CONTROL OF GENE EXPRESSION

=The Need for Control of Gene Expression:= Every cell in our body contains a complete set of all our genes (our genome). However, every cell only expresses a small fraction of these genes. This is actually what makes different cell types different from one another. For instance, our muscle cells will all express certain genes, while our brain cells will express a different set of genes. Obviously there may be genes involved in basic cellular functions that all cells express, but many genes will only be expressed by certain cell types at certain times. It would be a waste of energy for a cell to produce all of its proteins all the time. Indeed the build-up of unused proteins could even be toxic, for this reason many cells will only produce some of their proteins in response to certain signals (known as stimuli). For example: [image:http://i.imgur.com/AnQXi17.png] Immune system –Increases the expression of immune proteins such as antibodies in response to an infection. [image:http://i.imgur.com/snpRFJN.png] Developmental genes –Certain genes are only expressed during the early stages of foetal development, many others are expressed more / less during the different stages of development also. [image:http://i.imgur.com/C5BXB5t.png] Digestion – Glands associated with our stomach will alter the expression of genes coding for digestive enzymes and proteins that regulate blood sugar levels (such as insulin). As a result it is necessary for a cell to be able to switch genes on and off. The mechanisms for controlling gene expression are considerably different between prokaryotes and eukaryotes. =How Gene Expression is Controlled= Mechanisms of controlling gene expression focus on regulating the transcription of the gene into mRNA. [image:http://i.imgur.com/8v6RRGA.png] This makes sense as it would be wasteful for a cell to transcribe the DNA into mRNA only to then prevent it from being translated into a protein. Therefore, a cell will regulate which genes are transcribed at what time. Once a gene has been transcribed, the cell is committed, translation occurs and a protein is always produced. ==Promoters:== RNA polymerase is the enzyme that will transcribe the DNA into mRNA. Not every gene will start with the same sequence, thus a short sequence upstream (before) the gene/s is required in order for RNA polymerase to recognise where to start transcribing the gene. ==Gene Expression in Prokaryotes & Eukaryotes== Prokaryotes are '''polycistronic'''. This means that they will often express related genes together. A single control region will be followed by several genes, which are all transcribed into a single mRNA which is later translated into several proteins. Eukaryotes are monocistronic. In Eukaryotes every gene has its own control region. This means that; '''One Gene → One mRNA → One Protein'''