15-Gene Regulation

What is name given to a protein that binds to DNA and increases the rate of transcription?

An activator

What is the name given to a protein that binds to DNA and inhibits transcription?

A repressor

What are the different components of an operon?

Promoter

Operator

Set of genes

Terminator

If a bacterial cell is in an environment with tryptophan, will the trp operon be expressed?

No

Why is the trp operon not expressed when tryptophan is present?

Because tryptophan binds to the tryptophan repressor and activates it so that it binds to the operator and prevents transcription.

Why is the trp operon expressed when tryptophan is absent?

Because when tryptophan is absent the tryptophan repressor is inactive and not bound to the operator. This allows RNA polymerase to bind to the promoter and transcribe the trp operon.

Under what conditions is the lac operon expressed?

-glucose +lactose (when glucose is absent and lactose is present)

What molecule binds to the lac repressor?

Allolactose

When allolactose binds to the repressor, what is the effect?

When allolactose binds to the lac repressor in inactivates the repressor so that is is not bound to the operator and inhibiting transcription.

When will allolactose be present?

Anytime lactose is present. Lactose is converted into allolactose (a lactose isomer) during a side reaction in the bacterial cell.

What is CAP?

CAP is an activator that controls the expression of the lac operon.

What small molecule binds to CAP?

cAMP (cAMP is a nucleotide)

When cAMP binds to CAP, what is the effect?

When cAMP binds to CAP it activates CAP which will then bind to the CAP site and activate transcription.

What happens if cAMP is not bound to CAP?

Then CAP is inactive and can't bind to the CAP site to activate transcription.

What controls whether cAMP is bound to CAP?

The presence or absence of glucose. Glucose inhibits the production of cAMP. So, if glucose is present, cAMP levels will be low and cAMP will not bind to CAP. If glucose is absent, cAMP levels will be high and cAMP will bind to CAP.

Why is the lac operon expressed in the -glucose +lactose situation?

Because glucose is absent, cAMP levels will be high and cAMP will bind to CAP and CAP will bind to the CAP site to activate transcription. Additionally, because lactose is present, allolactose will be present and bind to the lac repressor to inactivate it so that the repressor will not inhibit trnascription.

There are differences in gene regulation in eukaryotes when compared to bacteria, what 3 did we discuss?

1. The segments of DNA that control the expression of a gene in a eukaryote can be very far away from the gene it regulates.

2. In eukaryotes, you can regulate transcription by regulating the general transcription factors.

3. In eukaryotes, changes in chromatin structure are used to control gene expression.

What effect does DNA methylation usually have on gene expression in eukaryotes?

It usually inhibits gene expression.

What type of RNAs can regulate gene expression by preventing translation of the mRNA?

miRNA and siRNA