Mastering Biology: DNA replication: synthesis and repair
For the experiment shown in Figure 15.2 (Hershey-Chase), which additional piece of evidence would prove that the viral capsids were shaken off the bacterial cells by the agitation step?
Examination of the pellet by electron microscopy to show that no capsids are present.
Who demonstrated that DNA is the genetic material of the T2 phage?
Hershey and Chase
You isolate an infectious substance capable of causing disease in plants, but you do not know whether the infectious agent is a bacterium, virus, viroid, or prion. You have four methods at your disposal to analyze the substance and determine the nature of the infectious agent. I. Treat the substance with enzymes that destroy all nucleic acids and then determine whether the substance is still infectious. II. Filter the substance to remove all elements smaller than what can be easily seen under a light microscope. III. Culture the substance on nutritive medium, away from any plant cells. IV. Treat the sample with proteases that digest all proteins and then determining whether the substance is still infectious. If you already know that the infectious agent was either bacterial or viral, which method(s) listed above would allow you to distinguish between these two possibilities?
II. Filter the substance to remove all elements smaller than what can be easily seen under a light microscope. III. Culture the substance on nutritive medium, away from any plant cells.
Researchers found E. coli that had mutation rates one hundred times higher than normal. Which of the following is the most likely cause of these results?
The proofreading mechanism of DNA polymerase was not working properly.
DNA polymerases use ______ as their substrate.
deoxyribonucleoside triphosphates (dNTPs)
How many copies of DNA polymerase III are in the replisome?
2
Put the following steps of DNA replication in chronological order. 1. Single-stranded binding proteins attach to DNA strands. 2. Hydrogen bonds between base pairs of antiparallel strands are broken. 3. Primase binds to the site of origin. 4. DNA polymerase binds to the template strand. 5. An RNA primer is created.
2, 1, 3, 5, 4
For the Meselson-Stahl experiment shown in Figure 15.5, what would be the composition of the DNA after three generations?
3/4 low-density and 1/4 intermediate-density
Hershey and Chase used ___ to radioactively label the T2 phage's proteins.
35S
At a specific area of a chromosome, the sequence of nucleotides below is present where the chain opens to form a replication fork: 3' C C T A G G C T G C A A T C C 5' An RNA primer is formed starting at the underlined T (T) of the template. Which of the following represents the primer sequence?
5' A C G U U A G G 3'
In an analysis of the nucleotide composition of DNA, which of the following will be found?
A + C = G + T
In analyzing the number of different bases in a DNA sample, which result would be consistent with the base-pairing rules?
A + G = C + T
For a science fair project, two students decided to repeat the Hershey and Chase experiment, with modifications. They decided to label the nitrogen of the DNA, rather than the phosphate. They reasoned that each nucleotide has only one phosphate and two to five nitrogens. Thus, labeling the nitrogens 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
What would be the consequence(s) for DNA synthesis if primase were defective?
Both leading and lagging strand synthesis would be incomplete.
The radioactive isotope 32P labels the T2 phage's ___
DNA
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
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.
What is the role of DNA ligase in the elongation of the lagging strand during DNA replication?
It joins Okazaki fragments together.
What would be the consequence(s) for DNA synthesis if DNA ligase were defective?
Lagging strand synthesis would be incomplete; leading strand synthesis would be largely unaffected.
Which type of DNA repair is most likely to be compromised if there is a failure in the system that allows repair enzymes to distinguish old and newly synthesized DNA?
Mismatch repair
What is a major difference between eukaryotic DNA replication and prokaryotic DNA replication?
Prokaryotic chromosomes have a single origin of replication, while eukaryotic chromosomes have multiple origins of replication.
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 appears to be a dark side to telomerase activity with regard to human health?
Telomerase is active in most cancer cells.
Why does telomerase have to have a built-in template for DNA synthesis?
Telomerase is involved in adding DNA to the end of the lagging strand
In a healthy cell, the rate of DNA repair is 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 to a cancerous cell.
Which statement is correct concerning DNA synthesis catalyzed by DNA polymerases?
The new DNA strand is synthesized in the 5'—>3' direction; the template strand is read in the 3'—>5' direction.
Of the following, which is the most current description of a gene?
a DNA sequence that is expressed to form a functional product: either RNA or polypeptide
After allowing phages grown with bacteria in a medium that contained 32P and 35S, Hershey and Chase used a centrifuge to separate the phage ghosts from the infected cell. They then examined the infected cells and found that they contained _____, which demonstrated that _____ is the phage's genetic material.
labeled DNA...DNA
Which one of the following is least likely to cause mutations in DNA?
light from an incandescent bulb
Which of the following cells have reduced or very little active telomerase activity?
most normal somatic cells
DNA replication is highly accurate. It results in about one mistake per billion nucleotides. For the human genome, how often would errors occur?
on average, six times each time the entire genome of a cell is replicated
Semiconservative replication involved a template. What is the template?
one strand of the DNA molecule
Telomere shortening is a problem in which types of cells?
only eukaryotic cells
Within a double-stranded DNA molecule, adenine forms hydrogen bonds with thymine and cytosine forms hydrogen bonds with guanine. This arrangement _____.
permits complementary base pairing
Topoisomerase works to ______.
prevent twisting of DNA in front of the replication fork
The epsilon (ε) subunit of DNA polymerase III of E. coli has exonuclease activity. How does it function in the proofreading process? The epsilon subunit _____.
removes a mismatched nucleotide
The primers used for DNA synthesis are ______.
short RNA sequences
Which of the following help(s) to hold the DNA strands apart while they are being replicated?
single-strand DNA binding proteins
In the polymerization of DNA, a phosphodiester bond is formed between a phosphate group of the nucleotide being added and _____ of the last nucleotide in the polymer.
the 3' OH
What provides the energy for the polymerization reactions in DNA synthesis?
the deoxyribonucleotide triphosphate substrates
The leading and the lagging strands differ in that _____.
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
The polarity of a DNA strand refers to the fact that ______.
the two ends of the strand have different chemical groups
In E. coli, what is the function of DNA polymerase III?
to add nucleotides to the 3' end of a growing DNA strand
Viruses ___.
use the host cell to copy themselves and make viral proteins
