Question

1. An inducer may affect gene expression by binding: (Select all that apply)

A. Directly to a consensus sequence on DNA

B. An activator protein to remove it from the DNA

C. A repressor protein to remove it from the DNA

D. A corepressor

E. An activator protein to help it bind DNA

F. A repressor protein to help it bind DNA

2.What is the main difference between structural genes and control sequences in bacteria? Cite examples for both.

3.How is repression of biosynthetic pathways different than repression of a catabolic pathway? Why does repression of these two types of pathways work differently?

Answer 1

1. Answer : C and E

Inducers regulate gene expression either by disabling repressors or enabling activators.

When the inducer binds to the repressors, it prevents the repressors from binding to the operator .so that repressor protein is removed from the DNA and transcription begins.

When the inducer binds to the activators, it enables them to bind to the activation sequence and activates target gene.So that activator proteins bind to the DNA at operator site and promote transcription. When inducer is removed , the transcription stops.

2. Regulatory sequences or control sequences are genes that control the expression of one or more genes.It also regulate the expression of structural genes.For example, Trp operon contain a repressive regulatory protein ,trpR which has a promotor where RNA polymerase binds and helps to synthesis regulatory protein mRNA.

Structural genes are genes that code for specific RNA or proteins which have structural or functional importance.It determines the structure of polypeptide chain . For example, structural gene Lac Y codes for beta galactoside permease which allows the lactose to enter the cell in lac operon .

3. In Repression of biosynthetic pathway , repressors binds to the end product of the pathway whereas in Repression of catabolic pathway, the repressors bind to the substrate if that pathway.

When repressors Binds to the end product , the affinity towards operators are increased and the substrate build up to high-level so no energy is wasted in biosynthetic pathway. When repressors binds to the substrate, the affinity towards operators are decreased and this removes the repressors from operator and energy is needed to make enzymes in catabolic pathway.