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Gene Regulation in Eukaryotes
The latest estimatesare that a human cell, a eukaryotic cell, contains some 21,000 genes.
- Some of these are expressed in all cells all the time. These so-called housekeeping genes are responsible for the routine metabolic functions (e.g. respiration) common to all cells.
- Some are expressed as a cell enters a particular pathway of differentiation.
- Some are expressed all the time in only those cells that have differentiated in a particular way. For example, a plasma cell expresses continuously the genes for the antibody it synthesizes.
- Some are expressed only as conditions around and in the cell change. For example, the arrival of a hormone may turn on (or off) certain genes in that cell.
How is gene expression regulated?
There are several methods used by eukaryotes.
- Altering the rate of transcription of the gene. This is the most important and widely-used strategy and the one we shall examine here.
- However, eukaryotes supplement transcriptional regulation with several other methods:
- Altering the rate at which RNA transcripts are processed while still within the nucleus. [Discussion of RNA processing]
- Altering the stability of messenger RNA (mRNA) molecules; that is, the rate at which they are degraded. [Link to discussion of RNA interference]
- Altering the efficiency with which ribosomes translate the mRNA into a polypeptide. [Examples]
Protein-coding genes have
- exons whose sequence encodes the polypeptide;
- introns that will be removed from the mRNA before it is translated [Discussion];
- a transcription start site
- a promoter
- the basal or core promoter located within about 40 base pairs (bp) of the start site
- an “upstream” promoter, which may extend over as many as 200 bp farther upstream
- enhancers
- silencers
Adjacent genes are often separated by an insulator which helps them avoid cross-talk between each other’s promoters and enhancers (and/or silencers).
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