Transcript Gene Expression

Gene Control
Chapter 11
Prokaryotic Gene Regulation
 Operons, specific sets of clustered genes, are the controlling unit
 Promoter: sequence where RNA polymerase binds
 Requirement for initiation of transcription
 Operator: sequence between the promoter and start of gene
 Repressor binds to the operator
 RNA polymerase can’t move toward the gene
 Removed by signal molecules that bind to repressor
 Shape changes  can’t bind
 Activators make the promoter more accessible
 Assists unzipping of DNA at promoter site
 Signal molecule binding regulates
 Repressors and activators aren’t antagonistic
Lac Operon as a Model of Control
• Production of enzymes to break down milk sugar
• An activator and repressor have roles
• Conditions tightly controlled
– Lactose must be high, but no
other sugar present
– [Lactose] and [glucose]
Trp Operon
• AA tryptophan necessary for
all life
• Only repressor has role
• Opposite mechanism of lac
operon
– Repressor can’t bind without
Trp
• RNA polymerase binds
– Trp binds to repressor =
shape change
• RNA polymerase blocked
Eukaryotic Gene Regulation
• Chromatin structure
– Promoters blocked from transcription factors
• DNA wraps around proteins called histones, forming
nucleosomes
• Changes during cell cycle
– Methylation (-CH3) of DNA ensures ‘off’ genes stay off
• Transcriptional control
– Transcription factors (major)
• Activators bind to enhancers
• DNA bends to allow interaction of enhancer with promoter
so RNA polymerase can bind
– 1 promoter per gene
• Silencers prevent
Eukaryotic Gene Regulation (cont.)
• mRNA processing
– Alternate RNA splicing
• Change how exons recombine
• 1 gene can code many polypeptides
• Translational control
– Life span of mRNA
• Enzymes degrade or allow to be translated
– Prokaryotic is short lived = quick adaptation
– Eukaryotic varies
– Protein requirements
• Protein structure
– Levels of protein folding (1°, 2°, 3°, 4°)
– Cleavage of proteins
– Selective breakdown/denaturation