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REGULATION OF GENES
The Operon Discovered in the bacterium, E. coli
Used as a model for gene regulation
An operon is a set of genes and the switches that control the expression of those genes
Jacob and Monod Discovered the operon in the 1940’s
Found two types: inducible (Lac) operon and repressible (tryptophan) operon
The Lac operon is switched off until it is induced to turn on
The tryptophan operon is always in the on position until it is no longer needed
The Lac Operon Lactose is not available to bacteria as an
energy source So, the genes necessary to utilize lactose are
not transcribed
In order for E. coli to utilize lactose, three structural genes must be transcribed in order to produce the enzymes necessary for the breakdown of lactose into glucose and galactose
The Lac Operon The three enzymes necessary are b-
galactosidase, permease and transacetylase These enzymes are coded for by thee
structural genes in the Lac operon
In order for this transcription to occur, RNA polymerase must bind to DNA at the promoter
The Lac Operon If a repressor binds to the operator,
RNA polymerase is prevented from binding to the promoter and transcription of the structural genes is prevented or blocked
The relationship between RNA polymerase and the repressor is an example of noncompetitive inhibition Both substances are competing for two active
sites, one of which blocks the other
The Lac Operon If allolactose, similar to lactose, is present, it acts
as an inducer or allosteric effector
It binds to the repressor, causing the repressor to change shape or conformation
Now, the repressor can no longer bind to the operator and RNA polymerase is free to bind to the promoter
Structural genes are now transcribed and lactose is utilized
Repressed Lac Operon
Tryptophan Operon Repressible; continuously switched on
unless turned off by a corepressor
Consists of five structural genes that code for the enzymes necessary to synthesize the amino acid tryptophan
Tryptophan Operon The repressor molecule encoded by the
regulator gene is initially inactive
RNA polymerase is free to bind to the promoter and transcribe the structural genes, resulting in tryptophan production
Tryptophan Operon When the inactive repressor combines
with a specific corepressor molecule (tryptophan), it changes shape and binds to the operator
This prevents RNA polymerase from binding to the promoter and blocks the further production tryptophan
If tryptophan levels are high, no more is needed, so no more is made
Tryptophan Operon Tryptophan acts as an allosteric
effector
This is an example of a negative feedback mechanism
Prions Prions are not cells and are not viruses
Misfolded versions of a protein normally found in the brain
If prions enter a normal brain, they cause all of the normal versions of the protein to misfold in the same way
Prions Prions are infectious and cause several
brain diseases Scrapie in sheep
Mad cow disease in cattle
Creutzfeldt-Jakob disease in humans
Transposons Transferring genetic elements sometimes
called jumping genes
Discovered by Barbara McClintock
Some transposons jump in a cut-and-paste fashion from one part of the genome to another
Transposons Others make copies of themselves that
move to another region of the genome, leaving the original behind
Two types: insertion sequences and complex transposons
Insertion Sequences Consist of one gene that codes for
transposase, an enzyme that moves the sequence from one place to another
Causes a mutation if it lands within a DNA region that regulates gene expression
Complex Transposons Longer than insertion sequences and
include extra genes Antibiotic resistance or seed color
McClintock hypothesized the existence of transposons when she saw patterns in corn color that made sense only if some genes were mobile