Prokaryotic Gene Expression (6.4)

Question 1

Structure of an Operon:

Answer 1

Promoter: A DNA sequence where RNA polymerase binds to initiate transcription of the operon.

Operator: A DNA region that acts as a switch. It is located between the promoter and the structural genes, and it is where a repressor protein can bind to block transcription.

Repressor: A protein that can bind to the operator and prevent transcription. The repressor is often regulated by an inducer or co-repressor molecule.

Structural genes: These are the genes that code for proteins, often enzymes, that carry out a particular function (e.g., breaking down a sugar or synthesizing an amino acid).

Question 2

Inducible vs. Repressible operons

Answer 2

Inducible operon - Usually turned “off” . Turned on when the metabolic pathway is needed (high lactose) (eg - lac operon)
Repressible operon - Usually turned “on”. Turned off when the metabolic pathway is no longer needed (high tryptophan) (eg - trp operon)

Question 3

lac Operon

be able to describe what it is and how it regulates the expression of genes that code for enzymes that help break down lactose in bacteria.

Answer 3

The lac operon is an inducible operon in E. coli that is involved in the breakdown of lactose. It consists of the following components:

Repressor (LacI): The gene that codes for the lac repressor, which binds to the operator and prevents transcription.

Regulation Mechanism:

When lactose is absent, the repressor (LacI) binds to the operator, blocking transcription of the lac genes.

When lactose is present, lactose (or more specifically, allolactose, an isomer of lactose) binds to the repressor, causing it to change shape and release from the operator. This allows RNA polymerase to transcribe the genes needed to break down lactose.

Question 4

trp Operon

be able to describe what it is and how it regulates the expression of genes that code for enzymes that help make tryptophan.

Answer 4

The trp operon is a repressible operon in E. coli that regulates the synthesis of the amino acid tryptophan.

Repressor (TrpR): This gene encodes a repressor protein that, in its inactive form, cannot bind to the operator. However, when tryptophan levels are high, tryptophan itself binds to the repressor, activating it.

When tryptophan levels are low, the repressor remains inactive and the operon is transcribed, allowing the cell to produce tryptophan.

Regulation Mechanism:

When tryptophan levels are high, tryptophan binds to the repressor, activating it. The repressor then binds to the operator, blocking transcription and stopping the production of more tryptophan.