Download HW10 Answer Key

1) (10 Points) Compare and contrast the function of nucleosomes in the regulation of eukaryotic transcription to the
function of the lac repressor in regulation of lac operon transcription.
Nucleosomes and the lac repressor function similarly in regulation of transcription in that both
inhibit transcription when bound to DNA near/on the promoter of genes and the binding of both
can be altered in order to allow transcription to occur. They differ in several respects. For
example, the lac repressor binds to a specific DNA sequence, the operator, whereas
nucleosomes assemble on DNA of any sequence. They also differ in the mechanism that controls
their binding to/assembly on DNA. The lac repressor is prevented from binding by the presence
of lactose, which causes the lac repressor to change structure and become unable to bind to the
operator DNA sequence. The presence of nucleosomes is regulated by "chromatin remodelling"
proteins that act in several different ways to either remove nucleosomes from DNA, alter the
location of nucleosomes (on or off of promoters), or alter the structure of nucleosomes (making
promoter sequences more or less available)
10 points if inhibitory function and ability to be regulated are described. 5 points
if effort is made but both aspects of regulation are not addressed or one or both
statements are incorrect. Zero of no effort.
2) (10 Points) Two different cell types in an animal express slightly different forms of the same protein. This protein is
encoded by a single gene. The two proteins are identical in sequence in their amino-terminal 200 amino acids and their
carboxyl-terminal 200 amino acids but they differ in the sequence of their middle 100 amino acids.
a) What stage of gene expression (see figure 18.1) is regulated to control expression of this gene?
pre-mRNA processing or pre-mRNA splicing. mRNA splicing also accepted. 5
Points if correct. 2.5 for effort, 0 for no effort.
b) Draw a figure that illustrates how the two proteins are encoded by a single gene.
See next page for key. I've drawn three ways that I can think of that show
understanding. Any one of these three is worth 5 Points. Otherwise, 2.5 for effort, 0
for no effort.
pre-mRNA
exon 1
intron 1
exon 2
intron 2
exon 1
exon 2
intron 3
exon 4
alternate splicing pathway
one splicing pathway
mRNAs
exon 3
exon 1
exon 4
exon 3
exon 4
part of intron 1 or intron 2
exon 1
pre-mRNA
intron 1
exon 2A
one splicing pathway
mRNAs
exon 1
exon 2A
exon 1
intron 1A
intron
exon 2B
intron 2
alternate splicing pathway
exon 3
exon 2A
exon 3
exon 1
exon 2B
exon 3
exon 3
intron 2A
pre-mRNAs
exon 1
intron 1B
one splicing pathway
mRNAs
exon 1
exon 2A
exon 3
exon 2B
exon 3
intron 2B
alternate splicing pathway
exon 1
exon 2B
exon 3
3) (10 Points) Gene X is transcribed in liver cells but not in muscle cells. If liver-cell-specific transcription of gene X is due
to the function of an enhancer, how would a loss of function mutation in the enhancer affect transcription of gene X in
liver and muscle?
The enhancer functions in liver but not muscle, therefore, loss of enhancer function only affect
liver cells. The mutation would prevent expression of the gene in liver cells and would have no
effect in muscle cells. The gene would not be expressed in either cell types.
10 Points if correct (need to address the effect of the mutation in both cell types), 5 points for
effort, zero for no effort.
4) (10 Points) Gene X is transcribed in liver cells but not in muscle cells. If liver-cell-specific transcription of gene X is due
to the function of a silencer, how would a loss of function mutation in the silencer affect transcription of gene X in liver
and muscle?
The silencer functions in muscle but not liver, therefore, loss of silencer function only affects
muscle cells. The mutation would allow expression of the gene in muscle cells and would have
no effect in liver cells. The gene would be expressed in both cell types.
10 Points if correct (need to address the effect of the mutation in both cell types), 5 points for
effort, zero for no effort.
5) (10 Points) Genes X and Z are located on the same chromosome but are separated by an insulator. Gene X is
expressed in liver cells but not in muscle cells whereas gene Z is expressed in muscle cells but not in liver cells. If livercell-specific expression of gene X is due to the function of an enhancer and muscle-cell-specific expression of gene Z is
due to the function of an enhancer, how would a loss of function mutation in the insulator affect transcription of genes
X and Z in liver and muscle cells?
The enhancer for gene X functions in liver but not muscle, whereas the enhancer for gene Z
functions in muscle but not liver. The insulator normally prevents the gene X enhancer from
affecting gene Z and also prevents the gene Z enhancer from affecting gene X. The mutation
would allow both enhancers to affect the expression of the both genes: the liver enhancer would
promote expression of X and Z in liver and the muscle enhancer would promote expression of X
and Z in muscle. Both genes would be expressed in both cell types.
10 Points if correct (need to address the effect of the mutation in on both genes in both cell
types), 5 points for effort, zero for no effort.