Question 1

In an analysis of the nucleotide composition of DNA,which of the following will be found in a given DNA molecule?

Accepted Answer

DC) The amount of adenine (A) is equal to thymine (T), and the amount of guanine (G) is equal to cytosine (C) in a DNA molecule. Therefore, when you add A + C, it equals G + T because of base pairing rules.D) This is just a restatement of option C but with the pairs switched around. It still reflects the base pairing rules where A pairs with T and G pairs with C, making the total amounts equal on both sides.

Question 2

A mutation is identified in a single-celled eukaryote called a protist in which DNA primase does not assemble RNA primers on the lagging strand during DNA replication.Which of the following statements correctly describes a possible result of this mutation?

Accepted Answer

In eukaryotic DNA replication, both the leading and lagging strands require RNA primers to be assembled by DNA primase before DNA polymerase can synthesize new DNA. Without RNA primers on the lagging strand, DNA polymerase cannot start replicating the DNA strand, resulting in incomplete replication of the lagging strand. Therefore, the lagging strand will not be replicated normally.

Question 3

Which of the following characteristics of the structure of DNA allows it to carry a large amount of hereditary information?

Accepted Answer

The sequence of bases (adenine, thymine, guanine, and cytosine) along the DNA molecule can vary in countless ways, allowing for a large amount of hereditary information to be encoded. The other choices (B, C, and D) are important characteristics of the structure of DNA, but they don't directly relate to the amount of hereditary information it can carry.

Question 4

After mixing a heat-killed,phosphorescent (light-emitting)strain of bacteria with a living,nonphosphorescent strain,you discover that some of the living cells are now phosphorescent.Which observation(s)would provide the best evidence that the ability to phosphoresce is a heritable trait?

Accepted Answer

The fact that descendants of the living cells are also phosphorescent provides the best evidence that the ability to phosphoresce is a heritable trait, as it suggests that the genetic material responsible for this trait has been passed down from generation to generation. Options A and B suggest the transfer of genetic material, but do not provide evidence of heritability. Option C may indicate a strong expression of the trait, but does not provide evidence of heritability.

Question 5

Which of the following statements correctly describes how the leading and the lagging strands of DNA formed during DNA replication differ?

Accepted Answer

The leading strand is synthesized continuously in the same direction as the movement of the replication fork, while the lagging strand is synthesized in the opposite direction in short fragments called Okazaki fragments which are ultimately stitched together.

Question 6

In which order do the necessary enzymes act to repair a thymine dimer by nucleotide excision repair?

Accepted Answer

In nucleotide excision repair, the damaged section of DNA is first recognized and excised by a nuclease enzyme. Then, a DNA polymerase fills in the gap by adding new nucleotides, using the intact strand as a template. Finally, DNA ligase seals the nick to complete the repair. Therefore, the correct order of enzyme action is nuclease, DNA polymerase, and DNA ligase. Choice B is incorrect because helicase is not involved in nucleotide excision repair, and RNA primase is not needed because RNA is not involved in DNA repair. Choice C is also incorrect because DNA ligase acts last in the repair process, after the gap has been filled in by DNA polymerase.

Question 7

What is meant by the description "antiparallel" regarding the two strands of nucleic acids that make up DNA?

Accepted Answer

Antiparallel refers to the opposite directionality of the two strands of DNA. One strand runs in a 5' to 3' direction, while the other runs in a 3' to 5' direction, resulting in a parallel yet opposite arrangement.

Question 8

Suppose you are provided with an actively dividing culture of Escherichia coli bacteria to which radioactive thymine has been added.What would happen if a cell replicates once in the presence of this radioactive base?

Accepted Answer

During DNA replication, the newly synthesized strand will contain the radioactive thymine since it was incorporated by the replicating DNA polymerase. Therefore, both daughter cells will have radioactive DNA. Option A is incorrect because both daughter cells should have radioactive DNA. Option B is incorrect because the radioactive thymine will be incorporated into the DNA in at least one of the daughter cells. Option C is incorrect because only the thymine will be radioactive, not all four bases.

Question 9

Which of the following statements most accurately describes transformation in bacteria?

Accepted Answer

Transformation in bacteria refers to the uptake and integration of external DNA into the bacterial cell. This process was first observed by Frederick Griffith in 1928, who found that dead bacteria could transfer genetic material to live bacteria, thereby transforming their phenotype. Transformation is one of the three mechanisms by which bacteria can exchange genetic material, along with conjugation (direct cell-to-cell transfer) and transduction (transfer via bacteriophage).

Question 10

Which of the following statements correctly describes one characteristic of DNA replication in eukaryotes?

Accepted Answer

In eukaryotes, DNA replication occurs in a semiconservative manner, where each strand of the original double helix serves as a template for the synthesis of a new complementary strand. However, due to the antiparallel nature of DNA, the two strands have to be replicated in different ways. The leading strand is replicated continuously in the 5' to 3' direction, whereas the lagging strand is replicated discontinuously in the opposite 3' to 5' direction, resulting in the formation of short Okazaki fragments. These fragments are then joined together by DNA ligase to form a complete strand. Therefore, option C is the correct statement describing DNA replication in eukaryotes.

Question 11

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?

Accepted Answer

The helicase is responsible for unwinding the DNA strands at the origin of replication, allowing replication to occur. If the helicase is altered due to the mutation in dnaB, the DNA strands will still separate but replication will not progress because the replication machinery will not be able to move along the DNA strands.

Question 12

Which of the following statements describes one difference between DNA replication in prokaryotes and DNA replication in eukaryotes?

Accepted Answer

Prokaryotic chromosomes have a single origin of replication, whereas eukaryotic chromosomes have many. This is because prokaryotic genomes are typically smaller and simpler than eukaryotic genomes, and therefore have less DNA to replicate. Eukaryotic genomes have multiple origins of replication in order to start and complete the replication process more efficiently.

Question 13

What is the function of DNA polymerase III in replication in E.coli?

Accepted Answer

DNA polymerase III is a key enzyme in DNA replication in E.coli. Its main function is to add nucleotides to the 3' end of a growing DNA strand, thus extending the strand in the 5' to 3' direction. This process requires a template strand to base-pair with and a primer to initiate the addition of nucleotides. DNA polymerase III has high processivity, meaning that it can add many nucleotides before dissociating from the template strand. This makes it well-suited for the rapid and accurate replication of the entire E.coli genome during cell division.

Question 14

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?

Accepted Answer

In DNA, the amount of cytosine (C) equals the amount of guanine (G), and the amount of adenine (A) equals the amount of thymine (T). If cytosine is 42%, guanine is also 42%, making up 84% of the DNA. The remaining 16% must be split equally between adenine and thymine, so thymine is 8%.

Question 15

In his transformation experiments,which of the following results did Griffith observe?

Accepted Answer

Griffith observed that mixing a heat-killed pathogenic strain of bacteria with a living nonpathogenic strain can convert some of the living cells into the pathogenic form. This was demonstrated in his experiments with Streptococcus pneumoniae, where he mixed a heat-killed virulent strain with a non-virulent strain and observed that some of the non-virulent cells became virulent. This process was later identified as transformation, which involves the uptake and incorporation of foreign DNA by a bacterial cell. The other options listed are not accurate descriptions of Griffith's observations.

Question 16

Why does a new DNA strand elongate only in the 5' to 3' direction in replication?

Accepted Answer

DNA polymerase can only add nucleotides to the free 3' end. The enzyme adds new nucleotides to the 3' end of the template strand, which creates a free 3' hydroxyl group for the next nucleotide to be added. As a result, elongation can only occur in the 5' to 3' direction.

Question 17

In trying to determine whether DNA or protein was the genetic material,Hershey and Chase made use of which of the following facts about these two types of molecules?

Accepted Answer

Hershey and Chase used the fact that DNA contains phosphorus, whereas protein does not. They labeled the DNA and protein of bacteriophages with different radioactive isotopes of sulfur and phosphorus, respectively, and found that only the DNA was transferred to the bacterial host, indicating that DNA was the genetic material.

Question 18

Which of the following characteristics of the structure of DNA was determined directly from X-ray diffraction photographs of crystallized DNA?

Accepted Answer

The double helical structure of DNA was determined directly from X-ray diffraction photographs of crystallized DNA. The other options listed cannot be determined from X-ray diffraction photographs.

Question 19

Which of the following statements correctly describes one of Chargaff's rules regarding DNA?

Accepted Answer

Chargaff's rule states that the percentages of adenine and thymine are roughly equal, as are the percentages of guanine and cytosine, in the DNA of a given species. This rule is important because it helped Watson and Crick determine the base pairing rules that govern the structure of DNA, which in turn led to the development of our current understanding of DNA replication and transcription.

Question 20

Which enzyme in E.coli catalyzes the elongation of a DNA strand in the 5' → 3' direction?

Accepted Answer

DNA polymerase III is the enzyme in E.coli that catalyzes the elongation of a DNA strand in the 5' → 3' direction.

Quiz 13: The Molecular Basis of Inheritance