BIO206H5 Study Guide - Final Guide: Lac Repressor, Allolactose, Repressor

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18 Feb 2016
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Michelle Chyjek (1001326371)
BIO206 – TUT0113
Regulation of Gene Expression Worksheet
1.
2. The lacI gene product is a repressor protein. It prevents transcription of the lac operon.
3. The repressor protein will bind to the operator sequence, located between the promoter and
the start site of transcription on the lac operon. Initially, a dimer is formed, and, in the absence of
lactose, it binds to the operator region of the operon. There are 3 binding sites for the dimer to
bind: one main operator site and two auxiliary sites. If two of these sites are occupied, the DNA
is looped such that the dimers can form a tetramer. This bend of the DNA prevents the binding
of the RNA polymerase.
4. When present, lactose is transported into the cell. Once inside, the β-galactosidase (lacZ
gene product) converts the lactose into allolactose. Allolactose then binds to the repressor (the
lacI gene product) causing an allosteric change in the shape of the repressor, making it
incapable of binding to the operator sequences, and therefore transcription will be able to occur.
5. The lacY gene encodes the permease which transports the lactose into the cell. This is
crucial because it is only within the cell that lactose can be broken down by the β-galactosidase
into allolactose, which allows transcription to occur.
6. The basal level of gene expression is the minimum constant rate of transcription of a gene
that a cell requires to survive. These genes are under positive control mechanism and their
proteins code for basic metabolic machinery and other proteins necessary for cell life. The basal
level of gene expression is also careful to always transcribe enough new mRNAs to replace
those being degraded.
7. When glucose levels begin to decrease, adenylate cyclase begins produce cAMP, which is
made from ATP. Cytosolic cAMP binds to the catabolite activator protein (CAP). Activated CAP
(CAP with cAMP bound) binds to the CAP site and recruits the RNA polymerase to begin
transcription. In the presence of glucose and lactose, the lac repressor does not bind. However,
since glucose is present, cAMP is low, so cAMP-CAP does not bind to the CAP site in the
operator. As a result, RNA polymerase does not bind efficiently to the lac promoter and only a
fragment of a lac mRNA in synthesized. In the presence of lactose and absence of glucose,
maximal transcription of the lac operon occurs. This is because in this situation, the lac
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Document Summary

Bio206 tut0113: the laci gene product is a repressor protein. It prevents transcription of the lac operon: the repressor protein will bind to the operator sequence, located between the promoter and the start site of transcription on the lac operon. Initially, a dimer is formed, and, in the absence of lactose, it binds to the operator region of the operon. There are 3 binding sites for the dimer to bind: one main operator site and two auxiliary sites. If two of these sites are occupied, the dna is looped such that the dimers can form a tetramer. This bend of the dna prevents the binding of the rna polymerase: when present, lactose is transported into the cell. Once inside, the -galactosidase (lacz gene product) converts the lactose into allolactose. These genes are under positive control mechanism and their proteins code for basic metabolic machinery and other proteins necessary for cell life.

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