Structural Biochemistry, Gene Regulation

Gene expression in organisms is carefully regulated to allow them to adapt the differing conditions and most importantly to prevent wasteful overproduction of unneeded proteins which would make the organism not productive. Organisms regulate expression of their genes for the reasons of developmental changes, cellular specialization and adaptation to the new environment.The expression of genetic information in a given cell or organism is neither random nor fully pre-programmed. The information in an organisms genome must be regulated in an orderly fashion during development and yet must be available to direct the organisms responses to changes in internal and external conditions.

In prokaryotes gene expression is almost entirely controlled at the level of transcription. The expression of specific genes may be actively inhibited or stimulated through the effects of proteins that bind to DNA or RNA. Unlike the prokaryotic control systems the eukaryotic mechanisms must contend with much larger amounts of DNA that is packaged in inaccessible structures. Non expressed DNA is typically highly condensed in a form known as heterochromatin. An extreme example is the complete inactivation of one of the two chromosomes in female mammals known as Barr bodies.
Virtually every stage of a proteins existence from transcription to posttranslational modification offers opportunities for regulation. Posttranslational control such as alternative mRNA splicing can yield multiple proteins from a single gene. Additional control of eukaryotic gene expression is effected by variable rates of mRNA degradation and regulation of translation initiation.

There are over 25000 genes in our human genome. Each gene codes for a unique protein. Regulation of the expression of proteins occurs at the level of transcription from DNA in to RNA. The regulation takes place at a specific position on the DNA template, called promoter, this is the place where RNA polymerase looks for and knows where to start the transcription of the gene. Sometimes a group of related genes cluster together to form an operon and copied into mRNA molecule.

For example the genes involved in the transport and breakdown of food is highly regulated genes. The food we eat needs a digestive enzyme in our mouth to breakdown the sugar to facilitate the absorption by our body. The genes which code for these enzymes are not expressed when arabinose is absent, they only expressed when arabinose is present in their environment. When arabinose is present in the environment, bacteria take it up. and the arabinose interacts directly with araC which bound to the DNA. The interaction causes araC to change its shape which in turn promotes the binding of RNA polymerase and the genes are grascribed. the enzymes are produced.They break down arabinose and eventually the arabinose runs out. In the absence of arabinose the araC returns to its original shape and transcription is shut off. This is a good example of gene regulation.

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