TPR Review Exam 1 C/P # 58

[basophilic]

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Which of the following is true regarding the specific enzymes involved in DNA replication?

A)DNA topoisomerases function to produce supercoils during DNA replication.

B)DNA polymerase can only initiate DNA replication following the placement of a RNA primer by the enzyme DNA primase.

C)DNA ligase functions by catalyzing phosphodiester bond formation between the ends of two strands of DNA.

D)Telomerase preserves genetic information found in DNA by adding a sequence of DNA onto the 5’ ends of DNA strands.

A and D are obviously wrong. However, C is the right answer.
B: I thought this was correct because the enzyme that lays down the primer may be an RNA polymerase, but it is called a DNA primase, at least according to Wikipedia and a textbook source.

C: This one seemed ambiguous and so I felt this was less correct than B. Sure DNA ligase joins separate strands longitudinally. But the wording of the answer choice could be interpreted to mean ligating two complementary strands (and you usually see "two strands" to mean complementary ones); hence, I felt this was "more wrong" than B.
Anyone agree/disagree with my rationale?

 

[bee17]

At first glance, I would have chosen B as well. After some intense googling, I think B is wrong because of the word "only." It turns out that some primers are made of DNA, not RNA, so to say that DNA polymerase can only initiate replication following an RNA primer would be false.

 

[basophilic]

At first glance, I would have chosen B as well. After some intense googling, I think B is wrong because of the word "only." It turns out that some primers are made of DNA, not RNA, so to say that DNA polymerase can only initiate replication following an RNA primer would be false.

Wow, never knew that; I guess that makes it an Extreme, which is of course never good on MCAT.
However, TPR 's rationale for eliminating B was because the answer choice says DNA primase rather than RNA primase.

 

[bee17]

hmmm. Then I have to say it's a bad question, because I have seen sources use both "DNA primase" and "RNA primase" to describe the same enzyme. Perhaps that is why most sources, including my biochem textbook, just refer to it as primase!

 

[yeezuswest]

In the biochem textbook I'm using (Lehninger 6e) there is a primase involved with DNA replication that synthesizes an RNA primer, as well as a specific RNA primase that is mentioned in primer synthesis at telomeric ends. In both cases, an RNA primer is synthesized, and so it would be safe to assume an RNA primase would be involved in synthesis of an RNA primer.