Answers to Open-Ended Questions
Hoefnagels Essentials 2/e
Chapter 7
Answers to Mastering Concepts
7.1
1. How did Watson and Crick decipher the structure of DNA?
They combined evidence from several other researchers. Erwin Chargaff’s work revealed
that DNA contains equal amounts of adenine and thymine and equal amounts of cytosine
and guanine. Maurice Wilkins’s and Rosalind Franklin’s X-ray diffraction photo of a
crystal of DNA was also important in determining the overall shape.
2. Describe DNA’s components and three-dimensional structure.
A DNA molecule is composed of subunits called nucleotides. Each nucleotide is
composed of a deoxyribose sugar bonded to a phosphate group and a nucleotide base
(adenine, thymine, cytosine, or guanine). The three-dimensional structure of DNA is a
double helix, which resembles a twisted ladder.
7.2
1. What is the relationship between a gene and a protein?
A gene is a strand of DNA that encodes a protein.
2. How do transcription and translation use genetic information?
Transcription uses the information in DNA to produce a strand of mRNA, and translation
uses this mRNA to produce a protein.
3. What are the three types of RNA, and how does each contribute to protein synthesis?
Messenger RNA (mRNA) carries the instructions for building the protein; transfer RNA
(tRNA) carries the appropriate amino acid to the ribosome; and ribosomal RNA (rRNA)
is the major component of a ribosome, which is the structure where amino acids are
assembled into polypeptides.
7.3
1. What happens during each stage of transcription?
The steps of transcription are initiation, elongation, and termination. During initiation,
enzymes unzip the DNA, and RNA polymerase binds to the promoter. During elongation,
RNA polymerase uses the DNA template strand to add complementary nucleotides to the
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growing RNA strand. During termination, synthesis of the RNA molecule ends and the
DNA molecule is “zipped” back into its double helix form.
2. Where in the cell does transcription occur?
Transcription takes place at the DNA, which is inside the nucleus of a eukaryotic cell.
3. What is the role of RNA polymerase in transcription?
RNA polymerase is an enzyme that uses a DNA template to bind additional nucleotides
to a growing chain of RNA.
4. What are the roles of promoter and terminator sequences in transcription?
The promoter signals the start of a gene, and the terminator signals the end of a gene.
RNA polymerase recognizes the promoter and terminator, so it starts and stops
transcription at the correct positions along the DNA template strand.
5. How is mRNA modified before it leaves the nucleus of a eukaryotic cell?
Before mRNA leaves the nucleus of a eukaryotic cell, a cap is added to one end of the
molecule, a poly A tail is added to the other end, introns are removed, and exons are
spliced together.
7.4
1. What happens in each stage of translation?
In initiation, ribosomal subunits bind to mRNA, and a tRNA carrying the first amino acid
attaches to the first codon. In elongation, the ribosome moves along the mRNA, adding
new amino acids to the growing polypeptide. In termination, the ribosome reaches a stop
codon and releases the last tRNA and the polypeptide. The ribosomal subunits then
dissociate from the mRNA.
2. Where in the cell does translation occur?
Translation occurs at ribosomes, which are either free in the cytoplasm or attached to the
rough ER.
3. How are polypeptides modified after translation?
Polypeptides must be folded to become functional proteins. In addition, sometimes amino
acids are cut out of the chain, and sometimes multiple polypeptides join together.
7.5
1. Which steps in protein synthesis require energy?
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All steps of protein synthesis require energy.
2. Why do cells regulate which genes are expressed?
Protein production costs a lot of energy; the regulation of gene expression avoids the
production of unnecessary proteins and therefore saves energy.
3. How does a repressor protein help regulate the expression of a bacterial operon?
If the repressor protein binds to the operator, RNA polymerase cannot transcribe the
genes in the operon; on the other hand, if the repressor is not bound to the operator, the
genes can be transcribed. In the case of the lac operon, the absence of lactose means the
repressor is bound to the operator, so the genes remain “off.” If lactose is present, it binds
to the repressor protein, which detaches from the DNA and allows the genes to be
transcribed.
4. What is the role of transcription factors in gene expression?
Transcription factors bind to certain DNA sequences to regulate transcription, for
example by preparing a promoter site to bind RNA polymerase. A gene is not expressed
in the absence of transcription factors.
5. What are some mechanisms by which eukaryotic cells control gene expression?
Transcription factors control gene expression. Cells can also keep DNA coiled, so it
cannot be transcribed, or they can attach methyl groups that inactivate genes. After
transcription, different combinations of introns can be removed. mRNA can be confined
to the nucleus or rapidly degraded. Proteins can also be degraded or modified in
processing.
7.6
1. What are the types of mutations, and how does each alter the encoded protein?
In a substitution mutation, one DNA base is replaced with another. The mutation may
have no effect on the resulting protein (silent mutation), change one amino acid, or create
a stop codon in the middle of the mRNA. Insertions and deletions add or remove
nucleotides; they often shift the “reading frame” of a gene. Such a frameshift mutation
may alter many amino acids in the protein, drastically changing its shape and function.
An insertion of three nucleotides adds one amino acid to the encoded protein, and a
deletion of three nucleotides removes one amino acid.
2. What causes mutations?
Mutations are caused by DNA replication errors, errors during meiosis, and exposure to
chemicals or radiation.
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3. In what ways are mutations important?
Some mutations cause diseases. Mutations also produce genetic variability, which is the
raw material of evolution. Scientists induce mutations to learn how genes normally
function and to develop new varieties of crop plants.
7.7
1. How are viruses similar to and different from cells?
Viruses and cells both contain genetic material and proteins. However, viruses are
smaller than cells and contain no ribosomes or organelles. Moreover, viruses do not have
metabolism and they cannot replicate on their own; they must infect a living host cell to
replicate.
2. What features do all viruses share?
All viruses contain genetic information (either DNA or RNA) and are surrounded by a
protein coat.
3. Describe the five steps in viral replication.
(1) Attachment: virus adheres to host cell receptor. (2) Penetration: virus enters the cell.
(3) Synthesis: host cell produces multiple copies of the viral genome and proteins. (4)
Assembly: viral genetic information is packaged in a protein coat. (5) Release: new
viruses leave host cell.
4. What is the source of energy and raw materials for the synthesis of viruses in a host
cell?
The source of energy and raw materials is the host cell’s ATP and its stores of
nucleotides and amino acids.
7.8
1. Compare and contrast lysogenic and lytic viral infections.
A lysogenic viral infection is similar to a lytic viral cycle in that both begin with
attachment and penetration. In a lytic infection, the remaining steps (synthesis, assembly,
and release) occur immediately, destroying the host cell. In contrast, a lysogenic virus
inserts its DNA into the host chromosome, where it is carried on into daughter cells.
Eventually an environmental change triggers a switch to the lytic pathway, destroying the
infected host cells.
2. What is a latent animal virus?
In a latent infection, a virus has infected a host cell, but the viral genetic information is
not being expressed. The host cell is therefore not producing new viruses.
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3. Describe how HIV replicates in host cells.
After attachment and penetration, the HIV enzyme reverse transcriptase copies the viral
RNA to DNA. The viral DNA then inserts itself into the host cell's DNA. New HIV
particles are synthesized and assembled inside the host cell and are released by budding.
A new virus acquires its envelope from the host cell’s membrane as it emerges.
4. How are some latent viral infections linked to cancer?
Some latent viruses signal host cells to divide continuously, which increases the number
of infected cells but can also cause cancer.
5. How do viruses enter plant cells and spread within a plant?
One of the most common ways for viruses to enter plant cells is by hitching a ride on the
contaminated mouthparts of plant-feeding insects. The viruses spread within the plant via
plasmodesmata or in vascular tissue.
6. What are some symptoms of viral infection in plants?
Some symptoms include small dead spots, mottled leaves, abnormal growth, or even
streaking in some flowers.
7.9
1. How are viral infections treated and prevented?
Viral infections are difficult to treat because the viruses infect living host cells. The best
weapon is the vaccine; however, some antiviral drugs can prevent viral replication in
infected cells.
2. Explain why antiviral drugs are difficult to develop.
Virus genomes replicate and mutate rapidly, so they are genetically diverse. For many
types of viruses, developing effective drugs has so far proved impossible, since at least
some viral strains are typically unaffected.
7.10
1. How are viroids and prions different from viruses?
All viruses have genetic information and a protein coat. Viroids consist only of RNA and
therefore lack a protein coat. Prions are abnormally shaped proteins with no genetic
information.
2. How do viroids and prions cause disease?
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Viroids interfere with the production of important proteins. An abnormal prion protein
“recruits” normal proteins to refold into the abnormal configuration, triggering a chain
reaction that results in cell death.
Write It Out
1. Describe the three-dimensional structure of DNA.
DNA is a double helix that resembles a twisted ladder. In this molecule, the rails of the
ladder are alternating units of deoxyribose and phosphate, and the ladder’s rungs are A-T
and G-C base pairs joined by hydrogen bonds.
2. What is the function of DNA?
The function of much of the DNA in a cell is not known, but some of it encodes the cell’s
RNA and proteins.
3. Write the complementary DNA sequence of each of the following base sequences:
a.
AGGCATACCTGAGTC
b.
GTTTAATGCCCTACA
c.
AACACTACCGATTCA
The complementary sequences are:
a) TCCGTATGGACTCAG
b) CAAATTACGGGATGT
c) TTGTGATGGCTAAGT
4. Arrange the following objects in order from smallest to largest: nucleotide,
nitrogenous base, gene, nucleus, cell, codon, chromosome.
From smallest to largest, the order is nitrogenous base, nucleotide, codon, gene,
chromosome, nucleus, and cell.
5. List the three major types of RNA and their functions.
Messenger RNA (mRNA) carries the information that specifies a protein. Ribosomal
RNA (rRNA) combines with proteins to form a ribosome, the physical location of protein
synthesis. Transfer RNA (tRNA) molecules are “connectors” that bind mRNA codons at
one end and specific amino acids at the other. Their role is to carry each amino acid to the
ribosome at the correct spot along the mRNA molecule.
6. Some people compare DNA to a blueprint stored in the office of a construction
company. Explain how this analogy would extend to transcription and translation.
Transcription would be the process of scanning or copying the blueprints so that the
contractor would have a set at the construction site. Translation would be the process of
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the contractor directing the assembly of all the raw materials at the site into the finished
building.
7. Where in a eukaryotic cell do transcription and translation occur?
In a eukaryotic cell, transcription occurs in the nucleus, and translation occurs at
ribosomes in the cytoplasm or the rough ER.
8. Write the sequence of the mRNA molecules transcribed from the following template
DNA sequence: G G A A T A C G T C T A G C T A G C A
The complementary sequence is: CCUUAUGCAGAUCGAUCGU
9. How many codons are in the mRNA molecule that you wrote for question 8?
A codon has three nucleotides, so the 19-nucleotide sequence in question 8 has six
codons (plus one leftover nucleotide).
10. The roundworm C. elegans has 556 cells when it hatches. Each cell contains the
entire genome but expresses only a subset of the genes. Therefore, the cells “specialize”
in particular functions. List all of the ways that a roundworm cell might silence the
unneeded genes.
An individual roundworm cell can keep some of its DNA coiled or attach methyl groups
to inactivate genes. Transcription factors needed for transcription might not be available.
After transcription, different combinations of introns can be removed. mRNA can be
confined to the nucleus or rapidly degraded. The proteins can also be degraded.
11. If a gene is like a cake recipe, then a mutated gene is like a cake recipe containing an
error. List the major types of mutations, and describe an analogous error in a cake recipe.
Substitution mutation: instead of baking powder, the recipe lists baking soda. Insertion (3
nucleotides): the recipe lists one extra ingredient. Deletion (three nucleotides): the recipe
leaves out one ingredient. Frameshift: the word spacing is altered, e.g., flour, wate, regg,
ssuga, rsalt, etc.
12. A protein-encoding region of a gene has the following DNA sequence:
TTTCATCAGGATGCAACA
Determine how each of the following mutations alters the amino acid sequence:
a.
substitution of an A for the T in the first position
b.
substitution of a G for the C in the seventeenth position
c.
insertion of a T between the fourth and fifth DNA bases
d.
insertion of a GTA between the twelfth and thirteenth DNA bases
e.
deletion of the first DNA nucleotide
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a. Instead of encoding the amino acid lysine, the codon would recruit a release factor
protein.
b. Instead of incorporating the amino acid cysteine, the protein would incorporate serine.
c. Frameshift mutation; valine is replaced by aspartic acid, and the remainder of the
protein is disrupted.
d. Insertion mutation; the amino acid histidine is added within the protein.
e. Frameshift mutation; the entire protein is disrupted.
13. How might the effects of a mutation in a gene’s promoter differ from the effects of a
similar mutation in the gene’s protein-encoding region? How might the effects differ if
the mutation occurs in an intron versus an exon? Does the type of mutation affect your
answer?
A mutation in a gene’s promoter might prevent transcription factors or RNA polymerase
from binding; the gene would not be transcribed. In contrast, if a mutation occurs in a
gene’s protein-encoding region, then part of or all of the gene will be transcribed;
however, the amino acid sequence of the polypeptide produced in translation may be
different from expected. On the other hand, a silent mutation will not affect the amino
acid sequence, nor will a mutation in an intron. Also, if a substitution mutation occurs
that changes the first codon to signal a release factor, then none of the gene will be
transcribed.
14. The amount of melanin in the skin is controlled by genes, yet melanin is not a protein.
How can this be?
Many proteins are enzymes that control the abundance of other types of molecules in
cells. For example, enzymes in some cells synthesize pigment molecules called melanin.
Genes control the presence and amount of these enzymes in the cell.
15. Describe the basic parts of a virus and how each contributes to viral replication.
The two basic parts of a virus are the protein coat and genetic information. The protein
coat protects the virus and (in non-enveloped viruses) attaches to host cell receptors. The
genetic information encodes the viral molecules that are synthesized and assembled in the
host cell.
16. Your biology lab instructor gives you a petri dish of agar covered with visible
colonies. Your lab partner says the colonies are viruses, but you disagree. How do you
know the colonies are bacteria?
Viruses cannot produce colonies on agar because they require living host cells to
reproduce. Many bacteria, on the other hand, can live and reproduce using the nutrients
present in agar.
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17. Imagine a hybrid virus with the protein coat of virus X and the DNA of virus Y.
Will a host cell infected with this hybrid virus produce virus X, virus Y, a mix of virus X
and Y, or hybrid viruses? Explain your answer.
The host cell would produce virus Y since the capsid of virus X would contain no genetic
instructions.
18. Why do antibiotics such as penicillin kill bacteria but leave viruses unharmed?
Viruses lack the cell walls, ribosomes, and enzymes that antibiotic drugs target.
19. Search the Internet for information about the injectable flu vaccine (a “flu shot”).
Why is the flu shot administered annually when many other vaccines last for years? Is it
possible for a flu shot to cause influenza?
The influenza virus mutates so rapidly that this year’s vaccine will be ineffective against
next year's virus. The injectable flu vaccine consists of killed viruses, which cannot cause
influenza.
Pull It Together
1. Why is protein production essential to cell function?
Cell structure and function depend on proteins. Enzymes are proteins and are required
for almost all chemical reactions to occur within a cell. Without enzymes, the cell could
not synthesize ATP, which the cell uses for energy. In addition, proteins embedded
within cell membranes have several important functions such as adhesion, cell
recognition, and transport of water-soluble molecules; without protein production, new
cell membrane proteins could not be produced when the cell divides.
2. Where do promoters, terminators, stop codons, transcription factors, RNA
polymerase, and ribosomes fit into this concept map?
Both “Transcription factors” and “RNA polymerase” can connect with the phrase “bind
to” to “Promoters.” Both “Promoters” and “Terminators” can connect with the phrase
“are non-coding sequences of” to “DNA.” “Promoters” can also connect with the phrase
“signals the starting point for ” to “Transcription.” “Terminators” can also connect with
the phrase “signals the end point for” to “Transcription.” “Stop codons” can connect with
the phrase “ends the process of” to “Translation.” “Translation” can connect with “occurs
at” to “Ribosomes.”
3. How would viruses fit into this concept map?
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“Viruses” can connect with the phrase “rely on a host for” to “Transcription” and
“Translation”. “Viruses” can also connect with “genomes are sometimes” to “RNA.”
4. Use the concept map to explain why a mutation in DNA sometimes causes protein
function to change.
A mutation is a change in a DNA sequence. If the mutation leads to a change in the
encoded amino acid sequence, the protein’s shape could be altered or destroyed.
Therefore, mutations could lead to changes in protein function. (A gene that undergoes a
silent mutation, however, encodes the same amino acid sequence. The protein’s function
therefore does not change.)
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McGraw-Hill Education.