Monday, October 06, 2008.
10:43 PM |
The Question: "Explain what is a lac operon and how it controls the regulation of gene expression in response to changing concentrations of lactose and glucose in a growth medium."The Answer: "Lac operon is a cluster of genes that controls the regulation of gene expression in response to different concentrations of lactose and glucose in a growth medium. Negative gene regulation is activated in the presence of repressors, which determines whether transcription activity is being carried out, while positive gene regulation is activated in the presence of inhibitors, which determines the rate of transcription.
Structural genes include lac Z, which encodes beta-galactosidase whose function is to convert disaccharides lactose to monosaccharides glucose and galactose. The second gene is lac Y, specifying the primary protein permease, whose function is to facilitate the entry of lactose into the cell. Lac A, which codes for transacetylase, has a function believed to be the removal of toxic by-products of lactose digestion from the cell. The structural genes are closely linked in the order Z-Y-A and are transcribed as a single unit, resulting in polycistronic mRNA.
When lactose is absent, lac I codes for an allosteric repressor protein which binds to the operator and represses RNA polymerase action. the lac repressor is said to be in its active configuration as the binding inhibits transcription.
When lactose is present, however, beta-galactosidase converts lactose to allolactose, an inducer which binds to the allosteric repressor protein and inactivates the repressor. Even though the lac operon is uninduced, low levels of beta-galactosidase is still being produced, a.k.a basal levels of expression. As the allolactose binds to the repressor, it alters the conformation of the repressor, nullifying its ability to bind to the operator. RNA polymerase gains access to the promoter region and produces a single polycistronic mRNA for the enzymes of the lactose pathway.
when lactose is present, glucose scarce, cyclic AMP (cAMP) accumulates and binds to a regulatory protein cAMP receptor protein (CRP), an activator of transcription. The protein assumes its active shape and binds upstream to the lac repressor. This facilitates binding of RNA polymerase to the promoter and hence transcription is initiated.
When lactose is present, glucose present, cAMP concentration falls and CRP disengages from the operator where it was bound to. Transcription activity is inhibited as RNA polymerase can no longer bind to the promoter region.
Hence, the lac operon can be under dual control, negative gene regulation and positive gene regulation respectively."The above is an answer to the Biology question posted. Jol is having BIO exam tomorrow!
Loves, derrick out.
