Download Answers to End-of-Chapter Questions – Brooker et al ARIS site

Answers to End-of-Chapter Questions – Brooker et al ARIS site
Chapter 13
Test Yourself Questions
1. Genes that are expressed at all times at relatively constant levels are known as ____ genes.
a. induced
b. repressed
c. positive
d. constitutive
e. structural
Answer: d. Constitutive genes, also known as housekeeping genes, are expressed at constant levels at
all times. The gene products are necessary for basic functions of the cell.
2. Which of the following is not considered a level of gene regulation in prokaryotes?
a. transcriptional
b. RNA processing
c. translational
d. post-translational
e. All of the above are levels at which prokaryotes are able to regulate gene expression.
Answer: b. Though RNA processing, including intron removal, is common in eukaryotes, it is not a
process carried out by prokaryotes.
3. Transcription factors that bind to DNA and stimulate transcription are
a. repressors.
b. small effector molecules.
c. activators.
d. promoters.
e. operators.
Answer: c. Activators demonstrate positive control of transcription by binding to the DNA to increase the
rate of transcription.
4. In prokaryotes the unit of DNA that contains multiple structural genes under the control of a single
promoter is called ___________. The mRNA produced from this unit is referred to as ____________
mRNA.
a. an operator, a polycistronic
b. a template, a structural
c. an operon, a polycistronic
d. an operon, a monocistronic
e. a template, a monocistronic
Answer: c. An operon is a region of prokaryotic DNA that contains the gene sequence for more than one
gene and is regulated by a single promoter. The mRNA that is transcribed from the operon will contain
the code for more than one polypeptide chain is referred to as a polycistronic mRNA.
5. In the lac operon, the repressor protein binds to ___________ to inhibit transcription.
a. the promoter
b. the operator
c. the CAP site
d. the enhancer
e. lactose
Answer: b. Binding of the repressor protein to the operator region prevents the RNA polymerase from
transcribing the structural genes of the lac operon.
6. The presence of _________ in the medium prevents the CAP from binding to the DNA, resulting in
_____________ in transcription of the lac operon.
a. lactose, an increase
b. glucose, an increase
c. cAMP, a decrease
d. glucose, a decrease
e. lactose, a decrease
Answer: d. Glucose inhibits the activity of adenyl cyclase resulting in a reduction of cAMP. CAP must be
bound to cAMP to bind to DNA to activate transcription. If cAMP levels are too low, CAP cannot bind to
the DNA and transcription levels would drop.
7. The trp operon is considered _______ operon because the structural genes necessary for
tryptophan synthesis are not expressed when the levels of tryptophan in the cell are high.
a. an inducible
b. a positive
c. a repressible
d. a negative
e. both c and d
Answer: c. Repressible operons are usually associated with enzymes that are involved in biosynthesis.
The genes of the trp operon are involved in tryptophan synthesis. The presence of sufficient tryptophan
would repress the expression of this operon. When tryptophan levels drop, the operon would be
expressed to bring the level of tryptophan back to normal levels.
8. Response elements that function to increase transcription levels in eukaryotes are called.
a. promoters.
b. silencers.
c. enhancers.
d. transcriptional start sites.
e. activators.
Answer: c. The binding of activator proteins to enhancers increases the rate of transcription.
9. DNA methylation in many eukaryotic organisms seems to relate to
a. increased translation levels.
b. introns that will be removed.
c. regions of DNA that do not contain structural genes.
d. decreased transcription levels.
e. response elements that are not necessary for transcription.
Answer: d. DNA methylation correlates with decreased gene expression in many eukaryotes.
10. _________________________ refers to the phenomenon where a single type of pre-mRNA may
give rise to multiple mRNAs due to different patterns of intron and exon removal.
a. Spliceosomes
b. Variable expression
c. Alternative splicing
d. Polycistronic mRNA
e. Induced silencing
Answer: c. Alternative splicing results in one gene coding for multiple gene products. The different
patterns of intron and exon removal allows for more than one mRNA to be produced from a single gene
sequence, thereby increasing the amount of information that can be stored in the DNA.
Conceptual Questions
1. List the components of the lac operon and explain the function of each.
Answer: LacA, lacY and lacZ are structural genes that code for enzymes involved in the catabolism of
lactose. The operator is a binding site for the repressor protein. The CAP site is a DNA sequence
recognized by an activator protein. The promoter is a binding site for RNA polymerase. Though not a
part of the lac operon itself, lacI encodes the lac repressor that binds to the operator site.
2. Explain the difference between inducible and repressible operons.
Answer: In an inducible operon, the presence of a small effector molecule causes transcription to occur.
In repressible operons, a small effector molecule inhibits transcription. The effects of these small
molecules are mediated through regulatory proteins that bind to the DNA. Repressible operons usually
encode anabolic enzymes and inducible operons encode catabolic enzymes.
3. Explain how DNA methylation inhibits transcription.
Answer: The addition of methyl groups to CpG islands, especially near the promoters of eukaryotic
genes, may prevent an activator from binding to an enhancer element or may convert chromatin from an
open to a closed conformation.
Experimental Questions
1. What were the key observations made by Jacob, Monod and Pardee that led to the development of
their hypotheses regarding the lacI gene and the regulation of the lac operon?
Answer: The first observation was the presence of some bacterial strains that had constitutive
expression of the lac operon. Normally, the genes are only expressed when lactose is present. These
mutant strains expressed the genes all the time. The researchers also observed that some of these
strains had mutations in the lacI gene. These two observations were key to the development of
hypotheses explaining the relationship between the lacI gene and the regulation of the lac operon.
2. What were the hypotheses proposed by the researchers explaining the function of the lacI and the
regulation of the lac operon?
Answer: The researchers proposed two hypotheses. One hypothesis was that the lacI gene produced a
repressor protein that interacts with the DNA to stop transcription. The other hypothesis was that the
lacI was an operator, a binding site for a repressor protein. In either case, a mutation in the lacI would
disrupt the regulation of the lac operon.
3. How did Jacob, Monod, and Pardee test each of their hypotheses? What were the results of the
experiment? How do these results support the hypothesis that the lacI gene produces a repressor
protein and does not function as an operator?
Answer: The researchers used F’ factor to introduce the wild-type lacI into the cell. In this case, the cells
that contained the F’ factor had both a mutant copy of the gene and a normal copy of the gene.
Depending on how the introduction of the F’ factor affected the expression of the lac operon, the
researchers would be able to determine whether the lacI produced a protein that interacts with the DNA
or functioned as a binding site on the DNA for some other regulatory protein.
By creating a merozygote with an F’ factor with a normal copy of the lacI gene, regulation of the lac
operon was restored. The researchers concluded that the normal gene produced adequate amounts of
a diffusible protein that could interact with the operator on the chromosomal DNA as well as the F’ factor
DNA and regulate transcription. If the lacI gene had been an operator, the F’ factor would not be able to
restore normal gene regulation because the operator is a DNA sequence that only regulates genes that
are adjacent to it. Introducing a wild-type operator could not restore normal gene regulation.
Collaborative Questions
1. Discuss the different points at which gene regulation can take place to produce a functional protein.
Answer: First, it can occur during transcription when a gene is used to synthesize mRNA. Many factors
can influence this process, one being the environmental conditions inside and outside the cell. It can
also occur during the process of translation, the conversion of mRNA information into a polypeptide.
The process of translation can be influenced by the cells’ internal environment and whether inhibitory
molecules are present or not. Finally, it can also occur at the post-translation level. After a protein is
made, it can then be modified to become a fully functional protein.
2. Discuss several factors which control the regulation of transcription in eukaryotes.
Answer:
a. Activator proteins may stimulate the ability of RNA polymerase to initiate transcription.
b. Repressor proteins may inhibit the ability of RNA polymerase to initiate transcription.
c. Activator proteins may make it easier for RNA polymerase to recognize and transcribe a region where
a gene is located.
d. DNA methylation usually inhibits transcription. This is done by either hindering activator proteins or by
causing DNA to become more compact therefore making it more difficult to transcribe.