Solutions For An Introduction to Genetic Analysis Chapter 11 Problem 9P

The structural genes such as Z, Y, and A codes for proteins like β-galactosidase, lactose permease, and transacetylase respectively.

The repressor protein synthesized by gene ‘i’ binds to operator to inhibit transcription. If the bacterium is living in the absence of lactose and lactose is suddenly introduced into the bacterium's environment, then immediately the high concentration of lactose in the cell binds to the repressor and makes it unfit for binding. RNA (ribonucleic acid) polymerase can then transcribe the structural genes.

In the promoter region (P) the RNA polymerase binds and then begins the transcription. The other region present is known as operator (O). E. coli cells have several DNA -binding proteins that are called as lac repressors that can bind to the O region. When the lac repressor binds to the O region RNA polymerase cannot start the transcription. As a result, binding of repressor protein turns off the operon by preventing the transcription of the genes.

The operator gene is cis-acting, which means it controls only those structural genes that are present on the same DNA molecule. O^C mutants are characterized by changes in the DNA sequence of the operator. Thus, changes in DNA sequence impairs the binding of lac repressor to the operator and cannot turn off the dominant structural genes.