Question

1. Describe how bacteria regulate genes through operons. 2. What is the difference between a repressible and inducible genetic system? 3. Expression of the Lac operon is under the controls of (1) the Lacl repressor and (2) CAP. Consider each of the following conditions and determine which factors will be acting on the operon. Predict the rate of gene expression. Conditions Lac Operon fon" or "off" CAP present" or "absent" at Lac Operon Predict the rate of gene expression (none, low, high) Glucose, High Lactose Glucose, Low Lactose Glucose, High Lactose Glucose, Low Lactose

Answer 1

1.Bacterial genes are often found in operons. Genes in an operon are transcribed as a group and have a single promoter.Each operon contains regulatory DNA sequences, which act as binding sites for regulatory proteins that promote or inhibit transcription.Regulatory proteins often bind to small molecules, which can make the protein active or inactive by changing its ability to bind DNA.Some operons are inducible, meaning that they can be turned on by the presence of a particular small molecule. Others are repressible, meaning that they are on by default but can be turned off by a small molecule.In bacteria, related genes are often found in a cluster on the chromosome, where they are transcribed from one promoter (RNA polymerase binding site) as a single unit. Such a cluster of genes under control of a single promoter is known as an operon. Operons are common in bacteria.

2.Inducible genetic system are turned on in reponse to a metabolite (a small molecule undergoing metabolism) that regulates the operon. E.g. the lac operon is induced in the presence of lactose (through the action of a metabolic by-product allolactose). It consists of specific small molecule that binds to a bacterial regulatory protein and changes its shape so that it cannot bind to an operator, thus switching an operon on.If  lactose absent, repressor active, operon off (blocks RNA polymerase from binding to operator) and if lactose present, repressor inactive, operon on (RNA polymerase binds to operator).

Repressible genetic system are switched off in reponse to a small regulatory molecule. E.g., the trpoperon is repressed in the presence of tryptophan.It consists of protein that inhibits gene transcription
-In case of prokaryotes, this protein binds to the DNA in or near the promoter.when bound to operator, no production of tryptophan.Too much tryptophan present, bounds to repressible operon, which then binds to operator

3.a)Lac operon is ON and CAP is Absent and the rate of gene expression is low in glucose,high lactose

b)Lac Operon is OFF and CAP is Present and the rate of gene expression is high in glucose,low lactose