Horizontal Gene Transfer

Horizontal Gene Transfer for schools and colleges

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Horizontal DNA transfer is the exchange of genes between two cells of the same generation, as opposed to from parent to progeny.

Normally, genes and the characteristics they code for are passed down from parent to progeny. This is called vertical gene transfer and is why you have half of the charecteristics of your mother, and half of your father. Bacteria and some lower eukaryotes are unique in that they can pass DNA from one cell of the same generation to another. We refer to this as Horizontal Gene Transfer (see diagram below).

DNA transfer

There are three ways for bacteria to transfer their DNA horizontally:

Conjugation-

Conjugation is the transfer of DNA directly from one cell to another through cell-cell contact. The DNA transferred by conjugation often involve plasmids. Plasmids are circular pieces of DNA that can replicate in the bacterial cell, independently of the chromosome. The conjugative transfer of plasmids is carried out by cell surface structures that act like syringes, injecting the plasmid into neighbouring cells.

You can watch a video describing conjugation by clicking here.

Transformation-

Unlike humans, bacteria are capable of taking up DNA directly from their enviroment and incorporating it into their genomes.This process is known as natural transformation. This DNA usually comes from dead bacteria lysing (splitting open) and releasing their genetic contents into the surrounding area.

Transduction-

Transduction is the transfer of DNA from one cell to another by a virus. These viruses are known as bacteriophage and they specifically infect bacteria. Bacteriophage don't have the machinery to replicate their own genomes or express their own genes, so instead, they hijjack the bacterial machinery to do so.  Host cells will continue to express phage proteins and replicate the phage genome forming new virus particles. This process continues until the cell is so full of phage particles that it splits open (lyses), releasing phage into the surrounding area. This is known as the lytic cycle. Some phage can switch between this life cycle and a state of lysogeny, where they combine their genome with the bacterial chromosome, and remain silent for many generations. When lysogenic phage remove (excise) their genomes from the host chromosome, they occasionally take small sequences of bacterial DNA with them. Phage genome containing bacterial DNA is then packaged into phage coat proteins to form a complete, recombinant virus particle. When these phage lyse the bacterial cell and re-infect a new host, they take bacterial DNA with them.

Click here to watch a narrated animation about phage transduction.

HGT

The diagram above demonstrates the three ways bacteria can acquire DNA horizontally. In this example, the bacteria are acquiring genes which make them resistant to antibiotics. Source: Nature, Furuya and Lowy (2006).

Conjugation is the transfer of circular DNA called plasmids through cell to cell contact. Transformation is the uptake of 'free' DNA from the environment. Transduction is the transfer of DNA by bacteria-specific viruses called bacteriophage.

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