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The control tasks of the genome
The genome is responsible for controlling cellular tasks such as response to environmental conditions, the cell division cycle and cell differentiation. Each of these require the regulation of gene expression in both space and time.
Throughout its lifetime, a cell must respond to many different types of environmental signals. Single-celled bacteria are able to detect and move towards nutrient sources, they also react to changes in temperature and acidity. Multicellular cells must also respond to chemical signals emitted by neighbouring cells in the organism. These external signals are transmitted to the genome via a series of chemical reactions known as signal transduction pathways [50].
As well as responding to external signals, the genome is also subject to internal control. The cell cycle plays the role of a cell's internal clock [88]. In order for an organism to develop, each embryogenic cell goes through a process of growth, replication and division. During cell growth, a cell increases in size. Its entire genome is then replicated to produce two identical copies. When the cell divides, each of its daughter cells contains one complete copy of the genome. The signals that tell a cell when to switch from growth to replication and from replication to division are controlled by a subset of genes that regulate timing.
Each cell of a multicellular organism contains identical genetic information (with some rare exceptions). The feature that distinguishes cells of different types is the set of genes that are active in a particular cell. This pattern activation determines which proteins are produced, and hence the functional properties of the cell. When an egg cell is initially fertilised, it is fully undifferentiated and has the potential to become any type of cell. As an organisms developmental program unfolds, its cells divide and undergo physical and chemical changes that result in their final fates (for example, as blood or skin cells) becoming more specified [142]. The role of the gene regulatory network in this process is to integrate the internal dynamics of the cell and external signals from the environment and other cells to control the differentiation process.
Next: Further reading Up: Biological background Previous: The basics of gene Nic Geard 2004-05-06