Regulation of Gene Expression: The Operon

Question 1

What is an operon

Answer 1

• An operon is a gene cluster found in bacterial cells

- It consists of an operater, a promoter, and a operon genes

Question 2

What is the operator’s function in the operon?

Answer 2

The operator works as an on/off switch for the expression of the genes in the operon. It is a binding site for a protein that can block the promoter preventing transcription from occuring.

Question 3

Describe the repressor inducer relationship in operons

Answer 3

A repressor is a protein that is created from regulatory genes for a specific operon. The repressor binds to the operator blocking the promoter in the operon. This prevents RNA polymerase from binding to the promoter and transcribing the structural genes in the operon.

An inducer binds to the repressor and induces a change in shape for the repressor that makes it stop binding the operator. The promoter is now available for RNA polymerase and transcription can now begin.

Question 4

What is the regulatory region and what is the coding region in the operon?

Answer 4

The regulatory region consists of the repressor and the operator

The coding region consists of the structural genes.

Question 5

Describe the Lac Operon and how it is regulated. This is a must know system for the DAT

Answer 5

The Lac-operon is an inducible system in E. Coli that encodes for the enzymes Permease, Transacetylase, and B-Galactosidase. These enzymes are needed for lactose breakdown.

Only when lactose is present, does E.Coli use lactose as an energy source. When glucose levels are low there is a higher amount of transcription and when they are high there is a lesser amount of transcription. The operon is under dual control.

1. Allolactose an isomer of lactose (it’s present when lactose is around) and it binds to the repressor and acts as an inducer to remove the repressor from the operator. (This is a negative control by the lac repressor protein). Once this is removed transcription can occur.
2. When glucose levels are low, cyclic AMP starts to accumulate and bind to CAP (catabolic activator protein). Once bound, CAP has the proper conformation to bind to the upstream part of the promoter and recruit RNA polymerase to bind more readily to the promoter and start transcription. (This is a positive control by CAP) This significantly increases the rate of RNA polymerase binding to the promoter.

This is a catabolic operon, because it creates enzymes when a substance that will be catabolized enters the cell

Question 6

Describe the Trp Operon and how it is regulated

Answer 6

Trp is the Tryptophan operon. It consists of 5 structural genes that help create the amino acid tryptophan.

It is a repressible operon, meaning that it is on in its default state, but can be turned off if a corepressor is present.

Tryptophan is the corepressor in this operon. When present, it binds to an inactive repressor changing the conformation, and allowing it to bind to operon, blocking transcription.

This is an anabolic operon, because it assists in the building of protein

Question 7

What percent of DNA codes for proteins?

Answer 7

Only 2% of DNA codes for proteins. The rest code for RNA or nothing at all.

Question 8

Describe chromatin structure regulation

Answer 8

• Genes in heterochromatin are highly condensed and not normally expressed.
• Histones are acetylated allowing for less tight packing of the chromatin, hence allowing for more transcription. The positive NH2+ on the lysine is acetylated removing the positive charge from the amino acid. This weakens the attraction to the negatively charged phosphate groups of DNA. Acetylation of histones = More transcription
• DNA methylation: When CH3 groups are added, tigher packing occurs, thus we see a reduction in gene expression. Inactivated genes are heavily methylated and certain genes are activated by demethylation. This is usually seen on Cytosine and sometimes Adenine. The methyl group is added to the nitrogenous base. It often occurs on promoters to prevent transcription from occuring.

Question 9

Describe the RNA regulators of gene expression

Answer 9

MicroRNA molecules (miRNA) and small interfering RNA molecules (siRNA) can bind to the mRNA.

These two molecules can:

• Degrade mRNA
• Bind to mRNA and therefore block translation.

Question 10

What is the structure of a human histone?

Answer 10

A histone consists of an octomer of mirror image core histones. H2A, H2B, H3, and H4. These are wrapped by core DNA

It also contains H1 that binds to the nucleosome and the linker DNA between histones.

Histones are rich in Lysine and Arginine, making them basic and having a strong positive charge that pulls in tightly the negatively charged phosphate groups of the DNA.

Histones are composed of tails that stick out away from the main histone body. These are what get attacked to modify the histone. They can undergo methylation, acetylation, ubiquitination, phosphorylation and others.