Control of Gene Expression

What is present in every nucleus-containing eukaryotic or prokaryotic cell? As a result of this, what has to happen?

1. The entire genome of the organism

2. Regulation of expression of genes and of rate of synthesis of protein products (enzymes + hormones)

Why does regulation of expression of genes need to happen? (4)

1. Ensures only right genes are expressed in each cell as diff cells have diff jobs (heart cells don’t need to make same proteins as liver/skin cells)

2. Conserves energy & resources

3. Allows cell development, specialisation, & to work in a coordinated way

4. Cells can turn genes on/off in response to signals (e.g. hormones, environmental changes), allows organisms to adapt

What are the mechanisms called within cells to ensure correct genes are expressed in correct cell at correct time?

Regulatory mechanisms

• Control which genes are expressed at different points in time

• Controlled by regulatory genes

What are the types of regulatory mechanisms?

1. Transcriptional

2. Post-Transcriptional

3. Translational

4. Post-Translational

What are transcription factors & what is their role in Transcriptional Control?

Proteins that bind to the specific regions (promoter) of DNA to control transcription of genes

• Can initiate or inhibit transcription (switch genes on/off)

Can the shape of transcriptional factors change?

Yes

• shape can be altered by binding of other molecules, like hormones

If the transcription factor can’t bind to the promoter, what happens?

RNA polymerase can’t attach, transcription doesn’t occur. Gene is off.

How does the extent to which DNA is wound allow transcription to occur?

Tightly bound → Heterochromatin

• Transcription reduced; RNA polymerase can’t access genes

Loosely bound → Euchromatin

• Genes can be freely transcribed

What is an ‘operon’?

Group of genes under the control of same regulatory mechanism (transcriptional) and are expressed at the same time

Promoter

DNA polymerase attachment site

Operator

Reversibly binds to repressor protein

What are structural and regulatory genes?

1. Structural gene codes for protein that has function within a cell

2. Regulatory genes code for proteins that control expression of structural genes e.g. repressor & activator proteins

In terms of the lac operon, what are the examples of the structural genes and regulatory gene?

Structural genes

• lacZ, lacY, lacA

Regulatory gene

• laci (I)

• Codes for repressor protein

  • Prevents transcription of structural genes in absence of lactose

What happens when there’s no lactose?

1. Laci gene expressed & repressor protein is transported

2. Repressor protein binds to operator

3. RNA polymerase can’t bind to promoter region

4. Transcription of structural genes doesn’t occur

   • Structural genes not made to break down lactose

   • No lactase enzyme synthesised

What happens when lactose is present?

1. Lactose binds to repressor, changing its shape so it can’t bind to operator site

2. RNA polymerase binds to promoter region

3. Transcription occurs, all structural genes expressed

4. Lactase produced, lactose broken down

What happens when an effector molecule binds to a repressor protein?

Helps repressor bind to operator and prevent transcription of structural genes

What needs to be done to produced the increased quantity of enzymes needed to metabolise lactose?

cAMP’s receptor protein CRP needs to be binded to cAMP

The transport of glucose into an E. coli cell decreases the levels of cAMP,

reducing the transcription of the genes responsible for the metabolism

of lactose. If both glucose and lactose are present then it will still be

glucose, the preferred respiratory substrate, that is metabolised.

ExonsC