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16.8 Control of gene expression - Coggle Diagram
16.8 Control of gene expression
Gene control in prokaryotes
Case study: beta-galactosidase (lactase)
Production of lactase controlled by
lac
operon
In low concentrations of lactose, transcription of the gene is switched off
In high concentrations of lactose, transcription of the gene is switched on
lac
operon has 3 structural genes
lacZ
codes for beta-galactosidase
lacY
codes for permease (allows lactose to enter the cell)
lacA
codes for transacetylase
Transcription of all genes controlled by same promoter
All genes transcribed at same time
Operator region is upstream of structural genes
When no lactose present in growth medium
Repressor protein binds to operator region
This inhibits RNA polymerase from binding to promoter region
No transcription of structural genes occurs
When lactose present in growth medium
Lactose is taken up by the bacterium (using permease)
Lactose binds to repressor protein
This distorts its shape and prevents it from binding to operator on DNA
RNA polymerase is no longer inhibited, and mRNA is synthesised, leading to the production of lactase
lac
operon has regulatory gene
Codes for repressor protein
Protein has two binding sites - one for lactose and one for DNA
Definitions
Structural gene - codes for a protein used by a cell
Regulatory gene - codes for a protein that helps control expression of other genes
Operon - a cluster of genes controlled by the same promoter (can involve structural and regulatory genes)
Transcription factor - a molecule which affects whether a gene is transcribed or not
Inducible enzyme - synthesised only when substrate is present
Repressible enzyme - normally synthesised unless prevented
Transcription factors in eukaryotes
General transcription factors are necessary for transcription to occur
Some transcription factors ensure that different genes are activated in the right order during the growth of a baby
Transcription factors allow responses to environmental stimuli (e.g. switching on certain genes in high temperatures)
Gibberellin
DELLA proteins inhibit binding of PIF transcription factor
Gibberellin binds to gibberellin receptor, which then binds to an enzyme and activates it
Activated enzyme breaks down DELLA, so PIF is no longer inhibited and can bind to promoter region of DNA and allow transcription of mRNA coding for amylase