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Biology Question on the molecular basis of inheritance

In E.coli, the lac operon gets switched on when

A

lactose is present and it binds to the repressor

B

repressor binds to operator

C

RNA polymerase binds to the operator

D

lactose is present and it binds to RNA polymerase

Answer

lactose is present and it binds to the repressor

Explanation

Solution

A lac operon is made up of regulatory DNA sequences and three genes that encodes different products. Regulatory proteins bind to the DNA sequences controlling the transcription. The three DNA sequence regions in a lac operon are:

  • Promoter
  • Operator
  • CAP Binding Site

When lactose is absent, a repressor protein binds to the operator region preventing transcription and thus keeping the operon off. However, when lactose is present, it binds to the repressor protein altering its shape and thus dissociating it from the operator. Once the repressor is released, RNA polymerase can bind and carry out transcription of genes. Thus, lactose is needed to turn on the lac operon.