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GENE REGULATION
Definition
• The mechanisms that determine the types
of cells in which a gene will be transcribed.
• When the mRNA will be exported to the
cytoplasm.
• When and how often the mRNA will be
translated,
• and the duration of time before the mRNA
is degraded.
Regulation categories
• Negative regulation
• Positive regulation
• Negative regulation is more common in
prokaryotes, positive regulation in
eukaryotes.
Negative regulation
• Means that the default state of
transcription is (ON) unless a repressor
turns it (OFF).
Repressible transcription
• Repressible transcription needs a corepressor to bind with the aporepressor to
make an active repressor. Transcription
wont occur.
Inducible transcription
• Inducible transcription needs an inducer to
bind with the repressor and inactivate it, so
the repressor loses its capability to bind
DNA. Transcription will occur.
Positive regulation
• Means that the default state of
transcription is (OFF), unless a protein
turns transcription ON, which is called
transcriptional activator protein. It binds
the activator binding site on DNA.
Autoregulation
• Means that the protein product of a gene
regulates its own transcription.
• Most genes include positive and negative
regulatory elements.
Constitutive synthesis
• Transcription and translation of a gene
takes place at a constant rate independent
of the presence or absence of any inducer
or repressor molecules.
Operator
• It is a regulatory region (or binding site) in
DNA for the repressor protein to control
gene transcription.
Operon
• A collection of genes regulated by an
operator and repressor.
Promoter
• A DNA sequence at which RNA
polymerase binds and initiate transcription.
• RNA polymerase is an enzyme
responsible for transcription of DNA to
RNA.
TATA box
• The base sequence 5’-TATA-3’ in the DNA
of a promoter.
• TATA-box-binding protein:
A protein that binds to the TATA motif in the
promoter region of a gene.
Not only ON-OFF system
Ex:(Tryptophan biosynthesis)
• trpR gene produces trp aporepressor
(regulatory protein) . It does not bind to the
operator unless it is first bound to
tryptophan. That means botn form trp
repressor.
• Insufficient tryptophan means that
aporepressor wont bind trp operator and
transcription occurs.
• Trytophan acts as co-repressor.
Transcriptional activator proteins
• Helix-turn-helix motif
• DNA-binding motif.
Enhancers
• Enhancers are found at a variety of
locations around the gene they regulate.
• Short (less than 20 base pairs)
Silencers
• Short sequences of DNA.
• They promote the assembly of large
protein complexes that prevent
transcription.