Genes code for proteins, and proteins build bodies: a salmon fry hatching from an egg.
The function of genes is to pass on the information necessary to build proteins - and bodies - from one generation to the next. A newly fertilised egg cell has a collection of genes that contains all information needed to transform it from a single cell into an embryo and then an adult. The process that changes a single cell into a new person (or a new frog, or a new oak tree) is called development.
During the course of development, complex structures develop from simple ones. A single cell transforms itself into an adult organism. How does something complicated come from something simple? And how do genes control this process?
Creating an organism from a single cell involves three important processes:
- Cell division: cells divide to produce more cells.
- Cell differentiation: cells change into different types of cell to do specific jobs in the body, from nerve cells to muscle cells.
- Morphogenesis: groups of cells move and change their shape to produce the structure of the organism.
Genes play a vital role in controlling all of these processes.
Genes contain the information a cell needs to make proteins - a bit like a recipe for a living thing. Different genes contain the information needed to make different proteins, and different proteins do different jobs in the cell. The proteins a cell makes decide what kind of cell it becomes, and there are some 350 different types of cell in an adult human being.
Cells change into different types of cell because of changes in the way their genes work. Some genes are activated (switched on), and some genes are inactivated (switched off). As a result, the cell produces a specific set of proteins. So, for example, a nerve cell produces only the proteins needed to make a nerve cell, and a muscle cell produces only the proteins needed to make a muscle cell.
But how do cells switch their genes on and off? And, more importantly, how do they 'know' which genes to switch on and which genes to switch off? The answer lies in special control genes that produce proteins that control the activity of other genes. So, for example, homeotic or homeobox genes control whole sets of other genes to set out the basic body plan of the embryo, separating the front from the back, and producing the right body structure in the right place.
One way in which genes can influence the activity of other genes is through the production of proteins called transcription factors, which stick to special control sites in the DNA at the start of a gene to switch them on and off. (See Gene expression and regulation for more details.)
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