Biology 1002B Lecture Notes - Chromosome, Allolactose, Lac Operon

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15 Mar 2013
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14.1: Regulation of Gene Expression in Prokaryotic Cells
Wednesday, March 06, 2013
7:52 PM
Proks tend to be single-celled, relatively simple, with generation times measured in minutes
Prok cells undergo rapid and reversible alteration in biochemical pathways that allows them
to adapt quickly to changes in their environment
The operon is a unit of transcription
A cluster of prok genes and the DNA sequence involved in their regulation
Brings multiple genes under the control of a single operator
The promoter is a region where the RNA pol beings transcription
The operator is a binding sequence for a regulatory pn,
A gene that is separate from the operon encodes the regulatory pn
Repressors, regulatory pns, bind to the DNA and reduce the likelihood that genes will
get transcribed
Activators, regulatory pns, bind to the DNA and increase the likelihood that genes will
get transcribed
Many operons are controlled by more than one regulatory mechanism
The transcription unit is the cluster of genes that are transcribed into a single mRNA
Typically, the pns encoded by genes in the same operon catalyze steps int eh same
process, such as enzymes acting in sequence in a biochemical pathway
The lac operon for lactose metabolism is transcribed when an inducer inactivates a repressor
The metabolism of lactose as an nrg source involves 3 genes:
lacZ
lacV
lacA
These are adjacent ot one another on the chromosome
The lac operon is controlled by a regulatory pn called the lac repressor
Encoded by the regulatory gene lacI which is nearby but separated from the lac
operon and is synthesized in the active form
When lactose is absent:
The Lac repressor binds to the operator, thereby blocking the RNA pol from
binding tot eh promoter
Repressor binding is a kind of equilibrium
The repressor occasionally comes off
As a result, there is always a low concentration of lac operon gene products in the
cell
In the presence of lactose
The lac operon is turned on and all three enzymes are synthesized rapidly
Lactose is converted to allolactose, an isomer of lactose
Allolactose is an inducer for the lac operon, it binds to the lac repressor altering
its shape so that the repressor can no longer bind to the operator DNA
RNA pol is then able to bind to the promoter and transcribe the 3 genes at a
dramatically elevated rate
The lac operon is called an inducible operon because an inducer molecule
increases its expression
As lactose is used up, the regulatory system switches the operon off
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Document Summary

14. 1: regulation of gene expression in prokaryotic cells. Proks tend to be single-celled, relatively simple, with generation times measured in minutes. Prok cells undergo rapid and reversible alteration in biochemical pathways that allows them to adapt quickly to changes in their environment. The operon is a unit of transcription. A cluster of prok genes and the dna sequence involved in their regulation. Brings multiple genes under the control of a single operator. The promoter is a region where the rna pol beings transcription. The operator is a binding sequence for a regulatory pn, A gene that is separate from the operon encodes the regulatory pn. Repressors, regulatory pns, bind to the dna and reduce the likelihood that genes will get transcribed. Activators, regulatory pns, bind to the dna and increase the likelihood that genes will get transcribed. Many operons are controlled by more than one regulatory mechanism.

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