Question 1

What is a major difference between eukaryotic DNA replication and prokaryotic DNA replication?

Accepted Answer

Prokaryotic chromosomes have a single origin of replication, while eukaryotic chromosomes have multiple origins of replication. This means that eukaryotic DNA replication requires a more complex process of coordinating multiple replication forks, while prokaryotic DNA replication can proceed from a single origin in both directions until completed. The other options are not accurate differences between prokaryotic and eukaryotic DNA replication.

Question 2

Refer to the figure associated with this question. What bases will be added to the primer as DNA replication proceeds? The bases should appear in the new strand in the order that they will be added starting at the 3' end of the primer.

Accepted Answer

The answer of  Refer to the figure associated with...

Question 3

In the accompanying figure, which is the template strand?

Accepted Answer

The template strand is the strand of DNA that is used as a template to produce a complementary RNA strand during transcription. In the accompanying figure, the template strand is the bottom strand, which runs from right to left and has the sequence 5'-ACTTGGC-3'. The top strand, which runs from left to right and has the sequence 3'-TGAACCG-5', is the non-template or coding strand. Among the choices a to d, only choice A matches the sequence of the template strand. Therefore, the correct answer is A.

Question 4

In the accompanying figure, which structure causes the two strands of DNA to separate?

Accepted Answer

Structure B, helicase, is responsible for separating the two strands of DNA by breaking the hydrogen bonds between the complementary base pairs. This allows for replication or transcription to occur. Structures A, C, and D are not directly involved in strand separation.

Question 5

Which of the following cells have reduced or very little active telomerase activity?

Accepted Answer

Most normal somatic cells have reduced or very little active telomerase activity. This is due to the fact that telomerase activity is usually turned off in these cells in order to prevent unlimited cell division which can lead to cancer. On the other hand, germ cells and cancer cells usually have higher levels of active telomerase activity.

Question 6

The DNA of telomeres has been highly conserved throughout the evolution of eukaryotes. This most likely reflects ________.

Accepted Answer

The high conservation of telomere DNA throughout evolution suggests that telomeres serve a critical function. Telomeres protect the ends of chromosomes from degradation and fusion, and are necessary for proper cell division and genome stability. Mutations in telomere sequences could have detrimental effects on these processes, which may explain why telomere DNA is conserved.

Question 7

Eukaryotic telomeres replicate differently than the rest of the chromosome. This is a consequence of which of the following?

Accepted Answer

The answer of Eukaryotic telomeres replicate differently than the rest...

Question 8

The leading and the lagging strands differ in that ________.

Accepted Answer

The leading strand is synthesized in the same direction as the movement of the replication fork, while the lagging strand is synthesized in the opposite direction. This is because DNA synthesis can only occur in the 5' to 3' direction, and the replication fork moves in a 3' to 5' direction. The leading strand is synthesized continuously, while the lagging strand is synthesized in short fragments that are ultimately stitched together.

Question 9

Polytene chromosomes of Drosophila salivary glands each consist of multiple identical DNA strands that are aligned in parallel arrays. How could these arise?

Accepted Answer

Polytene chromosomes arise when DNA replication occurs without separation of the resulting identical sister chromatids, resulting in parallel arrays of multiple identical strands. This process is called endoreplication.

Question 10

In E. coli, what is the function of DNA polymerase III?

Accepted Answer

DNA polymerase III is the main polymerase involved in extension of the leading and lagging strands during DNA replication. It adds nucleotides to the 3' end of the growing DNA strand.

Answer: E
 After the leading and lagging strands are synthesized by DNA polymerase III, the two strands must be rejoined to form a circular, double-stranded DNA molecule. This process is called ligation, and it is carried out by enzymes called ligases. However, DNA polymerase III does play a role in repairing DNA damage by replacing missing nucleotides, and it is involved in a range of other DNA metabolic processes as well.

Question 11

At a specific area of a chromosome, the following sequence of nucleotides 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?

Accepted Answer

RNA primers are synthesized using the DNA template strand, and they are complementary and antiparallel to the template. The sequence starting from the underlined T in the template is 5' AGGCT... 3'. The complementary RNA sequence would be 5' UCCGA... 3', but since the primer starts at the T, the sequence is 5' UUAGG 3', which matches option D.

Question 12

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?

Accepted Answer

Adding telomerase would eliminate telomere shortening and thus extend the replicative capacity and lifespan of cells. This can retard cellular aging by preventing the accumulation of damaged or dysfunctional cells.

Question 13

What is the role of DNA ligase in the elongation of the lagging strand during DNA replication?

Accepted Answer

DNA ligase joins the Okazaki fragments together on the lagging strand during DNA replication. It does not synthesize RNA nucleotides, lengthen telomeres, unwind the parental double helix, or stabilize unwound parental DNA.

Question 14

What provides the energy for the polymerization reactions in DNA synthesis?

Accepted Answer

The deoxyribonucleotide triphosphate substrates provide the necessary energy for polymerization reactions in DNA synthesis. As each nucleotide is added to the growing DNA strand, two phosphates are cleaved off, releasing energy that drives the reaction forward. ATP provides energy in other cellular processes but is not directly involved in DNA polymerization. DNA polymerase is the enzyme that facilitates the polymerization reaction but does not provide energy, and breaking the hydrogen bonds between complementary DNA strands requires energy rather than providing it.

Question 15

Which of the following would you expect of a eukaryote lacking telomerase?

Accepted Answer

Telomerase is responsible for adding telomeres, repetitive DNA sequences, to the ends of chromosomes to prevent the loss of important genetic information during DNA replication. Without telomerase, chromosomes will gradually shorten with each cell division. In gametes, this reduction in length can have significant consequences for the development of offspring. Therefore, a eukaryote lacking telomerase would be expected to have a reduction in chromosome length in gametes. The other options are not directly related to telomerase function.

Question 16

Which of the following help(s) to hold the DNA strands apart while they are being replicated?

Accepted Answer

Single-strand DNA binding proteins (SSB proteins) help to hold the DNA strands apart while they are being replicated. They bind to and stabilize single-stranded DNA, preventing it from re-forming base pairs with the complementary strand. This allows the replication machinery to access and copy the individual strands. Primase (A) is an enzyme that synthesizes short RNA primers that are necessary for DNA polymerase to initiate replication. Ligase (B) is an enzyme that catalyzes the joining of DNA fragments, and is not involved in holding the strands apart during replication. DNA polymerase (C) is the enzyme responsible for synthesizing new strands of DNA and proofreading them for errors, but it does not directly hold the strands apart. Nuclease (E) is an enzyme that breaks down nucleic acids, and is not involved in DNA replication.

Question 17

How does the enzyme telomerase meet the challenge of replicating the ends of linear chromosomes?

Accepted Answer

Telomerase catalyzes the addition of repetitive DNA sequences called telomeres to the ends of linear chromosomes, compensating for the shortening that could occur during replication without telomerase activity. This ensures that the ends of the chromosomes are replicated without loss of genetic material and maintains chromosome integrity.

Question 18

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.

Accepted Answer

The correct sequence for DNA replication is: 2) Hydrogen bonds between base pairs are broken to open the double helix. 1) Single-stranded binding proteins stabilize the unwound DNA. 3) Primase binds to the site of origin to synthesize a short RNA primer. 5) The RNA primer is created, providing a starting point for DNA polymerase. 4) DNA polymerase binds to the template strand to begin adding nucleotides.

Question 19

What is a telomere?

Accepted Answer

Telomeres are the repetitive DNA sequences found at the ends of linear chromosomes. They function to protect the genetic information during cell division and prevent the loss of important genetic material.

Question 20

Identify the lagging strand during duplication of DNA starting from a double helix in the accompanying figure.

Accepted Answer

The lagging strand is the template strand that is replicated in the opposite direction of the replication fork movement. In the figure, the fork movement is from right to left, and the template strand on the right is replicated in the 5' to 3' direction continuously as the leading strand. The template strand on the left, however, is replicated in the opposite direction, so the new strand synthesis is in the opposite direction of the replication fork movement, resulting in Okazaki fragments. Therefore, the template strand on the left is the lagging strand, and the correct choice is C.

Quiz 15: Dna and the Gene: Synthesis and Repair