13-2 Notes: Gene Regulation
It was originally believed the human genome contained a million genes. This was then reduced to
100,000. We now know that less than 23,000, genes exist in the human genome. This is also the same
number and types of genes that most mammals contain. The differences lie in how genes are regulated
and spliced.
Gene Regulation has become the hottest area of research in biology. Of course, prokaryotes and
eukaryotes do it differently, as their genomes differ in many dramatic ways.
PROKARYOTIC REGULATION
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Francois Jacob and Jacques Monod proposed a mechanism for the control of gene expression
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Structural Genes = Gene that codes for a polypeptide
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Operon= A regulated cluster of adjacent structural genes with related functions
o
Has a single promoter region, so one RNA polymerase will transcribe all structural genes
on an all-or-none basis
o
Produces polycistronic mRNA = a large mRNA molecule that is a transcript of several
genes
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o

Translated into several polypeptides
Grouping structural genes into operons is advantageous because:

Expression of these genes can be coordinated

Metabolic pathway genes can be controlled by one switch
Operator = a DNA segment located between the promoter and structural genes, which controls
access of RNA polymerase to structural genes



o
Acts as an on/off switch
o
By itself (“naked”) the operator is on; it is switched off by binding of a repressor protein
Repressor = Specific protein that binds to an operator and blocks transcription of the operon
o
Blocks attachment of RNA Polymerase
o
Encoded by regulatory genes
Regulatory Genes= genes that code for repressor or regulators of other genes
o
Is located outside the operon
o
Has its own promoter
Activity depends upon the presence of key metabolites in the cell = Corepressor
o
May be inducers or repressors

Corepressor = a molecule, usually a metabolite, that binds to a repressor protein, causing the
repressor to change into its active conformation
Negative Controls

Repressible operons = operons which can have their transcription inhibited. Usually associated
with anabolic processes, (e.g., trp operon  See diagram).

Inducible operons = operons which can have their transcription stimulated. Usually associated
with catabolic processes. (e.g. lac operon  See diagram).
Positive Controls

Positive control = an activator molecule interacts directly with the genome to increase the rate of
transcription
Ex. In the lac Operon
o
Cyclic AMP (cAMP) builds up in bacterial cells when glucose is absent
o
Binds to CAP (catabolite activator protein)
o
The CAP/cAMP complex binds to the CAP binding site next to the lac operon
o
This bends the DNA, exposing the promoter to RNA polymerase, which makes it easier
for RNA polymerase to bind

This increases the rate of transcription, so its production increases tremendously
when glucose is absent
o
When glucose is present, the lac operon may still be induced to be on, but does not get
transcribed nearly as quickly. (Cells prefer to metabolize glucose.)
o
CAP affects other operons in the same way