BSC 1010 Chapter Notes - Chapter 16: Lac Operon, Lac Repressor, Trp Operon

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8 Feb 2017
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Chapter 16: Control of Gene Expression
Regulatory proteins: any of a group of proteins that modulates the ability of RNA polymerase to bind to a promoter & begin
Apoptosis: programmed cell death
This idea of controlling the access of RNA polymerase to a promoter is common to both prokaryotes & eukaryotes, but the details
differ
Prokaryotic cells have been shaped by evolution to grow & divide as rapidly as possible
o Allows them to exploit transient resources
o Proteins turn over rapidly
allows these organisms to respond quickly to changes in their external environment by changing patterns of gene
expression
o the primary function of gene control is to adjust the cell’s activities to its immediate environment
Multicellular organisms (eukaryotes) have been shaped by evolution to be protected from transient changes in their
immediate environment
Major groove: the larger of the two grooves in a DNA helix, where the
paired nucleotides’ hydrogen bonds are accessible
Regulatory proteins can recognize & bind to regions in the major
groove
DNA-binding motif: a region found in a regulatory protein that is capable
of binding to a specific base sequence in DNA
A critical part of the protein’s DNA-binding domain
Domain is a functionally distinct part of the protein necessary
to bind to DNA in a sequence-specific manner
Share the property of interacting with specific sequences o bases,
usually through the major groove of the DNA helix
Helix-turn-helix: a common DNA-binding motif found in regulatory
proteins
It consists of two α-helices linked by a nonhelical segment (the
“turn”)
Structure reveals how proteins containing such motifs interact
with the major groove of DNA
Homeodomain motif: a special class of helix-turn-helix motifs found
in regulatory proteins that control development in eukaryotes
Were discovered when researchers began to characterize a set
of homeotic mutations
Zinc finger motif: a type of DNA-binding motif in regulatory proteins
that incorporates zinc atoms in its structure
Often occurs in clusters, the β sheets spacing the helical
segments so that each helix contacts the major groove
Effect is like a hand wrapped around the DNA with the fingers
lying in the major groove
The more zinc fingers in the cluster, the more the protein
associates with DNA
Leucine zipper motif: a motif in regulatory proteins in which two
different protein subunits associate to form a single DNA-binding
site
The proteins are connected by an association between
hydrophobic regions containing leucines (the “zipper”)
Each of these forms of control are mediated by regulatory proteins, but
the proteins have opposite effects
Positive control: a type of control at the level of DNA transcription initiation in which the frequency of initiation is increased
Activator proteins mediate positive control
o Activator: allosteric proteins that can bind to DNA & stimulate the initiation of transcription
Enhance the binding of RNA polymerase to the promoter to increase the frequency of transcription initiation
Are the logical & physical opposites of repressors
Effector molecules can either enhance or decrease activator binding
Negative control: a type of control at the level of DNA transcription initiation in which the frequency of initiation is decreased
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