Gene expression is the process of a gene producing a
certain functional gene product, such as a protein. We
have examined how a gene can determine the primary
structure of a protein. However, although all genes are
present in all cells, it is obvious that all genes are
not expressed all the time and in all tissues. That is,
gene expression is regulated. This can happen at various
cellular processes, but most often occurs at
transcription. We will take up few examples of gene
regulation.
- Gene Regulation in Prokaryotes:
In E. coli,
a set of three genes encoded by a single
polycistronic mRNA are turned on when lactose is
present in the medium (the lac operon). These three genes
produce proteins that are needed to utilize lactose
as a nutrient molecule. Turning on these genes means
that their polycistronic mRNA is being transcribed,
therefore the three proteins are being made.
However, when lactose is absent, these three genes
are turned off. That is, their polycistronic mRNA is
not being made. This is gene regulation at the
transcriptional level. Regulation occurs by an
interplay between the repressor protein, the
promoter, and the operator.
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