purine bases from the DNA backbone and single strand breaks.


Electrophoresis The movement of particles in an electrical field. Movement usually takes place through a semi-solid support such as agarose or polyacrylamide.

Electroporation A technique where an intense electric field is applied to cells which render the cell membranes permeable to DNA.

Embryo A young animal, or plant, while it is still contained within a protective structure such as a seed, egg, or uterus.

Embryogenesis The process by which the embryo is formed and develops, until it develops into a foetus.

Endonuclease An enzyme that breaks a phosphodiester bond at within a polynucleotide chain.

Enhancers Regions that increase the rate of transcription when bound to transcription factors. These sites may be thousands of nucleotides from the coding sequences or within an intron. Some enhancers are conditional and only work in the presence of other factors as well as transcription factors.

Enzyme A protein which has a 3 dimensional structure in which chemical groups are arranged on the surface so that the protein can bind a particular molecule (the substrate) and catalyse a chemical reaction which modifies the substrate.

Epigenetics The study of inherited changes in phenotype (appearance) or gene expression caused by mechanisms other than changes in the underlying DNA sequence; instead, non-genetic factors cause the organism's genes to behave (or "express themselves") differently. These changes may remain through cell divisions for the remainder of the cell's life and may also last for multiple generations.

ESTs see Expressed Sequence Tags

Euchromatin Chromatin that is less tightly bound during interphase; may be transcribed.

Eukaryote A cell organism that contains its genetic material in a membrane-bounded nucleus.

Eukaryotic cell A cell containing a membrane nucleus and other membrane bound organelles e.g. mitochondria and golgi apparatus.

Evolution A genetic and consequent phenotypic alteration in organisms from one generation to another.

Evolutionary tree A branching diagram or "tree" showing the inferred evolutionary relationships among various biological species or other entities based upon similarities and differences in their physical and/or genetic characteristics.

Excision repair The removal and replacement of nucleotides in damaged DNA.

Exon Those parts of a eukaryotic gene that are found in a mature mRNA, rRNA, or tRNA molecule.

Exonuclease An enzyme that digests DNA or RNA from the ends of the molecules.

Exonucleolytic proofreading Proofreading of a DNA sequence via the removal of the unpaired residues at the terminus by a separate catalytic site of DNA Polymerase.

Expressed Sequence Tags Short cDNA sequences that are derived from sequencing of all the mRNAs present in a cell. ESTs represent the expression profile of the cell at the time point of RNA isolation.

Expression cloning A recombinant DNA technique which allows isolation of a cDNA or genomic DNA based on positioning of the DNA downstream of a promoter in the vector so that the protein is expressed and can be detected directly in the cell.

Expression vector A DNA vector, which allows isolation of a cDNA or genomic DNA based on positioning of the DNA downstream of a promoter in the vector so that the protein is expressed and can be detected directly in the cell.

Extension The addition of nucleotides to primers - in the 5' to 3' direction to form a double-stranded copy of the target DNA.

Equatorial plane/plate Midline of cell, at right-angles to the axis formed by the centrosome during cell division.



F factor A large bacterial plasmid, known as the sex-factor which allows transfer of genetic material between two bacteria.

F plasmid see F factor

F1 The first filial generation; the first generation of descent from a given mating (usually between homozygous parents).

F2 The second filial generation produced by inter-breeding or self-pollinating the F1.

Footprinting see DNA Footprinting

Founder effect Unusual distributions of allele frequencies because the population was established by a very small number of individuals.

Frameshift mutations A mutation resulting from the addition or deletion of a single base pair to a gene. This leads to a shift in the translation reading frame. The mutation usually leads to nonfunctional proteins.

Functional genomics Use of the vast wealth of data produced by genomic projects (such as genome sequencing projects) to describe gene (and protein) functions and interactions.


G0 see Cell cycle

G1 phase In the eukaryotic cell cycle, the gap between the end of mitosis and the onset of the S phase.

G1 see G1 phase

G2 phase In the eukaryotic cell cycle, the gap between the S (synthesis) phase and the onset of mitosis.

G2 see G2 phase

Gamete The mature haploid sexual reproductive cell: the egg or the sperm.

Gel electrophoresis A semisolid matrix with holes corresponding to the sizes of macromolecules. After application of a potential field molecules are attracted to the opposite end of the gel and can be separated on the basis of their size as they pass through the gel.

Gene A unit of heredity which carries information from one generation to the next. A segment of the genome that codes for a functional product.

Gene amplification Creation of two or more copies of a particular gene. This allows for the synthesis of high levels of the RNA transcript (as in rRNA synthesis).

Gene cloning The incorporation of a gene into a vector followed by the amplification of the newly formed recombinant DNA molecule in an appropriate host cell.

Gene control Any of the methods used to regulate gene expression. This can include regulation at the transcriptional, RNA processing, RNA stabilisation, and translation stages. In a few cases, amplification or rearrangement of chromosomal segments affect gene expression.

Gene conversion Phenomenon in which one of the alleles of a gene is replace with another during meiotic recombination.

Gene duplication Non-homologous recombination can lead to two copies of a gene adjacent to each other on a chromosome.

Gene expression The process by which the RNAs and proteins expressed by genes exert their effects on the phenotype of an organism.

Gene family A set of genes derived from a single parent gene by gene duplication. The genes may or may not have identical sequences and may or may not be on the same chromosomes.

Genetic fingerprinting A technique employed by forensic scientists to assist in the identification of individuals on the basis of their respective DNA profiles.

Gene library A large collection of cloned DNA sequences from a single genome.

Gene mutation See mutation

Gene pool The complete set of unique alleles in a species or population.

Gene regulation see Gene control

Gene splicing The sequential cutting of precursor mRNAs, removal of fragments and rejoining of the remaining RNA sequences.

Gene therapy The treatment of inherited diseases by replacing defective genes with wild-type copies of the gene in affected individuals. There are two types of gene therapy namely somatic-and germ line gene therapy.

Genetic diversity Any variation in the nucleotides, genes, chromosomes, or whole genomes of organisms.

Genetic drift The change in the relative frequency in which a gene variant (allele) occurs in a population due to random sampling and chance: Genetic drift is an important evolutionary process, which leads to changes in allele frequencies over time.

Geneticist A scientist who studies genetics, the science of heredity and variation of organisms

General recombination Recombination that occurs between homologous chromosomes at homologous sites.

Genetic code The set of rules whereby the 64 nucleotide triplets (codons) in RNA are converted to the 20 amino acids in proteins.

Genetic counsellor A medical genetics expert who acts as a patient advocate as well as a genetic resource to physicians. Genetic counselors provide information and support to families who have members with birth defects or genetic disorders, and to families who may be at risk for a variety of inherited conditions.

Genetic disorder An illness caused by abnormalities in genes or chromosomes.

Genetic engineering see Genetic modification

Genetic load A measure of the cost of lost alleles due to selection (selectional load) or mutation (mutational load).

Genetic map The arrangement of genes or other identifiable sequences on a chromosome.

Genetic manipulation see Genetic modification.

Genetic modification The direct manipulation of an organism's genes. It different from traditional breeding, where the organism's genes are manipulated indirectly and artificial cloning, which involves the transfer of whole genomes, as genetic modification uses the techniques of molecular cloning and transformation to alter the structure and characteristics of genes directly.

Genetics The study of heredity and variation.

Genome The total genetic content of a cell or a virus.

Genome analysis The techniques needed to determine and compare the genetic sequence (e.g. DNA in the chromosomes and mitochondria).

Genomic DNA library A collection of clones each containing a single DNA fragment. The total collection of clones will contain all of the genomic DNA sequences of an organism.

Genomic DNA All the DNA sequences comprising the genome of a cell or a virus.

Genomic library see Genomic DNA library

Genomics The study of an organism’s genome (its entire genetic material in the form of RNA, DNA, genes and chromosomes).

Genomic imprinting A genetic phenomenon by which certain genes are expressed in a parent-of-origin-specific manner, independently of the classical Mendelian inheritance. Imprinted genes are either expressed only from the allele inherited from the mother or in other instances from the allele inherited from the father. Forms of genomic imprinting have been demonstrated in insects, mammals and flowering plants.

Genotype A description of the genetic constitution of an individual, either with respect to a single trait or with respect to a set of traits.

Germ cell Any precursor cell that can give rise to reproductive cells or gametes.

GTP (guanosine 5'-triphosphate) A nucleotide precursor for RNA synthesis which is also used in protein synthesis and signal-transduction pathways.

Guanine A nitrogen-containing base (a purine) found in DNA, RNA and GTP.

Guanosine A nitrogen-containing nucleoside found in DNA and RNA.

Gyrase see DNA gyrase


H bond see Hydrogen bond

Haplogroup A group of similar haplotypes that share a common ancestor with a single nucleotide polymorphism (SNP) mutation.

Haploid An organism or cell having only one copy of each chromosome and hence only one allele of each gene or genetic locus. Examples of haploid cells are gametes and bacteria.

Haplotype A combination of alleles at different places (loci) on the chromosome that are transmitted together. A haplotype may be one locus, several loci, or an entire chromosome depending on the number of recombination events that have occurred between a given set of loci. Also denotes a set of single-nucleotide polymorphisms (SNPs) on a single chromatid that are statistically associated.

Hardy-Weinberg model The central theoretical model in population genetics. It describes and predicts a balanced equilibrium in the frequencies of alleles and genotypes within a freely interbreeding population, assuming a large population size, no mutation, no genetic drift, no natural selection, no gene flow between populations, and random mating patterns.

Helicase see DNA Helicase

Helix A structure with a spiral shape e.g. the DNA double helix.

Helix-loop-helix A conserved DNA binding motif found in one class of eukaryotic transcription factors where two helices in a protein are separated by a short (5 - 10 amino acid) loop.

Helix-turn-helix A conserved DNA binding motif found in one class of prokaryotic transcription factors where two helices in a protein are separated by a short (3 - 5 amino acid) turn.

Heterochromatin Regions of chromatin that remain highly condensed and transcriptionally inactive.

Heterozygote A diploid cell or organism having two different alleles of a particular gene.

Heterozygous Possessing two different forms of a particular gene, one inherited from each parent. A person who is heterozygous is called a heterozygote or a gene carrier. Heterozygous is in contrast to homozygous, the possession of two identical copies of the same gene.

Hierarchical shotgun sequencing A technique, in which the genome is divided into relatively large sections that are mapped onto the appropriate chromosomes before being sequenced.

Highly repetitive DNA Short DNA sequences present in thousands to millions of copies in the genome.

Histones A set of small, highly conserved basic proteins, called H1, H2A, H2B, H3, and H4 which are complexed with DNA in the nucleosome.

Homeobox A conserved DNA sequence which codes for a DNA-binding domain (homeodomain) found in a class of transcription factors. The homeodomain is involved in regulating expression of genes expressed in different stages of development.

Homeodomain The conserved DNA-binding motif found in many developmentally important transcription factors which is coded by the homeobox.

Homeotic gene A gene which is involved in developmental regulation. Mutations in this class of genes causes cells in one region of the body to behave as if they were in another. This can result in conversion of one body part to another eg the antennapedia mutation in Drosophila where an antenna becomes a leg.

Homologous chromosomes Chromosomes of the same size, one from the male and one from the female parent, that carry the same pattern of genes, but not necessarily the same alleles.

Homologous Recombination see General recombination

Homology Protein or nucleic acid sequence or structure organ similarity that reflects a common evolutionary origin.

Homozygote A diploid cell or organism having two identical alleles of a particular gene.

Homozygous Describing an organism that carries two identical alleles for a given gene.

Hox genes A group of related genes critical for the assignment of the correct number and placement of embryonic segment structures (such as legs, antennae, and eyes) in multicellular eukaryotic organisms. The genes specify the anterior-posterior axis and segment identity of during early embryonic development.

Human Genome Project An international project whose goal is to map and sequence the entire human genome.

Hybridisation see Annealing

Hybrid vector A vector genetically engineered to a vector contain an additional DNA fragment.

Hydrogen bond A weak, non-covalent force between an electro-negative atom (usually oxygen or nitrogen) and a hydrogen atom which has a slight positive charge because it is covalently bonded to another electronegative atom.

Hydrophilic A chemical group that prefers to be in contact with water.

Hydrophobic bonding The association of nonpolar groups with each other in aqueous solution.

Hydrophobic interaction see Hydrophobic bonding

Hydrophobic A chemical group which prefers not to be in contact with water.

Hypothesis A possible answer to a question to which the answers are not known. Testable predictions can be generated from the hypothesis.


In situ From the Latin, meaning in the natural place. Experiments performed in situ are performed directly on cells or tissue.

In vitro mutagenesis see Site-Directed Mutagenesis

In vitro From the Latin meaning "in glass". Experiments performed in vitro are performed in the test tube.

In vivo From the Latin meaning "in the living organism". Experiments performed in vivo are performed on cells or tissue, not in the test tube.

Inducer A small molecule which causes a large increase in particular enzymes which are usually involved in metabolism of the small molecule.

Inducible gene A gene whose level of expression is directly dependent on the presence of compound (the inducer) in the system.

Induction An increase in the synthesis of an enzyme or series of enzymes in response to the appearance of a specific molecule (inducer) in the medium.

Inhibitor A molecule that binds to enzymes and decrease their activity.

Initiation complex In protein synthesis: A combination of a ribosome, an mRNA molecule, and the amino acid-initiator tRNA complex formed prior to the start of translation.

In DNA synthesis: A combination of RNA polymerase and a series of proteins involved in transcription initiation eg the TATA box binding protein bound together with DNA at the promoter.

Initiation factor One of a set of proteins required for the association of ribosomes, mRNA and tRNA prior to the start of protein synthesis.

Interphase The period between successive nucelar divisions during which the chromosomes are diffuse and the nuclear envelope is intact. It is during this period that DNA replicates and the cell is most active in transcribing and translating genetic information.

Intervening sequence see Intron

Intron Part of a primary transcript (or the DNA from which it is transcribed). The intron is removed during RNA processing (splicing) and is not found in the mature, functional RNA.

Inverted repeat A sequence present twice (but one is in reverse orientation to the other) in a DNA molecule. It has an axis of symmetry and is able to fold back on itself and two regions can base-pair with one another. An examples is GGCATT…..AAGCC.



Karyokinesis Division of the nucleus, usually an early stage in the process of cell division, or mitosis.

Karyotype Number, sizes, and shapes of the entire set of metaphase chromosomes of a eukaryotic cell nucleus.

Kilobase One thousand base pairs in a DNA molecule or 1,000 bases/nucleotides in ssDNA or RNA.

Kinase An enzyme that transfers the terminal (g) phosphate group from ATP to another molecule.

Kinetochore The protein structure on chromosomes where the spindle fibers attach during cell division to pull the chromosomes apart.

Knockout gene Specific replacement of a single normal allele with a mutated copy.


Lagging strand The new DNA strand synthesised discontinuously at the replication fork, in a 5' to 3' direction, gives rise to synthesis in an overall 3' to 5'.

Lambda phage vectors Recombinant viruses, containing the phage chromosome plus inserted 'foreign' DNA. In general, phage vectors can carry larger DNA sequences than plasmid vectors.

Leader sequence In proteins: the sequence of amino acids found at the N-terminal end responsible for directing the newly synthesised protein to a specific cellular compartment eg the endoplasmic reticulum.

Leading strand The new DNA strand synthesised continuously at the replication fork, in a 5' to 3' direction.

Leptotene First of five phases within prophase I, chromosomes start to condense.

Ligand A substance that is able to bind to and form a complex with a biomolecule to serve a biological purpose.

Ligase An enzyme that links the 3' OH of one nucleic acid strand with the 5' phosphate end of an adjacent strand to form a single molecule.

Ligation The covalent joining of two nucleic acicds.

Linkage map A linear or circular diagram that shows the relative positions of genes on a chromosome as determined by patterns of inheritance.

Linkage The degree to which genes are co-inherited due to being on the same chromosome.

Linker A short, synthetic, double-stranded DNA containing one or more restriction enzyme cleavage sites.

Locus (pl. loci) A position on a chromosome that is occupied by a given gene or one of its alleles.

DNA see Deoxyribonucleic AcidA DNA sequence directly repeated at each end of a retrotransposon or integrated retrovirus.

DNA see Deoxyribonucleic Acidsee Long terminal repeat

Lysogenic virus A virus that can replicate in one of two ways. It can grow and kill the cell or it can adopt an inactive (lysogenic) form, where it integrates its viral DNA into the bacterial chromosome.

Lysogenic A bacterium which contains an integrated copy of a viral genome in its chromosomal DNA.

Lytic infection A virus infection that leads to the breakdown of the host cell and release of many progeny virus particles.


M phase The period of the cell cycle when mitosis takes place.

M phase see Cell cycle

mRNA see Messenger RNA

Major and minor grooves The two types grooves within the detailed structure of DNA, the major groove has the nitrogen and oxygen atoms of the base pairs pointing inward toward the helical axis, whereas in the minor groove, the nitrogen and oxygen atoms point outwards; important because the major groove is more dependent on base composition and may be the site for protein recognition of specific DNA sequences or regions.

Map unit In eukaryotic genetics, one map unit corresponds to a recombinant frequency of 0.01 (i.e. 1 in 100 gametes are recombinant).

Mapping DNA Various techniques for determining the relative order of genes on a chromosome (genetic map) or the absolute position of known sequences (physical map).

Mapping In genetics, determining the order of genes on a chromosome and the distances between them.

MCS see Polylinker

Meiosis Division of a diploid nucleus to produce four haploid daughter cells. The process consists of one cycle of chromosome replication followed by two successive cell divisions.

Melting Temperature The temperature at which DNA is half denatured; AT rich DNA has a lower melting temperature than GC rich DNA, the abbreviation for melting temperature is Tm

Mendelian population A sexually interbreeding unit of plants or animals sharing a common gene pool.

Messenger RNA (mRNA) A transcript of one of the strands of DNA. It carries information to the ribosomes for the synthesis of proteins.

Metaphase Stage of mitosis at which chromosomes are fully condensed and attached to the mitotic spindle at its equator but have not yet started to segregate toward the opposite spindle poles.

Metaphase plate See equatorial plane/plate.

Microarray see DNA microarray.

Micropipette Instrument used to accurately measure and dispense small volumes of liquid. The capacity of a micropipette can range from less than 1µl to 1000µl (1ml).

Minor grooves see Major and minor grooves

Mismatch repair A repair system that removes the incorrect base in a mismatched base pair and inserts the correct one.

Missense mutation A mutation in a DNA sequence that alters a codon so that it specifies a different amino acid.

Mitochondria A membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 10 micrometers (µm) in diameter. Mitochondria are sometimes described as "cellular power plants" because they generate most of the cell's source of chemical energy.

Mitochondrial inheritance The inheritance of a trait encoded in the mitochondrial genome, always of maternal origin.

Mitosis Nuclear division in eukaryotes that leads to the formation of two daughter nuclei each with a chromosome complement identical to that of the original nucleus.

Mitotic apparatus See mitotic spindle

Mitotic spindle The spindle fibres that separates the chromosomes into the daughter cells during cell division.

Mitotic spindle The spindle fibres that separates the chromosomes into the daughter cells during cell division.

Molecular clock A technique in molecular evolution that uses fossil constraints and rates of molecular change to deduce the time in geologic history when two species or other taxa diverged.

Morphogens A substance governing the pattern of tissue development and, in particular, the positions of the various specialized cell types within a tissue. It spreads from a localized source and forms a concentration gradient across a developing tissue.

Morphogenesis Change of shape and movement of cells to produce the structure of an organism.

mRNA see Messenger RNA

Multicellular An organism which consists of more than one cell.

Multiple alleles A condition in which a particular gene occurs in three or more allelic forms within a population.

Multiple cloning site see Polylinker

Mutagen A physical or chemical agent that can alter the structure of bases in nucleic acids. The altered sequence brings about a heritable change (mutation) in an organism.

Mutagenesis A process by which a physical or chemical agent can alter the structure of bases in nucleic acids. The altered sequence brings about a heritable change (mutation) in an organism.

Mutant A gene, cell or organism that demonstrates a heritable change due to a mutation.

Mutation A heritable change in the nucleotide sequence of a chromosome. The extent of the change is usually small and extends over a single gene. It is identified because it leads to a change in gene function.


Nascent RNA RNA which is in the process of being synthesised.

National DNA Database A government database of DNA profiles which can be used by law enforcement agencies to identify suspects of crimes.

Natural selection A process by which heritable traits conferring survival and reproductive advantage to individuals, or related individuals, tend to be passed on to succeeding generations and become more frequent in a population, whereas other less favourable traits tend to become eliminated.

Nicked DNA Helix containing single stranded breaks.

Nick-translation A procedure for labelling DNA by nicking it with an endonuclease and then using E.coli DNA polymerase I to intiate DNA synthesis at this nick. The 5' to 3' nuclease activity associated with DNA polymerase degrades the DNA in front of it and synthesises DNA behind it so "translating" the nick along the DNA molecule.

Nitrogenous base see Base

Non-Mendelian inheritance patterns A general term that refers to any pattern of inheritance in which traits do not segregate in accordance with Mendel’s laws. These laws describe the inheritance of traits linked to single genes on chromosomes in the nucleus. Examples of non-Mendelian inheritance patterns include complex and multifactorial inheritance, mitochondrial inheritance and genomic imprinting.

Nonsense mutation A change in the DNA sequence that alters a codon from one that specifies an amino acid to one that signals termination of translation (a nonsense codon). There are three nonsense codons (UAG, UAA, or UGA).

Nontemplate strand see Sense strand

Northern blotting The transfer of RNA molecules from an agarose gel to a cellulose or nylon membrane by capillary action followed by probing with a labelled nucleic acid.

Nuclease An enzyme that degrades nucleic acids by cleaving phosphodiester bonds.

Nucleic acid A macromolecule composed of the sugars deoxyribose or ribose linked by phosphodiester bonds. The bases adenine, thymine, uracil, guanine, or cytosine (A, T, U, G, or C) are attached to the sugars. DNA and RNA are examples of nucleic acids found in cells.

Nucleohistone A complex of DNA and histone.

Nucleoid An irregularly-shaped region within the cell of prokaryotes which has nuclear material without a nuclear membrane and where the genetic material is localised.

Nucleolus A spherical body within the nuclei of eukaryotic cells. The nucleolus is involved in the synthesis of ribosomal RNA.

Nucleoplasm The solution containing small molecules within the nucleus of a cell. It does not include the larger structures such as chromosomes or nucleoli.

Nucleoside A small molecule composed of a purine or pyrimidine base linked to a pentose sugar (ribose or deoxyribose).

Nucleosome A complex of approximately 145bp of DNA duplex wrapped around an octamer of histone proteins (2x H2A, 2xH2B. 2x H3 and 2xH4).

Nucleotide A small molecule composed of a purine or pyrimidine base linked to a pentose sugar (ribose or deoxyribose) and one or more phosphate groups.

Nucleus The centrally located organelle in eukaryotic cells that is surrounded by a lipid bilayer and contains the cell's chromosomes.


Okazaki fragment A short segment of single stranded DNA that is an intermediate in DNA synthesis. The Okazaki fragment is base paired to the DNA template and covalently linked to the RNA primer at the 5' end.

Oligo see Oligonucleotide

Oligomer A polymeric molecule of intermediate size made up of between two and tens of a monomers.

Oligonucleotide A polynucleotide containing a small number (two to tens) of a nucleotide.

Oncogene A viral or cellular gene that is responsible for uncontrolled growth of animal cells.

Oocyte The egg-mother cell i.e. the cell that undergoes two meiotic divisions (oogenesis) to form the egg cell. Primary oocyte - before completion of the first meiotic division; secondary oocyte - after completion of the first meiotic division.

Open reading frame The sequence of codons, in DNA or RNA, that extends from a translation start codon to a stop codon.

Operon A group of genes found in bacteria which make up a co-regulated unit. This unit includes an operator, promoter and structural genes.

ORF see Open reading frame

Organelle A specialised subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer.

Ori see Origin of replication

Origin of replication A DNA sequence involved in the initiation of DNA replication. The enzyme helicase binds here and unwinds the DNA double helix so that DNA polymerase can bind here.



P arm The short arm of the chromosome (its name comes from the French for small- petit).

P site see Peptidyl (P) site

Pachytene Third of five phases within prophase I, crossing over between pairs of homologous chromosomes to form chiasmata (sing. chiasma).

Palindrome A sequence of bases that is the same as its complementary sequence (both read in the 5' to 3' direction) e.g. GAATTC.
Parental imprinting see Genomic imprinting
PCR see Polymerase chain reaction

Pedigree The pattern of inheritance of a genetic trait in a family.

Penetrance The proportion of individuals with a genotype who show the expected phenotype.

Peptidyl (P) site The binding site on a ribosome that contains the tRNA attached to the growing polypeptide chain.

Peptidyl transferase An enzyme activity contained in the large subunit of the ribosome which catalyses the formation of peptide bonds between amino acids.

Phage see Bacteriophage

Phagemids Chimaeric cloning vectors that contain nucleic acid sequences derived from both phage and plasmids.

Phenotype The observable characteristics of a cell or organism.

Phosphate group The functional group -OPO3H2.

Phosphodiester bond The grouping created by two hydroxyl groups which form ester linkages to the same phosphate group.

Phylogenetic tree A representation of lines of descent among organisms.

Pilus A tube like structure found on the surface of some bacteria which are used to adhere to other bacteria during conjugation.

Plaque assay A procedure for determining the number of infectious viral particles in a sample by infecting susceptible host cells with a diluted sample of virus and and then counting the clear areas of lysed cells (plaques) that are produced.

Plaque A circular clearing on a lawn of bacterial or cultured cells, resulting from cell lysis, production of phage or animal virus progeny and re-infection of adjacent cells.

Plasmid A small, circular autonomously replicating, extrachromosomal DNA molecule. Commonly used as a cloning vector.

Pluripotent Pluripotent stem cells can give rise to any fetal or adult cell type. However, alone they cannot develop into a fetal or adult animal because they lack the potential to contribute to extraembryonic tissue, such as the placenta.

Point mutation A single base change in DNA. Most easily detected when it occurs in a region coding for protein.

Polar see Hydrophilic

Polarity DNA synthesis uses 5' nucleotide triphosphates as precursor and proceeds by linking the 5' triphosphate of the incoming nucleotide to the 3' OH of the growing chain. Hence DNA strands have polarity ie a 5' and a 3' end.

Poly(A) polymerase An enzyme that adds consecutive adenosines to the 3' termini of eukaryotic mRNAs to generate poly(A) tails.

Polyadenylation The addition of poly(A) tails to mRNAs.

Polycistronic messenger RNA is found in prokaryotes and is derived from transcription of operons. The operon, and hence the mRNA, contains two or more cistrons (genes).

Polycloning site see Polylinker

Polylinker A synthetic fragment of DNA that has been inserted into a cloning vector. It contains a set of unique restriction enzyme cleavage sites.

Polymerase chain reaction A technique for the rapid production of millions of copies of a particular stretch of DNA. It involves multiple cycles of denaturation, hybridisation to oligonucleotide primers and DNA synthesis.

Polymerase An enzyme that catalyses the synthesis of a polymer (usually DNA or RNA) from monomers.

Polymorphic Occuring in a number of different forms e.g. genes occurring in a number of alleles.

Polymorphism DNA polymorphism: variable characteristic within a population of the same genes (e.g. hair colour). Presence of more than one sequence at a locus (allele) in a population.

Polynucleotide A linear polymer of nucleotides linked together by phosphodiester (5'-3') bonds.

Polypeptide A linear polymer of amino acids held together by peptide linkages.

Polyploid A cell or an organism in which the number of complete sets of chromosomes is greater than two.

Polyribosome A complex consisting of a single messenger RNA with several ribosomes synthesising polypeptide chains.

Polysome see Polyribosome

Population genetics The study of genetic variation within populations, involving the examination and modelling of changes in the frequencies of genes and alleles in populations over space and time.

Positional cloning The isolation of a DNA sequence (linked to a particular genetic characteristic) based on its map position in the genome.

Post-transcriptional processing The process of conversion of primary transcript RNA is into mature RNA in eukaryotic cells e.g. the conversion of precursor messenger RNA into mature messenger RNA (mRNA), which undergoes three main modifications: 5' capping, 3' polyadenylation, and RNA splicing, which occur in the cell nucleus before the RNA is translated.

Precursor mRNA Initial mRNA transcript (precursor mRNA or pre-mRNA) which contains introns and exons. These transcripts undergo RNA processing to form the mature mRNA species.

Preimplantation genetic diagnosis Procedures that are performed on embryos prior to implantation, sometimes even on oocytes prior to fertilization (also known as embryo screening).

Primary transcript see Heterogeneous nuclear RNA

Primase A RNA polymerase that synthesises short stretches of RNA for use as primers for DNA synthesis.

Primer A Short nucleic acid that base pairs to complementary sequenes on a single stranded DNA and serves as a starting point for DNA synthesis.

Proband The individual in a family to whom an inherited trait is first associated.

Probe A segment of single stranded nucleic acid which is labelled and then used to identify, by hybridisation, other nucleic acid molecules containing the complementary sequence.

Prokaryote Single celled organisms (e.g. bacteria and blue-green algae) that do not contain a nucleus or membrane-bound organelles.

Prokaryotic cell A cell that lacks membrane bound structures such as a nucleus.

Prometaphase Phase within meiosis where spindle apparatus is formed, and chromosomes attach to these spindle fibres.

Promoter A DNA sequence that signals RNA polymerase binding and the site of transcription initiation.

Proofreading The enzymatic checking for error in DNA replication such as mis-matched base pairs after incorporation of each nucleotide into the growing polynucleotide chain.

Prophase The first stage of nuclear division, during which chromosomes condense.

Protein A long-chain polymer of combinations of twenty different amino acids with characteristic properties.

Proteome The entire set of proteins expressed by a genome, cell, tissue or organism.

Proteome analysis Analysis of the entire complement of protein produced by an organism or system. This will include analysis of modifications made to a particular set of proteins over time and with distinct requirements, or stresses.

Proteomics The large-scale study of proteins, particularly their structures and functions.

Protoplast see Spheroplast

Pseudogene A DNA sequence, derived by gene duplication, that is very similar to a functional gene but contains sequence changes that prevents its expression.

Punnett square A diagram that is used to predict an outcome of a particular cross or breeding experiment.

Purine A basic compound, containing two fused heterocyclic rings, found in nucleic acids. The purines commonly found in DNA and RNA are adenine and guanine.

Pyrimidine A basic compound, containing one heterocyclic ring, found in nucleic acids. The pyrimidines commonly found in DNA are cytosine and thymine while in RNA thymine is replaced by uracil.


Q arm The long arm of a chromosome, named as q follows p in the alphabet (p is the short arm of the chromosome).

Quiescent The state of a cell when it is not dividing.


R factor see Antibiotic Resistance Factor

Reading frame see Open reading frame

Recessive Refers to that allele of a gene that is not evident in the phenotype when the dominant allele is present; also refers to the phenotype of an individual carrying two recessive alleles (a homozygote).

Recombinant DNA technology The application of molecular biological tools (restriction endonucleases, plasmids, and transformation) to the production of novel DNA molecules in vitro.

Recombinant DNA Any DNA molecule formed, usually in vitro, by joining DNA fragments from different sources.

Recombinant An individual, a cell or a DNA molecule in which genetic material originally present in two individuals ends up in the same individual, cell or DNA molecule. For example, because of recombination at meiosis, humans pass on genes from both parents in a single haploid gamete (egg or sperm).

Recombinant DNA technology see Genetic modification.

Recombination The production of gene combinations not found in the parents or original DNA molecules (see recombinant).

Regulatory gene A gene that codes for the synthesis of a regulatory macromolecule which is often a repressor protein.

Renaturation see Annealing

Repetitive DNA A DNA sequence that is repeated many times per genome.

Replication fork A site at which a DNA molecule is actively replicating. The fork forms at the origin of DNA replication in a single satranded area formed by the unwinding of the parent molecule. Two replication forks then move birectionally away from this origin.

Repressible enzyme An enzyme whose synthesis can be regulated by the presence or absence of a particular small molecule.

Repression The turning off of gene expression in response to the presence of a small molecule in the medium.

Repressor gene A gene that encodes a repressor.

Repressor Under certain condition e.g. the presence of a small molecule in the medium the repressor can bind to a specific operator and prevent transcription of the operon.

Resistance Factor see Antibiotic Resistance Factor

Restriction Endonuclease see Restriction enzyme

Restriction enzyme An endonuclease that recognises a specific short sequence (the restriction site) in a double stranded DNA molecule and cleaves the DNA molecule at or near that site.

Restriction enzyme mapping The compilation of map of known restriction sites within a sequence of DNA.

Restriction fragment A defined DNA fragment generated by cleavage with a restriction enzyme.

Restriction map A linear or circular physical map of a DNA molecule showing the positions of some restriction enzyme sites on the DNA.

Restriction mapping see Restriction enzyme mapping

Restriction site A DNA sequence that is cleaved by a restriction enzyme.

Retroposon A transposable DNA element that moves from one position in the genome to another via transcription of the DNA into RNA followed by reverse transcription of RNA into DNA and re-insertion into the genome. There are two families of retroposon called LINES and SINES and they both lack long terminal repeat sequences.

Retrovirus A virus that uses RNA as its genetic information but replicates via a DNA intermediate using reverse transcriptase to synthesise a DNA copy of its RNA genome.

Reverse transcriptase An enzyme that catalyses the synthesis of DNA (cDNA) from a RNA template.

Reverse transcription The process catalysed by reverse transcriptase in which an RNA template is used to synthesise a DNA strand.

Reverse transcriptase PCR (RT-PCR) A variant of polymerase chain reaction (PCR). In RT-PCR, however, an RNA strand is first reverse transcribed into its DNA complement using the enzyme reverse transcriptase, and the resulting cDNA is amplified using traditional PCR.

RFLP (Restriction Fragment Length Polymorphism) Genetic differences between populations which are seen by comparing the DNA fragments produced by digestion with restriction enzymes.

Ribonuclease An enzyme that digests an RNA molecule partially or completely forming ribonucleotides.

Ribonucleic Acid (RNA) A polynucleotide made up of nucleotides linked by phosphodiester bonds. The nucleotides contain adenine, guanine, cytosine and uracil and the sugar is ribose.

Ribose The five-carbon sugar (pentose) found in RNA.

Ribosomal RNA (rRNA) The RNA component of the ribosome, the protein manufacturing machinery of all living cells. Ribosomal RNA provides a mechanism for decoding mRNA into amino acids and interacts with tRNAs during translation by providing peptidyl transferase activity.

Ribosome A complex organelle made up of two or three different rRNA molecules and more than 50 proteins. It is composed of large and small subunits and is involved in protein synthesis.

Ribozyme An RNA molecule with which folds into a defined 3 dimensional structure and has catalytic activity.

RNA see Ribonucleic acid

RNA editing A Post-transcriptional processes that changes the sequence of an RNA molecule in a defined manner and changes the encoded protein.

RNAi (RNA interference) Specific degradation of RNA sequences that share sequence homology with double stranded RNA.

RNA polymerase An enzyme that catalyses the formation of RNA from a DNA template.

RNA processing Many, but not all RNAs, undergo a number of modifications after transcription. This can involve methylation, splicing, and addition of a cap and a poly(A) tail.

RNAse see Ribonuclease

RNA silencing Post-transcriptional control involving sequence-specific RNA degradation.

RNA splicing A process that results in the precise cutting of RNA, removal of introns and re-joining of exons to produce a fully functional RNA.

RNA transcript A complementary, antiparallel RNA strand, produces from a DNA template by RNA polymerase.

rRNA see Ribosomal RNAs


S phase The stage in the cell cycle during which DNA is replicated.

S see S phase

Segregation (genetic) The separation of alleles, or of homologous chromosomes, from one another during meiosis so that each of the haploid daughter nuclei produced by meiosis contains one or the other member of the pair found in the diploid mother cell, but never both.

Semiconservative replication The method by which DNA is synthesised. The two daughter duplex each contain one parental strand and one new strand.

Sense RNA A precursor or mature mRNA that contains the coding region of a gene that can translated to produce a polypeptide.

Sense strand That strand of DNA which is not transcribed. It will have the same sequence as the RNA produced from the gene except that the T in DNA is replaced by U in the RNA transcript.

Sequencing see Dideoxy (Sanger) sequencing.

Sex cells Sperm and eggs with only one copy of each chromosome, opposed to body cells that have two copies of each chromosome. See meiosis

Sex chromosome Chromosomes involved in sex determination (in humans X and Y).

Sex linkage Association of an hereditary trait with sex; the gene is usually on the X chromosome; often used synonymously with X-linkage.

Sex-linked or X-linked An inherited trait encoded by a gene on the X chromosome

Sigma factor A subunit found in prokaryotic RNA polymerase that is involved in the recognition of specific sites on DNA involved in initiation of RNA synthesis.
Silencers Regions bound by particular transcription factors (called repressors) that depress the rate of transcription.

Single nucleotide polymorphism see SNP

Single strand binding protein see Single-strand DNA-binding protein

Single-strand DNA-binding protein A protein that binds to single stranded DNA during replication preventing it from reforming double strands during the replication process.

siRNA Small interfering RNAs involved in RNA silencing.

Site-Directed Mutagenesis A technique which can be used to alter a specific nucleotide sequence to another predetermined sequence

Small nuclear RNAs (snRNAs) Small RNA molecules found in the nucleus of eukaryotic cells. Most snRNAs are components of the small nuclear ribonucleoprotein particles that are involved in recognition and excision of introns from pre-mRNAs.

SNP Genetic differences between populations which are seen by comparing the DNA sequences of a number of individuals. These single base differences are found, on average, every 1000bp and occur throughout the DNA, not only in coding sequences. They are useful in linkage analysis.

SnRNP see Small nuclear ribonucleoprotein particle

Solenoid A three dimensional coil; DNA is wrapped around histone proteins which in turn coil to form a solenoid.

Somatic Pertaining to the body, or body cells (rather than to germ cells).

Somatic cells Cells forming the body of an organism, as opposed to gametes (e.g. the spermatozoa and ova which fuse during fertilization in mammals).

Southern blotting The transfer of DNA molecules from an agarose gel to a cellulose or nylon membrane by capillary action originally developed by Edward Southern. Subsequently the positions of particular DNA fragments are identified by probing with a labelled nucleic acid.

Speciation The evolutionary process by which new biological species arise.

Spindle apparatus An array of microtubules resembling a spindle stretching from pole to pole of a dividing nucleus and required for movement of chromosomes at nuclear division.

Splicing see RNA splicing

Stacking energy The energy of (hydrophobic) interaction that gives rise to the face-to-face packing of base pairs in double stranded nucleic acids.

Start codon The mRNA triplet (AUG) that is recognised by the ribosome as a signal for the start of translation

Start site A start site for transcription.

Stem cell A primitive somatic cell which is able to divide and differentiate to produce other cells.

Stem cell research Creation of techniques to replace diseased cells and regenerate the body using stem cells.

Sticky ends Short, complementary, single-stranded regions produced by restriction enzymes. Digestion of DNA from different sources with the same restriction enzyme generate identical sticky ends and allow the two DNAs to be joined to each other.

Stop codons One of three codons in mRNA (UAG, UGA, UAA) that function as signals for the termination of translation by ribosomes.

Structural gene A gene that codes for the amino acid sequence of a polypeptide.

STSs (sequence-tagged sites) Short, unique DNA sequences which can be amplified by PCR. Their presence in particular DNA fragments is used to link physical and genetic maps.

Supercoiled DNA A coiled coil present in a DNA molecule in which the double helix is twisted on itself.
Syncytial specification. Cell differentiation is the result of interactions within cells, not between cells. Incomplete cell divisions after fertilization result in a cell containing many nuclei (a syncytium). Morphogens diffuse through the syncytium to set up concentration gradients that give cells information about their position: for example, along the anterior-posterior axis.

Syncytium A large cell-like structure filled with cytoplasm containing many nuclei. Most cells in eukaryotic organisms have a single nucleus; syncytia are specialized forms used by various organisms.


tRNA see Transfer RNA

Taq polymerase A heat-stable DNA polymerase used in the polymerase chain reaction which is isolated from the thermophilic bacterium Thermus aquaticus.

TATA box A conserved promoter sequence that determines the transcription start site. Found in prokaryotic and eukaryotic protein-coding genes the TATA box signals that the initiation point for transcription is nearby (10 - 35 bases) depending on whether the organism is prokaryotic or eukaryotic.

Telomerase An enzyme that adds DNA sequence repeats ("TTAGGG" in all vertebrates) to the 3' end of DNA strands in the telomere regions, which are found at the ends of eukaryotic chromosomes.

Telomeres Repeated DNA sequences found at the ends of eukaryotic chromosomes.

Telophase The final phase of mitosis or meiosis during which chromosomes become diffuse, nuclear envelopes reform and nucleoli begin to reappear.

Template One strand of DNA that is complementary the sequence of the RNA and is used to direct synthesis of a RNA during transcription. It is also the name given to the strand of DNA used during DNA replication.

Template strand see Template

Termination signal In transcription, a nucleotide sequence that specifies termination of RNA synthesis.

Thymidine One of the four nucleosides found in DNA.

Thymine A pyrimidine base found in DNA.

Topoisomerase An enzyme that adds or removes supercoils from a DNA duplex.

Totipotent A cell capable of differentiating into every type of cell found in an organism.

Transcription factor Any protein, excluding RNA polymerase, which can regulate transcription in eukaryotic cells. These can be general and affect transcription of all genes or specific and affect particular genes by binding to specific transcription elements.

Transcription unit A segment of DNA that contains transcription initiation and termination signals which flank a gene and is transcribed into a RNA molecule.

Transcription A process by which RNA polymerase catalyses the formation of a RNA using a DNA template.

Transduction Transfer of genes from one bacterium to another by a bacterial virus which carries host cell genes.

Transfection Refers to the introduction of foreign DNA (which may or may not be viral) into cultured eukaryotic cells.

Transfer RNA One of a family of low molecular-weight RNAs (approximately 75 bases) that transfer amino acids from the cytoplasm to the ribosome for protein synthesis.

Transformation Genetic alteration of bacteria through the in vitro incorporation of foreign DNA in bacterial cells.

Transgene A foreign or modified gene that has been stably incorporated into a plant or animal so that it is passed on to successive generations.

Transgenic A term applied to an organism that has been genetically altered by introducing DNA molecules (usually from a different organism) into them.

Transition see Base substitution

Translation The process of decoding a messenger RNA sequence into an amino acid sequence to give a functional protein.

Transposable element A segment of DNA, found in prokaryotes and eukaryotes, that moves from or gives rise to copies at another position on the same or different chromosome.

Transposon see Transposable element

Transversion see Base substitution

Triplet repeat Repeated, unstable, triplets of bases in a DNA sequence which, when expanded, may give rise to a genetic disease.

tRNA see Transfer RNA


Upstream The opposite direction to the movement of RNA polymerase during transcription ie towards the 5' phosphate containing end of a nucleic acid.

Uracil A pyrimidine base found in RNA but not in DNA. In DNA, uracil is replaced by thymine.


Vector A plasmid or virus that may be used to construct recombinant DNA molecules by insertion of a piece of DNA in recombinant DNA technology.

VNTR (variable number tandem repeat) A short, tandemly repeated sequence found in DNA in highly variable copy number.


Watson-Crick base pairs The normal type of hydrogen-bonded base pairs found in DNA or RNA. These are A-T, G-C, and A-U.

Western blotting A technique used in the detection of specific proteins in a mixture. The proteins are first separation by polyacrylamide gel electrophoresis, blotted on to a membrane and then specific proteins detected by labelled antibodies.

Whole genome shotgun sequencing DNA is broken up randomly into numerous small segments, which are sequenced using the chain termination method to obtain reads. Multiple overlapping reads for the target DNA are obtained by performing several rounds of this fragmentation and sequencing.

Wild type Normal, non-mutant form of a macromolecule, cell, or organism.

Wobble A hypothesis which explains that base-pairing between a codon and anti-codon is not of the Watson-Crick type because of steric problems. The result is that the rules governing base pairing of 3' base in the codon and the 5' base in the anti-codon are less stringent.


X chromosome A chromosome associated with sex determination. In most animals, the male has one and the female has two X chromosomes.

Xenotransplantation The transplantation of living cells, tissues or organs from one species to another, such as from pigs to humans.

X-linked see Sex-Linked


Y chromosome A sex chromosome present in the male of many animal species.

YAC see Yeast Artificial Chromosome

Yeast Artificial Chromosome A circular cloning vector which can be converted to linear form. It is constructed from essential elements of yeast chromosomes. YACs can accommodate foreign DNA inserts up to 1000 kb in size and when linearised they will replicate as if they were natural chromosomes.


Z DNA A duplex DNA structure (normally found in regions of DNA sequence with a high G-C content) in which normal hydrogen bonding between the base pairs takes place but the helix is left-handed (zig-zagged) rather than right-handed.

Z form see Z DNA

Zinc finger DNA-binding motifs found in proteins. Some of the histidines or cysteines in the protein domains are bonded to a zinc ion. Zinc fingers are present in many eukaryotic transcription factors.

Zygote A fertilized egg; diploid cell resulting from fusion of the male and female gametes.

Zygotene Second of five phases within prophase I, homologous chromosomes become closely associated (synapsis) to form pairs of chromosomes (bivalents) consisting of four chromatids (tetrads).


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