Genetics Chapter 16 Questions

C1. What is the difference between a constitutive gene and a regulated
gene?

A constitutive gene is unregulated, which means that its expression level
is relatively constant. In contrast, the expression of a regulated gene varies under different
conditions. In bacteria, the regulation of genes often occurs at the level of transcri

C2. In general, why is it important to regulate genes? Discuss examples
of situations in which it would be advantageous for a bacterial cell
to regulate genes.

In bacteria, gene regulation greatly enhances the efficiency of cell
growth. It takes a lot of energy to transcribe and translate genes. Therefore, a cell is much
more efficient and better at competing in its environment if it expresses genes only when
th

C3. If a gene is repressible and under positive control, describe what
kind of effector molecule and regulatory protein are involved.
Explain how the binding of the effector molecule affects the
regulatory protein.

In this case, an inhibitor molecule and an activator protein are involved.
The binding of the inhibitor molecule to the activator protein would prevent it from
binding to the DNA and thereby inhibit its ability to activate transcription.

C4. Transcriptional regulation often involves a regulatory protein that
binds to a segment of DNA and a small effector molecule that
binds to the regulatory protein. Do the following terms apply to a
regulatory protein, a segment of DNA, or a small effect

A. Regulatory protein
B. Effector molecule
C. DNA segment
D. Effector molecule
E. Regulatory protein
F. DNA segment
G. Effector molecule

C5. An operon is repressible�a small effector molecule turns off
transcription. Which combinations of small effector molecules and
regulatory proteins could be involved?
A. An inducer plus a repressor
B. A corepressor plus a repressor
C. An inhibitor plus

B and C are correct. In both of these cases, the presence of the small
effector molecule will turn off transcription. In contrast, the presence of an inducer turns
on transcription.

C6. Some mutations have a cis-effect on gene expression, whereas
others have a trans-effect. Explain the molecular differences
between cis- and trans-mutations. Which type of mutation (cis or
trans) can be complemented in a merozygote experiment?

A mutation that has a cis-effect is within a genetic regulatory sequence,
such as an operator site, that affects the binding of a genetic regulatory protein. A ciseffect
mutation affects only the adjacent genes that the genetic regulatory sequence
control

***C7. What is enzyme adaptation? From a genetic point of view, how
does it occur?

The term enzyme adaptation means that a particular enzyme is made
only when a cell is exposed to the substrate for that enzyme. It occurs because the gene
that encodes the enzyme that is involved in the metabolism of the substrate is expressed
only when t

C8. In the lac operon, how would gene expression be affected if one of
the following segments was missing?
A. lac operon promoter
B. Operator site
C. lacA gene

A. No transcription would take place. The lac operon could not be expressed.
B. No regulation would take place. The operon would be continuously turned
on.
C. The rest of the operon would function normally but none of the transacetylase
would be made.

C9. If an abnormal repressor protein could still bind allolactose, but
the binding of allolactose did not alter the conformation of the
repressor protein, how would this affect the expression of the lac
operon?

It would be impossible to turn the lac operon on even in the presence of
lactose because the repressor protein would remain bound to the operator site.

C10. What is diauxic growth? Explain the roles of cAMP and the
catabolite activator protein (CAP) in this process.

Diauxic growth refers to the phenomenon in which a cell first uses up
one type of sugar (such as glucose) before it begins to metabolize a second sugar (such as
lactose). In this case, it is caused by gene regulation. When a bacterial cell is exposed to
b

***C11. Mutations may have an effect on the expression of the lac operon
and the trp operon. Would the following mutations have a cisor
trans-effect on the expression of the structural genes in the
operon?
A. A mutation in the operator site that prevents

A. Cis-effect. It would affect only the genes that are in the adjacent operon.
B. Trans-effect. This is a mutation that affects a protein that can move
throughout the cell.
C. Trans-effect. This is a mutation that affects a protein that can move
throughou

C12. Would a mutation that inactivated the lac repressor and prevented
it from binding to the lac operator site result in the constitutive
expression of the lac operon under all conditions? Explain. What
is the disadvantage to the bacterium of having a co

A mutation that prevented the lac repressor from binding to the operator
would make the lac operon constitutive only in the absence of glucose. However, this
mutation would not be entirely constitutive because transcription would be inhibited in
the prese

***C13. What is meant by the term attenuation? Is it an example of gene
regulation at the level of transcription or translation? Explain your
answer.

Attenuation means that transcription is ended before it has reached the
end of an operon. Because it causes an end to transcription, it is a form of transcriptional
regulation even though the translation of the trpL region plays a key role in the
attenuat

***C14. As described in Figure 16.12, four regions within the trpL mRNA
can form stem-loops. Let's suppose that mutations have been
previously identified that prevent the ability of a particular region
to form a stem-loop with a complementary region. For

A. Attenuation will not occur because loop 2-3 will form.
B. Attenuation will occur because 2-3 cannot form, so 3-4 will form.
C. Attenuation will not occur because 3-4 cannot form.
D. Attenuation will not occur because 3-4 cannot form.

***C15. As described in Chapter 15 , enzymes known as aminoacyl-tRNA
synthetases are responsible for attaching amino acids to tRNAs.
Let's suppose that tryptophanyl-tRNA synthetase was partially
defective at attaching tryptophan to tRNA; its activity was

A defective tryptophanyl-tRNA synthetase would make attenuation less
likely. This is because the bacterial cell would have a lower amount of charged tRNATrp.
Therefore, it would be more likely for the ribosome to stall at the tryptophan codons
found withi

***C16. The 3-4 stem-loop and U-rich attenuator found in the trp operon
(see Figure 16.12) is an example of ?-independent termination. The
mechanism of ?-independent termination is described in Chapter
14 . Would you expect attenuation to occur if the try

The addition of Gs and Cs into the U-rich sequence would prevent
attenuation. The U-rich sequence promotes the dissociation of the mRNA from the DNA,
when the terminator stem-loop forms. This causes RNA polymerase to dissociate from
the DNA and thereby ca

***C17. Mutations in tRNA genes can create tRNAs that recognize stop
codons. Because stop codons are sometimes called nonsense
codons, these types of mutations that affect tRNAs are called
nonsense suppressors. For example, a normal tRNAGly has an
anticod

If you look very carefully at the RNA sequence in Figure 16.12, you will
notice that a UAA codon is found just past region 2. Therefore, in this mutant strain, the
UGA stop codon at the end of region 1 could be read by the mutant tRNAGly and then
the ribo

***C18. Translational control is usually aimed at preventing the initiation
of translation. With regard to cellular efficiency, why do you think
this is the case?

It takes a lot of cellular energy to translate mRNA into a protein. A cell
wastes less energy if it prevents the initiation of translation rather than a later stage such
as elongation or termination.

C19. What is antisense RNA? How does it affect the translation of a
complementary mRNA?

Antisense RNA is RNA that is complementary to a functional RNA such
as mRNA. The binding of antisense RNA to mRNA inhibits translation.

***C20. A species of bacteria can synthesize the amino acid histidine so it
does not require histidine in its growth medium. A key enzyme,
which we will call histidine synthetase, is necessary for histidine
biosynthesis. When these bacteria are given hist

One mechanism is that histidine could act as corepressor that shuts down
the transcription of the histidine synthetase gene. A second mechanism would be that
histidine could act as an inhibitor via feedback inhibition. A third possibility is that
histidin

***C21. Using three examples, describe how allosteric sites are important
in the function of genetic regulatory proteins.

1. lac operon: The binding of allolactose causes a conformational change in the
repressor protein and removes it from the operator site.
2. ara operon: The binding of arabinose to AraC breaks the looping interaction
and leads to the activation of the ara

***C22. In what ways are the actions of the lac repressor and trp repressor
similar and how are they different with regard to their binding to
operator sites, their effects on transcription, and the influences of
small effector molecules?

The two proteins are similar in that both bind to a segment of DNA and
repress transcription. They are different in three ways. (1) They recognize different
effector molecules (i.e., the lac repressor recognizes allolactose, and the trp repressor
recogniz

C23. Transcriptional repressor proteins (e.g., lac repressor), antisense
RNA, and feedback inhibition are three different mechanisms that
turn off the expression of genes and gene products. Which of these
three mechanisms would be most effective in each o

A. Antisense RNA or a translational repressor would shut down protein synthesis the
fastest. A transcriptional repressor would also shut down the synthesis of mRNA, so it
would eventually shut down protein synthesis once all of the preexisting mRNA had be