Reugaltion Of Gene Expression - Operons

Question 1

Housekeeping genes

Answer 1

Genes that are being transcribed (on) almost all the time. E.g. ribosome components, enzymes used in basic metabolic pathways

Question 2

Stimuli

Answer 2

Prokaryotes respond to external stimuli

Eukaryotes respond to internal stimuli

Question 3

What is gene control?

Answer 3

The control over amount of gene product (RNA & protein) in the cell

Question 4

Regulation in eukaryotes

Answer 4

Cell specialisation in multicellular organisms

Cell differentiation is a result of differences in event expression

Different genes expressed in different cells

Question 5

Controlling gene product amount

Answer 5

1. Rate of transcription - rate mRNA is produced, faster produced = more product
2. Translation - rate of translation or number of ribosomes translating, fast/more = more product
3. MRNA degradation - rate mRNA is broken down - faster broken down = less production
4. MRNA processing - capping, polyadenylation, splicing, slower processing = less product

Question 6

Operons

Answer 6

A group of genes, that often take part in the same process, regulated together

E.g. lactose operon, arginine operon, tryptophan operon

Question 7

Parts of an operon

Answer 7

• Structural gene (s)
• Promoter
• Operator
• Regulator gene
• Repressor/activator

Question 8

Operon structure

Answer 8

Promoter - - - Operator - - - - gene 1 — gene 2 — gene 3 …

“Regulator gene” - located elsewhere on genome

“Repressor/ activator” - located elsewhere in the genome

Question 9

Regulation in bacteria - Negative control

Answer 9

• Simple on/off mechanism
• No control over quantity
• 2 types of operon - inducible and repressible

Question 10

Regulation of bacteria - Positive control

Answer 10

• More subtle control
• Control of quantities of gene product

Question 11

Inducible operons

Answer 11

Normallay off, induced only when needed

E.g. lac operon - 3 enzymes for catabolism of lactose

1. Repressor binds at operator = no transcription
2. Lactose binds to Repressor, comes OFF operator = transcription can proceed

Question 12

Repressible operons

Answer 12

Normally on, repressed (turned off) when not need

E.g. genes for amino acid biosynthetic enzymes Arginine operon and tryptophan operon

1. Operon is actively transcribing
2. Arginine (or tryptophan) is synthesised
3. If arginine or tryptophan accumulates beyond useful levels = arginine acts as corepressor
4. Corepressor + Repressor protein binds to operator = shut off operon

Question 13

Positive gene regulation

Answer 13

• Some operons are subject to positive control through stimulators activator proteins e.g, catabolise activator protein (CAP)
• When glucose is scarce = lac operon is activated by the binding of CAP
• When glucose levels increase = CAP detaches from the lac operon - turns off