Gene Control

Chapter 11

Prokaryotic Gene RegulationOperons, specific sets of clustered genes, are the controlling unit

Promoter: sequence where RNA polymerase bindsRequirement for initiation of transcription

Operator: sequence between the promoter and start of geneRepressor binds to the operator

RNA polymerase can’t move toward the geneRemoved by signal molecules that bind to repressor

Shape changes can’t bindActivators make the promoter more accessible

Assists unzipping of DNA at promoter siteSignal 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