MOLECULAR GENETICSThe Regulation of Gene Expression

The Operon Model

The Operon Model An operon is a stretch of DNA that

contains a set of one or more genes involved in a particular metabolic pathway.

An operator is a DNA sequence located within the promoter sequences.

Negative Regulation Negative gene regulation

This is where a protein interacts directly with the genome to turn off gene expression.

In an E. coli cell, a repressor binds to the operator region upstream from the lac enzymes.

This repressor prevents RNA polymerase from creating an mRNA transcript for the lac enzymes.

Negative Gene Regulation

Positive Gene Regulation What happens when glucose and lactose

are both available for the cell? A second mechanism ensures that the lac

genes are only expressed at a high level when there is no glucose available in the cell.

If there is no glucose, cyclic AMP (cAMP) will accumulate in the cell.

cAMP will bind to an activator. This activator will then bind to a site close

to the Plac promoter.

Positive Gene Regulation

Positive Gene Regulation This is an example of positive gene

regulation. The attachment of the activator makes it easier

for RNA polymerase to bind to the promoter. If more RNA polymerase binds to the promoter,

the lac genes will be transcribed at a higher rate.

Inducer molecule – a molecule that promotes transcription.

Because an inducer plays a role in both positive and negative gene regulation in the lac operon, it is said to be an inducible operon.

Dual Control of Gene Expression

Co-repression in the tryp Operon Under normal conditions, an E. coli cell produces

the enzymes necessary to synthesize the amino acid tryptophan.

However, if the cell already contains more than enough tryptophan, the cell does not need to waste any energy making more.

If tryptophan levels are high, tryptophan molecules will bind to a repressor protein and activate it.

The activated repressor protein will then bind to the operator and prevent transcription.

This is considered a repressible operon.

Co-repression in the tryp Operon

Gene Expression in Eukaryotes Eukaryotic organisms often contain

multiple tissue types and so, unlike prokaryotes, they do not need to be expressing large amounts of their genome in every cell.

As a result, eukaryotes are more dependent on mechanisms that keep gene expression turned off most of the time, and that turn on selected genes only as they are needed.

Gene Expression in Eukaryotes1. Pre transcriptional control

What DNA is exposed to transcription enzymes?

2. Transcriptional control What parts of the exposed DNA is

transcribed into pre-mRNA?3. Post-transcriptional control

How much pre-mRNA is processed into finished mRNA?

Gene Expression in Eukaryotes1. Translational control

How much of the mRNA is actually transported to ribosomes within the cytoplasm?

2. Post-translational control Once the polypeptide is complete, how

and when will the cell modify it to become a functional protein?

Will the cell allow the polypeptide to be broken down before it becomes a functional protein?

Gene Expression in Eukaryotes