bio 181 ch 16
Thymine makes up 28% of the nucleotides in a sample of DNA from an organism. Approximately what percentage of the nucleotides in this sample will be guanine?
22%
In an analysis of the nucleotide composition of a molecule of DNA, which of the following combinations of base pairs will be found?
A + C = G + T
In the late 1950s, Meselson and Stahl grew bacteria in a medium containing "heavy" (radioactive) nitrogen (13N) and then transferred them to a medium containing 14N (non-radioactive). Which of the results in the figure would be expected after one round of DNA replication in the presence of 14N?
D
Hershey and Chase set out to determine what molecule served as the unit of inheritance. They completed a series of experiments in which E. coli was infected by a T2 virus. Which molecular component of the T2 virus actually ended up inside the cell?
DNA
In DNA replication, the resulting daughter molecules contain one strand of the original parental DNA and one new strand. What is the explanation for this phenomenon?
DNA replication is semiconservative
In a nucleosome, the DNA is wrapped around
histones
Which of the following lists represents the order of increasingly higher levels of organization of chromatin?
nucleosome, 30-nm chromatin fiber, looped domain
Which of the following statements accurately describes one characteristic of histones?
Histone H1 is not present in the nucleosome bead; instead, it draws the nucleosomes together.
Use the figure to answer the following question. Referring to the figure, what bases will be added to the primer as DNA replication proceeds?
5′ A, G, A, C, G, A, C 3′
Cytosine makes up 42% of the nucleotides in a sample of DNA from an organism. Approximately what percentage of the nucleotides in this sample will be thymine?
8%
In his transformation experiments, what phenomenon did Griffith observe?
Mixing a heat-killed pathogenic strain of bacteria with a living nonpathogenic strain can convert some of the living cells into the pathogenic form.
After the first replication was observed in their experiments testing the nature of DNA replication, Meselson and Stahl could be confident of which of the following conclusions?
Replication is not conservative
Researchers found a strain of E. coli bacteria that had mutation rates one hundred times higher than normal. Which of the following statements correctly describes the most likely cause of these results?
The proofreading mechanism of DNA polymerase was not working properly.
Semiconservative replication involves a template. What is the template?
one strand of the DNA molecule
In the polymerization of DNA, a phosphodiester bond is formed between a phosphate group of the nucleotide being added and which of the following atoms or molecules of the last nucleotide in the polymer?
the 3' OH
What are telomeres?
the ends of linear chromosomes
In E. coli, what is the function of DNA polymerase III?
to add nucleotides to the 3' end of a growing DNA strand
In E. coli, which enzyme catalyzes the elongation of a new DNA strand in the 5' → 3' direction?
DNA polymerase III
Why does a new DNA strand elongate only in the 5' to 3' direction during DNA replication?
DNA polymerase can add nucleotides only to the free 3' end.
Which of the following investigators was (were) responsible for the discovery that in DNA from any species, the amount of adenine equals the amount of thymine, and the amount of guanine equals the amount of cytosine?
Erwin Chargaff
Which of the following statements correctly describes the structure of chromatin?
Heterochromatin is highly condensed, whereas euchromatin is less compact.
Which of the following structural characteristics is most critical for the association between histones and DNA?
Histones are positively charged
How does the enzyme telomerase meet the challenge of replicating the ends of linear chromosomes?
It catalyzes the lengthening of telomeres, compensating for the shortening that could occur during replication without telomerase activity.
The elongation of the leading strand during DNA synthesis
depends on the action of DNA polymerase.
In E. coli, to repair a thymine dimer by nucleotide excision repair, in which order do the necessary enzymes act?
nuclease, DNA polymerase, DNA ligase
E. coli cells grown on 15N medium are transferred to 14N medium and allowed to grow for two more generations (two rounds of DNA replication). DNA extracted from these cells is centrifuged. What density distribution of DNA would you expect in this experiment?
one low-density and one intermediate-density band
It became apparent to Watson and Crick after completion of their model that the DNA molecule could carry a vast amount of hereditary information. Which of the following characteristics of DNA is responsible for this?
sequence of bases
Which of the following types of molecules help to hold the DNA strands apart while they are being replicated?
single-strand DNA binding proteins
For a science fair project, two students decided to repeat the Hershey and Chase experiment, with modifications. They decided to radioactively label the nitrogen of the DNA, rather than the phosphate. They reasoned that each nucleotide has only one phosphate and two to five nitrogen atoms. Thus, labeling the nitrogen atoms would provide a stronger signal than labeling the phosphates. Why won't this experiment work?
Amino acids (and thus proteins) also have nitrogen atoms; thus, the radioactivity would not distinguish between DNA and proteins.
Use the figure to answer the following question. 4 test tubes, labeled: at the top, super 14 N DNA; in the middle super 14 N-super 15 N hybrid DNA; at the bottom super 15 N DNA. A, DNA helix at the bottom. B, DNA helices at top and bottom. C, helix at the top. D, helix in the center. A space probe returns with a culture of a microorganism found on a distant planet. Analysis shows that it is a carbon-based life-form that has DNA. You grow the cells in 15N medium for several generations and then transfer them to 14N medium. Which pattern in the figure would you expect if the DNA was replicated in a conservative manner?
B
Which of the following effects might be caused by reduced or very little active telomerase activity?
Cells age and begin to lose function.
Which of the following facts did Hershey and Chase make use of in trying to determine whether DNA or protein is the genetic material?
DNA contains phosphorus, whereas protein does not.
Suppose you are provided with an actively dividing culture of E. coli bacteria to which radioactive thymine has been added. What would happen if a cell replicates once in the presence of this radioactive base?
DNA in both daughter cells would be radioactive.
A biochemist isolates, purifies, and combines in a test tube a variety of molecules needed for DNA replication. When she adds some DNA to the mixture, replication occurs, but each DNA molecule consists of a normal strand paired with numerous segments of DNA a few hundred nucleotides long. What has she probably left out of the mixture?
DNA ligase
DNA contains the template needed to copy itself, but it has no catalytic activity in cells. What catalyzes the formation of phosphodiester bonds between adjacent nucleotides in the DNA polymer being formed during DNA replication?
DNA polymerase
What is the basis for the difference in how the leading and lagging strands of DNA molecules are synthesized?
DNA polymerase can join new nucleotides only to the 3′ end of a pre-existing strand, and the strands are antiparallel.
The lagging strand is characterized by a series of short segments of DNA (Okazaki fragments) that will be joined together to form a finished lagging strand. The experiments that led to the discovery of Okazaki fragments gave evidence for which of the following ideas?
DNA polymerase is an enzyme that synthesizes leading and lagging strands during replication only in one direction.
Which of the following molecular characteristics cause histones to bind tightly to DNA?
Histones are positively charged, and DNA is negatively charged.
What is the role of DNA ligase in the elongation of the lagging strand during DNA replication?
It joins Okazaki fragments together.
Within a double-stranded DNA molecule, adenine forms hydrogen bonds with thymine, and cytosine forms hydrogen bonds with guanine. What is the significance of the structural arrangement?
It permits complementary base pairing
Who performed classic experiments that supported the semiconservative model of DNA replication?
Meselson and Stahl
In E. coli, there is a mutation in a gene called dnaB that alters the helicase that normally acts at the origin of replication. Which of the following events would you expect to occur as a result of this mutation?
No replication fork will be formed
Which of the following characteristics would you expect of a eukaryotic organism that lacks the enzyme telomerase?
a reduction in chromosome length in gametes
Which of the following characteristics of eukaryotic telomeres cause them to replicate differently than the rest of the chromosome?
gaps left at the 5' end of the lagging strand template
You briefly expose bacteria undergoing DNA replication to radioactively labeled nucleotides. When you centrifuge the DNA isolated from the bacteria, the DNA separates into two classes. One class of labeled DNA includes very large molecules (thousands or even millions of nucleotides long), and the other includes short stretches of DNA (several hundred to a few thousand nucleotides in length). Which two classes of DNA do these different samples represent?
leading strands and Okazaki fragments
In DNA replication in E. coli, the enzyme primase is used to attach a 5 to 10 base ribonucleotide strand complementary to the parental DNA strand. The RNA strand serves as a starting point for the DNA polymerase that replicates the DNA. If a mutation occurred in the primase gene, which of the following results would you expect?
Replication would not occur on either the leading or lagging strand.
Telomere shortening puts a limit on the number of times a cell can divide. Research has shown that telomerase can extend the life span of cultured human cells. How might adding telomerase affect cellular aging?
Telomerase eliminates telomere shortening and retards aging.
What is meant by the description "antiparallel" regarding the two strands that make up the DNA double helix?
The 5' to 3' direction of one strand runs counter to the 5' to 3' direction of the other strand.
In a healthy eukaryotic cell, the rate of DNA repair is typically equal to the rate of DNA mutation. When the rate of repair lags behind the rate of mutation, what is a possible fate of the cell?
The cell can be transformed into a cancerous cell
If a cell were unable to produce histone proteins, which of the following results would be a likely effect on the cell?
The cell's DNA could not be packed into its nucleus.
Individuals with the disorder xeroderma pigmentosum are hypersensitive to sunlight, and mutations to the DNA in their skin cells are left uncorrected. Why are the mutations not corrected in individuals with this disorder?
The disorder causes cells to be unable to repair thymine dimers.
Which of the following statements correctly describes the difference between the leading and the lagging strands of DNA during DNA replication?
The leading strand is synthesized in the same direction as the movement of the replication fork, and the lagging strand is synthesized in the opposite direction.
Hershey and Chase used a DNA-based virus for their work. How might the results have been different if they had used an RNA virus?
With an RNA virus, radioactive RNA would have been in the final pellet.
Which of the following types of cells are affected most by telomere shortening?
only eukaryotic cells
A heat-killed, phosphorescent (light-emitting) strain of bacteria is mixed with a living, non-phosphorescent strain. Further observations of the mixture show that some of the living cells are now phosphorescent. Which of the following observations would provide the best evidence that the ability to phosphoresce is a heritable trait?
phosphorescence in descendants of the living cells
What is the function of the enzyme topoisomerase in DNA replication?
relieving strain in the DNA ahead of the replication fork caused by the untwisting of the double helix
Which of the following statements accurately describes the structure of a eukaryotic chromosome?
It is a single linear molecule of double-stranded DNA plus proteins.
Which of the following statements accurately describes the differences between DNA replication in prokaryotes and DNA replication in eukaryotes?
Prokaryotic chromosomes have a single origin of replication, whereas eukaryotic chromosomes have many.
The spontaneous loss of amino groups from adenine in DNA results in hypoxanthine, an uncommon base, opposite thymine. What combination of proteins could repair such damage?
nuclease, DNA polymerase, DNA ligase
