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Biology Chapter 18 - Application Questions
Terms in this set (32)
How does binding of the trp corepressor to its repressor
alter repressor function and transcription? What about
the binding of the lac inducer to its repressor?
Binding by the trp corepressor (tryptophan) activates the trp repressor, shutting
off transcription of the trp operon; binding by the lac inducer (allolactose) inactivates
the lac repressor, leading to transcription of the lac operon.
Describe the binding of RNA polymerase, repressors, and activators to the lac operon when both lactose and glucose are scarce. What is the effect of these scarcities on transcription of the lac operon?
is scarce, cAMP is bound to CAP and CAP is bound to the promoter, favoring
the binding of RNA polymerase. However, in the absence of lactose, the repressor
is bound to the operator, blocking RNA polymerase binding to the promoter.
Therefore, the operon genes are not transcribed.
A certain mutation in E. coli changes the lac operator so that the active repressor cannot bind. How would this affect the cell's production of β-galactosidase?
The cell would continuously
produce β-galactosidase and the two other enzymes for lactose utilization, even in
the absence of lactose, thus wasting cell resources.
Compare and contrast the roles of a corepressor and an inducer in negative regulation of an operon.
A corepressor and an inducer are both small molecules that bind to the repressor
protein in an operon, causing the repressor to change shape. In the case of
a corepressor (like tryptophan), this shape change allows the repressor to bind to
the operator, blocking transcription. In contrast, an inducer causes the repressor
to dissociate from the operator, allowing transcription to begin
. In general, what are the effects of histone acetylation
and DNA methylation on gene expression?
Histone acetylation is generally associated with gene expression, while DNA
methylation is generally associated with lack of expression.
Compare the roles of general and specific transcription
factors in regulating gene expression.
factors function in assembling the transcription initiation complex at the
promoters for all genes. Specific transcription factors bind to control elements
associated with a particular gene and, once bound, either increase (activators)
or decrease (repressors) transcription of that gene
Suppose you compared the nucleotide sequences
of the distal control elements in the enhancers
of three genes that are expressed only in muscle cells.
What would you expect to find? Why?
The three genes should
have some similar or identical sequences in the control elements of their enhancers.
Because of this similarity, the same specific transcription factors in muscle
cells could bind to the enhancers of all three genes and stimulate their expression
Once mRNA encoding a particular protein reaches the
cytoplasm, what are four mechanisms that can regulate
the amount of the protein that is active in the cell?
Regulation of translation initiation, degradation of the mRNA,
activation of the protein (by chemical modification, for example), and protein
Describe what must happen for a cell-type-specific gene to be transcribed
in a cell of that type.
must not be tightly condensed because it must be accessible to transcription
factors. The appropriate specific transcription factors (activators) must bind to
the control elements in the enhancer of the gene, while repressors must not be
bound. The DNA must be bent by a bending protein so the activators can contact
the mediator proteins and form a complex with general transcription factors at
the promoter. Then RNA polymerase must bind and begin transcription.
Compare miRNAs and siRNAs, including their functions.
Both miRNAs and siRNAs are small, single-stranded RNAs that associate with
a complex of proteins and then can base-pair with mRNAs that have a complementary
sequence. This base pairing leads to either degradation of the mRNA or
blockage of its translation. In some yeasts, siRNAs associated with proteins in a
different complex can bind back to centromeric chromatin, recruiting enzymes
that cause condensation of that chromatin into heterochromatin. Both miRNAs
and siRNAs are processed from double-stranded RNA precursors, but have subtle
variations in the structure of those precursors
Suppose the mRNA being degraded in
Figure 18.14 coded for a protein that promotes cell division
in a multicellular organism. What would happen if a
mutation disabled the gene for the miRNA that triggers
The mRNA would persist and
be translated into the cell division-promoting protein, and the cell would probably
divide. If the intact miRNA is necessary for inhibition of cell division, then division
of this cell might be inappropriate. Uncontrolled cell division could lead to formation
of a mass of cells (tumor) that prevents proper functioning of the organism
and could contribute to the development of cancer.
Why are miRNAs called noncoding RNAs? Explain how they participate
in gene regulation
miRNAs do not "code" for the amino acids of a protein—they are never translated.
Each miRNA associates with a group of proteins to form a complex. Binding
of the complex to an mRNA with a complementary sequence causes that mRNA to
be degraded or blocks its translation. This is considered gene regulation because it
controls the amount of a particular mRNA that can be translated into a functional
As you learned in Chapter 12,
mitosis gives rise to two daughter cells that are genetically
identical to the parent cell. Yet you, the product of
many mitotic divisions, are not composed of identical
Cells undergo differentiation during embryonic development, becoming different
from each other. Therefore, the adult organism is made up of many highly specialized
Explain how the signaling
molecules released by an embryonic cell can induce
changes in a neighboring cell without entering the cell.
(See Figures 11.15 and 11.16.)
By binding to a receptor on the receiving cell's surface and
triggering a signal transduction pathway, involving intracellular molecules such
as second messengers and transcription factors that affect gene expression
Why are fruit fly maternal effect genes also called eggpolarity
Because their products, made and deposited into the egg by the mother, determine
the head and tail ends, as well as the back and belly, of the embryo (and
eventually the adult fly)
In Figure 18.17b, the lower cell is synthesizing
signaling molecules, whereas the upper cell is expressing
receptors for these molecules. In terms of gene regulation
and cytoplasmic determinants, explain how these cells
came to synthesize different molecules
The lower cell is synthesizing signaling molecules
because the gene encoding them is activated, meaning that the appropriate specific
transcription factors are binding to the gene's enhancer. The genes encoding these
specific transcription factors are also being expressed in this cell because the transcriptional
activators that can turn them on were expressed in the precursor to this cell. A similar explanation also applies to the cells expressing the receptor proteins.
This scenario began with specific cytoplasmic determinants localized in specific
regions of the egg. These cytoplasmic determinants were distributed unevenly to
daughter cells, resulting in cells going down different developmental pathways
Describe the two main processes that cause embryonic cells to
head down different pathways to their final fates.
The first process involves cytoplasmic determinants, including
mRNAs and proteins, placed into specific locations in the egg by the mother. The
cells that are formed from different regions in the egg during early cell divisions
will have different proteins in them, which will direct different programs of gene
expression. The second process involves the cell in question responding to signaling
molecules secreted by neighboring cells. The signaling pathway in the responding
cell also leads to a different pattern of gene expression. The coordination of
these two processes results in each cell following a unique pathway in the developing
The p53 protein can activate
genes involved in apoptosis, or programmed cell death.
Discuss how mutations in genes coding for proteins that
function in apoptosis could contribute to cancer. (Review
Apoptosis is signaled by p53 protein when a cell has extensive DNA damage,
so apoptosis plays a protective role in eliminating a cell that might contribute to
cancer. If mutations in the genes in the apoptotic pathway blocked apoptosis, a
cell with such damage could continue to divide and might lead to tumor formation
Under what circumstances is cancer considered to have
a hereditary component?
When an individual has inherited an oncogene or a mutant allele of a
Cancer-promoting mutations are likely to
have different effects on the activity of proteins encoded
by proto-oncogenes than they do on proteins encoded
by tumor-suppressor genes. Explain.
A cancer-causing mutation in a proto-oncogene usually
makes the gene product overactive, whereas a cancer-causing mutation in a
tumor-suppressor gene usually makes the gene product nonfunctional.
Compare the usual functions of proteins encoded by protooncogenes
with those of proteins encoded by tumor-suppressor genes.
The protein product of a proto-oncogene is usually involved in
a pathway that stimulates cell division. The protein product of a tumor-suppressor
gene is usually involved in a pathway that inhibits cell division.
If a particular operon encodes enzymes for making an essential
amino acid and is regulated like the trp operon, then
the amino acid acts as a corepressor
Muscle cells differ from nerve cells mainly because they
. express different genes.
The functioning of enhancers is an example of
transcriptional control of gene expression
Cell differentiation always involves
the production of tissue-specific proteins
Which of the following is an example of post-transcriptional
control of gene expression?
the removal of introns and alternative splicing of exons
What would occur if the repressor of an inducible operon were
mutated so it could not bind the operator?
continuous transcription of the operon's genes
Absence of bicoid mRNA from a Drosophila egg leads to the
absence of anterior larval body parts and mirror-image duplication
of posterior parts. This is evidence that the product of
the bicoid gene
normally leads to formation of head structures.
Which of the following statements about the DNA in one of
your brain cells is true?
It is the same as the DNA in one of your liver cells
Within a cell, the amount of protein made using a given mRNA
molecule depends partly on
the rate at which the mRNA is degraded
Proto-oncogenes can change into oncogenes that cause cancer.
Which of the following best explains the presence of these potential
time bombs in eukaryotic cells?
Proto-oncogenes normally help regulate cell division
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