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BIOL308 Study Guide - Final Guide: Northern Blot, Structural Motif, Glutamine

5 pages25 viewsSpring 2018

Course Code
Dragana Miskovic
Study Guide

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1. You have discovered base changes in the promoter region of the operon in a
bacterial chromosome. Would you expect these changes to act in trans on
another copy of the operon? Explain your reasoning.
No, they would not act in trans on another copy of the operon because promoter regions
only affect the dna that they are on. To act in trans means to be able to affect the
expression of another molecule of dna, this would not be the case here. No gene
product formed as result of promotor region.
2. What are cis- elements? What are trans- factors? Give an example from the Trp
operon (or form the Ara-operon).
Cis elements can only act on the strand of dna that they are on (in TRP, the binding site
for the apo-repressor is a cis element). A trans factor is mobile and can interact with
other strands of genes and affect their expression (tryptophan as a co-repressor is an
example of a trans factor). In L-ara operon, AraO2 is a cis site which araC will bind to in
order to block transcription from occurring. In l-ara operon, arabinose is a trans factor
which can bind to arac protein, change its conformation so it binds to i1 and I2
3. Draw the diagram of the lac operon that illustrates negative control.
4. You have isolated a protein that binds to DNA in the region upstream of the
promoter sequence of the gene of interest. If this is a positive regulator (activator)
which would be true:
A) Loss of function mutation in the gene encoding this DNA binding protein
would cause constitutive expression
B) Loss of function mutation in the gene encoding this DNA binding protein
would result in lower or no expression.
Explain your reasoning.
It would lead to lower or no expression because a positive regulator involves
stimulating transcription of downstream gene by increasing ability for rnap to bind
with promotor region. Loss of this protein would reduce that stimulation of
transcription thus lower/no expression.
5. Discuss why are lac Oc mutants cis-acting.
Because the operator sequence on a gene only affects the expression on that strand. A
repression is not able to bind to the operator on this strand as it is mutated thus
expression is constitutive but it will not stop a repressor from bind to another gene with
a functional repressor. Thus, cis acting, only on it’s own strand and expression.
6. Discuss why are lac I- mutants trans-acting.
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Lac I mutants have mutated binding sites meaning they cannot bind to any operators,
whether they are on the same strand or on different strands. Trans acting means that
the factors are mobile and can affect other genes, which is the case in this. The
mutation is on a trans factor itself.
7. Discuss positive and negative regulation of L-ara operon.
Positive, when arabinose is present, ARAc binds to ara l1 and ara l2 which allows for
transcription to occur.
Negative, no arabinose present, arac binds to araO2 and ara L1, which causes dna to
loop and make the promoter region hidden from RNA polymerase.
8. Regarding the regulation of Trp operon, what do we call the amino acid
tryptophan? Why?
The amino acid trptophan is a co-repressor because on it’s own, it does not repress but
when it is bound to the aporepressor, it forms a repressor which can bind to the
operator and prevent transcription of more tryptophan.
9. What is meant by polycistronic mRNA? Give an example.
Polycistronic mRNA means that the same mRNA can code for many different proteins
such as in the case of the lacoperon where mRNA coded by the lac operon will give rise
to beta-gal, permease and transaccetylase
10. What is catabolite repression? What is the role of Catabolite Activator Protein?
Explain its action.
Catabolite repressison is the repression of catabolic pathways when the favoured
energy source is present in the cell. Catabolite activator protein has the role of positivily
regulating the lac operon when the cell is starving. It does this when no glucose is
present, CAP protein binds to cAMP produced by starving cell which can form a
complex with RNAP catalyzing transcription to occur.
11. Define: repressor, co-repressor, aporepressor and inducer.
Repressor binds to operator and blocks rnap from binding to the promoter blocking
transcription. A corepressor binds to an inactive repressor causing it to activate and
able to repress. Aporepressor is an inactive form of repressor, is not active when no
corepressor is present. Inducer is a molecule that when bound to the repressor, causes
it to dissociate from operator inducing transcription.
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