Question

In this problem you will explore how to solve problems involving partial diploid lac operon bacterial strains.

Bacterial strains that are "partially diploid" have two copies of the lac operon because they aquired a plasmid carrying just the lac operon region. One copy of the lac operon region is on the recipient's bacterial chromosome, and the other copy is on the P plasmid that was introduced into the cell by conjugation. Partial diploid genotypes are written with the P segment first and the recipient chromosome next.

You create a lac operon partial diploid with this genotype:

F' I⁺ P⁺ O^c Z^- Y⁺ / I^- P⁺ O⁺ Z⁺ Y^-

To determine which genes are transcribed and under what conditions, you need to first consider each genotype separately, and then together.

Part A

For each region of the lac operon on the P plasmid, I⁺ P⁺ O^c Z^- Y⁺, determine whether the region is wild type (that is, it produces a functional protein or it's a correct protein binding sequence) or whether the region is mutated.

• Select all that apply.
• The promoter sequence is correct/functional.
• The repressor protein is produced.
• The operator sequence is correct/functional.
• Beta-galactosidase is produced from the lacZ gene.
• Permease is produced from the lacY gene.

Part B

Use the information from Part A to determine how the operon is regulated for this genotype (I⁺ P⁺ O^c Z^- Y⁺).

• Operon is inducible; permease is produced only in the presence of lactose.
• Operon is repressible; permease is only produced in the absence of lactose.
• Operon is constitutive; permease is produced even when lactose is absent.
• Operon is noninducible; permease is never produced, even when lactose is present.

Answer 1

The lac operon contains two types of genes, which include structural genes and regulatory genes. The genes that can be transcribed into mRNA (messenger ribonucleic acid) come under structural genes. There are three structural genes present in the lac operon, they are lac z, lac y, and lac i. There are two regulatory genes present in lac operon-the operator and the promoter, which cannot be transcribed.

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 the 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 polymerase can then transcribe the structural genes.

A mutation in “i” and operator gene (lac-O) causes the constitutive expression of a protein. The mutation in O-gene modifies the structure of an operator gene. As a result, the repressor no longer binds to the operator, and RNA (ribonucleic acid) polymerase initiates transcription continuously. Similarly, if lac “i” is mutated, the repressor protein would not function properly, and the lac operon would be overexpressed.      

 

 

Part A:

Based on the above data, the correct option among the following is 1, 2, and 5.

Part B:

Based on the above data, the correct option among the following is 3.