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

• methylation makes histones more hydrophobic so they bind more tightly to each other causing DNA to coil more tightly

Loosely bound → Euchromatin

• Genes can be freely transcribed

• Acetylation or phosphorylation reduces +ve charge on histones causing DNA to coil less tightly

What is an ‘operon’?

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

• Under control of promoter

Promoter

RNA polymerase attachment site

Operator

Reversibly binds to repressor protein

What are structural and regulatory genes?

1. Structural gene codes for structural proteins or enzymes that has function within a cell (not involved in DNA reg.)

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?

1. 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.

Exons

Coding sequences of DNA

• Eventually translated into amino acids forming final polypeptide

Introns

Non-coding sequences of DNA

• Not translated - don’t code for amino acid

• If included in mRNA, resulting protein wouldn’t form properly; mayn’t function correctly

What is pre-mRNA

Product of transcription

• Contains exons & introns

• Will go in to form mature mRNA

What happens at post transcriptional level?

1. Splicing

2. mRNA editing

What happens during splicing? What does it ensure?

• Spliceosome cuts RNA at points where introns are and removes them

• Exons fuse (join together) to form mature mRNA ready to be translated

• Ensures only coding sections of mRNA are used to form proteins by translation

What is mRNA editing?

• Nucleotide sequence can be changed via base addition, deletion, or substitution

• Results in synthesis of diff proteins that may have diff function (same effect as point mutations)

  • Increases range of proteins that can be produced from single mRNA

What mechanisms regulate protein synthesis (Translational Control)?

1. Degradation of mRNA

   • More resistant it is, longer it lasts, so more protein made

   • Controls how much protein made

2. Inhibitory proteins bind to mRNA to prevent it binding to ribosome (no proteins made)

   • mRNA stays in cell

3. Initiation factors activated, aiding binding of mRNA to ribosomes

   • Increases rate of translation

What happens at post translational control?

Proteins synthesised are modified:

1. Non-protein groups added

   • E.g. inorganic ions, lipids, carb chains, phosphates

2. Amino acids modified & formation of bonds

   • E.g. more cysteine added if live in hot places

3. Folding / Shortening of Proteins

   • Secondary Structures, Tertiary Structures

4. Modification by cAMP

What are protein kinases and what does their role do?

1. Enzymes that catalyse addition of phosphate groups to proteins

2. This changes tertiary structure and the function of a protein

   • Activates or modifies the protein for its specific function

   • Are therefore important regulators of cell activity

What activates protein kinase and in turn what happens?

1. cAMP (secondary messenger)

2. Once activated, protein kinase activates other proteins

Lac Operon and cAMP

In the lac operon, cAMP binds to CRP to increase the rate of transcription of structural genes