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

Answers to End-of-Chapter Questions – Brooker et al ARIS site
Chapter 11
Test Yourself Questions
1. Why did researchers initially believe the genetic material was protein?
a. Proteins are more biochemically complex than DNA.
b. Proteins are found only in the nucleus, but DNA is found in many areas of the cell.
c. Proteins are much larger molecules and can store more information than DNA.
d. All of the above.
e. Both a and c.
Answer: a. Researchers knew that chromosomes were composed of proteins and DNA. Proteins are
polymers of amino acids, which there are 20 different types. DNA is a polymer of nucleotides, which
there are only 4 types. Researchers felt that the higher complexity of proteins was evidence of its ability
to store information.
2. Considering the components of a nucleotide, what component always determines whether the
nucleotide would be incorporated into a DNA strand or an RNA strand?
a. phosphate group
b. pentose sugar
c. nitrogenous base
d. both b and c
Answer: b. The pentose sugar is the component that determines to what type of nucleic acid the
nucleotide belongs. Ribose sugar would indicate RNA and deoxyribose sugar would indicate DNA.
3. Which of the following equations would be appropriate when considering DNA base composition?
a. %A + %T = %G + %C
b. %A = %G
c. %A = %G = %T = %C
d. %A + %G = %T + %C
Answer: d. Chargaff’s rule indicates a relationship between the % purines and % pyrimidines.
According to the DNA model proposed by Watson and Crick, base pairs consist of a purine and a
pyrimidine. This would indicate that a double-stranded DNA molecule should have equal percentages of
purines and pyrimidines.
4. If the sequence of a section of DNA is 5’-CGCAACTAC-3’, what is the appropriate sequence for the
opposite strand?
a. 5’-GCGTTGATG-3’
b. 3’-ATACCAGCA-5’
c. 5’-ATACCAGCA-3’
d. 3’-GCGTTGATC-5’
Answer: d. When determining the correct sequence of the opposite strand, one must consider the
proper base pairing and directionality of the strand. Considering the base pairing, adenine base pairs
with thymine and guanine base pairs with cytosine. In double-stranded DNA molecules, the two strands
are antiparallel, meaning the orientation of one strand is 5’ to 3’, while the orientation of the opposite
strand is 3’ to 5’.
5. Of the following statements, which is correct when considering the process of DNA replication?
a. New DNA molecules are composed of two completely new strands
b. New DNA molecules are composed of one strand from the old molecule and one new strand.
c. New DNA molecules are composed of strands that are a mixture of sections from the old
molecule and sections that are new.
d. None of the above.
Answer: b. DNA is replicated semi-conservatively, where each new molecule is composed of an old
strand, parental strand, and one new strand, daughter strand.
6. Meselson and Stahl were able to demonstrate semiconservative replication in E. coli by
a. using radioactive isotopes of phosphorus to label the old strand and visually determining the
relationship of old and new DNA strands.
b. using different enzymes to eliminate old strands from DNA.
c. using isotopes of nitrogen to label the DNA and determining the relationship of old and new
DNA strands by density differences of the new molecules.
d. labeling viral DNA before it was incorporated into a bacterial cell and visually determining the
location of the DNA after centrifugation.
Answer: c. Meselson and Stahl used different isotopes of nitrogen to determine the composition of
newly synthesized DNA molecules. By analyzing the density differences over several generations of
replication, Meselson and Stahl were able to present data that were consistent with only semiconservative replication.
7. During replication of a DNA molecule, the daughter strands are not produced in exactly the same
manner. One strand, the leading strand, is made toward the replication fork, while the lagging strand is
made in fragments in the opposite direction. This difference in the synthesis of the two strands is the
result of
a. DNA polymerase is not efficient enough to make two “good” strands of DNA.
b. the two parental strands are antiparallel, and DNA polymerase makes DNA only in the 5' to 3'
direction
c. the lagging strand is the result of DNA breakage due to UV light.
d. the cell does not contain enough nucleotides to make two complete strands.
Answer: b. The two parental strands are antiparallel and DNA polyermase makes DNA only in the 5' to
3' direction. Because DNA polymerase can add new nucleotides only to the 3’ end of the new strands,
the two daughter strands are “growing” in opposite directions. The leading strand follows the replication
fork and can be made as a continuous strand. The lagging strand grows in the opposite direction, so
synthesis has to be initiated many times along the parental strand, producing the Okazaki fragments.
8. Considering the different proteins involved in DNA replication and their functions, which of the
following does not represent an accurate relationship?
a. DNA helicase unwinds the DNA molecule.
b. DNA primase produces the Okazaki fragments of the lagging strand.
c. DNA topoisomerase reduces the coiling ahead of the replication fork.
d. Single-strand binding proteins prevent the double helix from re-forming.
Answer: b. DNA primase produces short RNA primers that allows DNA polymerase to attach DNA
nucleotides. DNA polymerase makes most of the Okazaki fragments.
9. Which of the following does not promote the fidelity of DNA replication?
a. DNA polymerase will not form bonds between nucleotides if there is a mismatched base.
b. DNA polymerase has the ability to recognize mismatched bases and remove them.
c. Hydrogen bonds that hold the two strands together are more stable in correctly matched
bases than between mismatched bases.
d. All of the above are correct.
Answer: d. The hydrogen bonding between correct bases increases the stability of the molecule and the
DNA polymerase activity increases the fidelity of DNA replication.
10. Most organisms have more than one form of DNA polymerase. Some forms play specific roles in
replication, while others play roles in DNA repair. What is the proposed mechanism for the evolution of
the multiple forms of DNA polymerase?
a. Increased copies of polymerase genes due to chromosome number changes over several
generations.
b. Species hybridization may introduce new forms of polymerases.
c. Gene duplication followed by mutations that alter the activity of the polymerase.
d. Mutations change other genes into DNA polymerases.
Answer: c. The process of gene duplication would provide multiple copies of a gene with the function of
synthesizing DNA. Over evolutionary time, variation among these genes may arise due to the
accumulation of mutations.
Conceptual Questions
1. List and explain the four characteristics of the genetic material.
Answer: The genetic material must contain the information necessary to construct an entire organism.
The genetic material must be passed from parent to offspring and from cell to cell during cell division in
multicellular organisms. The genetic material must be accurately copied. The genetic material must
contain variation that can account for the known variation within each species and among different
species.
2. Explain or describe the essential features of the Watson and Crick model of the structure of DNA.
Answer: Two long chains of polynucleotides are coiled around a central axis forming a helix. The two
chains run in opposite directions or are antiparallel. Hydrogen bonds between the bases in opposite
strands stabilize the structure. Adenine always pairs with thymine and cytosine always pairs with
guanine. The width of the double helix is relatively constant. One complete turn of the double helix is
composed of ten base pairs.
3. Explain the function of telomerase.
Answer: Telomerase is an enzyme that prevents chromosome shortening by attaching many copies of a
repeated DNA sequence to the ends of chromosomes or telomeres.
Experimental Questions
1. Avery, MacLeod, and McCarty worked with two strains of Streptococcus pneumoniae to determine
the biochemical identity of the genetic material. Explain the characteristics of the Streptococcus
pneumoniae strains that made them particularly well suited for such an experiment.
Answer: Previous studies had indicated that mixing different strains could lead to transformation or the
changing of a strain into a different one. Particularly, Griffith had shown that mixing heat-killed type S
with living type R would result in the transformation of the type R to type S. Though mutations could
cause the changing of the identity of certain strains, the type R to type S transformation was not due to
mutation but was more likely due to the transmission of a biochemical substance between the two
strains. Griffith recognized this and referred to the biochemical substance as the “transformation
principle.” If Avery, MacCleod and McCarty could determine the biochemical identity of this
“transformation principle,” they could identify the genetic material for this organism.
2. In the experiment of Avery, MacLeod, and McCarty, what was the purpose of using the protease,
RNase, and DNase if only the DNA extract caused transformation?
Answer: The researchers could not verify that the DNA extract was completely pure and did not have
small amounts of contaminating molecules, such as proteins and RNA. The researchers were able to
treat the extract with enzymes to remove proteins (using protease), RNA (using RNase) or DNA (using
DNase). Removing the proteins or RNA did not alter the transformation of the type R to type S strains.
Only the enzymatic removal of DNA disrupted the transformation, indicating that DNA is the genetic
material.
3. The Hershey and Chase experiment used radioactive isotopes to track the DNA and protein of
phages as they infected bacterial cells. Explain how this procedure allowed them to detect that DNA
was the genetic material of this particular virus.
Answer: In the case of the Hershey and Chase experiment, a radioactive isotope of sulfur was used to
label the protein in the viral protein coat. The DNA was labeled using a radioactive isotope of
phosphorus. This was an ideal way of labeling the different components, because sulfur is found in
proteins but not DNA and phosphorus is found in DNA but not proteins. By labeling the two candidate
molecules with the radioactive isotopes, Hershey and Chase could determine the genetic material by
seeing which isotope entered the bacterial cells.
Collaborative Questions
1. Discuss the reasons why there are very few errors made during DNA replication.
Answer: First, hydrogen bonds between mismatched pairs of nucleotides A-T and C-G are more stable
and form more readily. Second DNA polymerase is less likely to catalyze bond formation between
adjacent nucleotides if a mismatched base pair is formed. Third, DNA polymerase enzymatically
removes mismatched nucleotides therefore reducing the number of mistakes. Fourth, in addition to DNA
polymerase proofreading the DNA strand there are other enzymes which proofread and repair any
damaged DNA.
2. What was Frederick Griffith’s contribution to the study of DNA and why was it so important?
Answer: Griffith discovered something called the transformation principle and his experiments showed
the existence of biochemical genetic information. In addition he showed that this genetic information can
move form one individual to another of the same species. In his experiment Griffith took heat-killed type
S bacteria and mixed them with living type R bacteria and injected them into a live mouse which died
after the injection. By themselves, these two strains would not kill the mouse but when they were put
together, the genetic information from the heat-killed type S bacteria was transferred into the living type
R bacteria, thus transforming the type R bacteria into type S.