AP Biology : 6.2 Replication – Exam Style questions with Answer- MCQ

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Question

A model of a process involving nucleic acids is shown in Figure 1.

The figure presents a diagram of a replication fork that is growing to the right. Strands that are complementary to the separated template strands on the left side of the fork have already been synthesized. The template strands on the right side remain single stranded. The left end of the upper template strand is labeled 5 prime, and the right end is labeled 3 prime. Two sequential arrows that point from right to left represent the process of strand synthesis with nucleotides that are complementary to the upper template strand. The left end of each arrow is labeled 3 prime, and the right end of each is labeled 5 prime. The left end of the lower template strand is labeled 3 prime, and the right end is labeled 5 prime. A single long arrow that points from left to right represents the process of strand synthesis with nucleotides that are complementary to the lower template strand. The left end of the arrow is labeled 5 prime, and the right end is labeled 3 prime.

Figure 1. Model of a process involving nucleic acids

Which of the following best explains what process is represented in Figure 1 ?

A. New DNA strands are being synthesized in the 3‘ to 5‘ direction from their DNA templates.
B. New DNA strands are being synthesized in the 5‘ to 3‘ direction from their DNA templates.
C. A new RNA strand is being synthesized in the 3‘ to 5‘ end from its DNA template.
D. Two new RNA strands are being synthesized in both directions from their DNA templates.

▶️Answer/Explanation

Ans: B
New DNA strands are synthesized in the 5‘ to 3‘ direction from the DNA template, and this is depicted in the diagram. The leading strand is being synthesized on the bottom, while the lagging strand is being synthesized in fragments on top.

Question

Figure 1 shows some relevant details of a model of how a deoxynucleotide, in this case dTMP, is added to a growing strand of DNA

.

The figure presents a diagram of adding d T M P to a growing strand of D N A. A D N A template that consists of nine nucleotides is shown. Five nucleotides have already been added to the complementary strand that is growing from left to right along the template. The left end of the growing strand is labeled 5 prime, and the right end is labeled 3 prime. The left end of the template strand is labeled 3 prime, and the right end is labeled 5 prime. An incoming d T T P has three phosphate groups. Arrows indicate that the two outermost phosphate groups split off as the d T M P with the remaining phosphate group is about to join to the 3 prime end of the growing strand.

Figure 1. Model showing details of adding a deoxythymidine monophosphate (dTMP) nucleotide to a growing strand of DNA

The features of this model provide evidence for which explanation of why all growing strands are synthesized in a 5′ to 3′ direction?

A. The two strands need to be antiparallel to bond properly.
B. Thymine and adenine would not bond properly if the strand grew from 3′ to 5′.
C. The translation of mRNA occurs in the 5′ to 3′ direction; therefore, the growing DNA strand must also grow in the 5′ to 3′ direction.
D. The phosphate group, attached to the 5′ carbon of the dTMP, forms a covalent bond with the oxygen atom attached to the 3′ carbon of the growing strand.

▶️Answer/Explanation

Ans: D
Synthesizing a growing strand requires the interaction of the phosphate groups found at the 5′ location on the new deoxynucleotide with the OH group attached to the 3′ carbon of the existing end of the growing strand. These specific interactions and the release of diphosphate explain the directionality requirement for a growing strand of DNA.

Question

Figure 1 illustrates a model of the molecules involved in DNA replication and their placement relative to each other.

The figure presents a segment of D N A. The D N A is coiled on the left side of the figure but separated into two single strands on the right side. The right end of the upper separated strand is labeled 3 prime, and the left end of the same strand, in the coiled region, is labeled 5 prime. The right end of the lower separated strand is labeled 5 prime, and the left end of the same strand, in the coiled region, is labeled 3 prime. A circle labeled Topoisomerase is wrapped around the coiled portion of the D N A close to the single stranded region on the right. A triangle labeled Helicase is positioned in the single stranded region at the point where the coiled strands are separating. A very short segment of R N A is bound to the far right end of the upper separated single strand of D N A.

Figure 1. Model including molecules involved in DNA replication

Which of the following correctly explains where DNA replication will begin on the strand oriented 5‘→3‘, reading from left to right?

A. DNA replication will be randomly initiated along the unwound portion of the DNA strand since base pairing will occur.
B. DNA replication cannot occur since there is already RNA base pairing with the template strand.
C. DNA replication will be initiated immediately to the left of the RNA, since DNA polymerase requires an RNA primer.
D. DNA replication will be initiated at the site of the topoisomerase since that is where DNA begins to uncoil.

▶️Answer/Explanation

Ans: C
The RNA primer marks the beginning of DNA synthesis in the 5‘ to 3‘ direction. There is only one RNA primer present on the top strand in the diagram so DNA replication will begin there.

Question

N-15, also known as heavy nitrogen, is an isotope of nitrogen that is heavier than the isotope that is typically found in nature, N-14. Conducting
chemical reactions in the presence of different isotopes of nitrogen allow a scientist to follow nitrogen atoms in a metabolic pathway. In a classic experiment, Meselson and Stahl allowed parent DNA (containing N-15) to replicate in the presence of N-14.
After one round of DNA replication, which of the following results would support the statement “DNA replication is semiconservative”?
(A) All DNA molecules have one strand containing N-14 and one
strand containing N-15.
(B) 50% of the DNA molecules only contain N-14, and 50% of the
DNA molecules only contain N-15.
(C) All DNA molecules only contain N-15.
(D) All DNA molecules only contain N-14.

▶️Answer/Explanation

Ans:

(A) After one round of DNA replication, every molecule of DNA
would contain one parent strand (containing N-15) and one newly
synthesized strand (containing N-14). Choice (B) is incorrect because it
describes the expected result from conservative DNA replication, not
semiconservative replication. Choices (C) and (D) are both incorrect because after one round of DNA replication, no DNA molecules would
contain solely N-15 or N-14.

Question

N-15, also known as heavy nitrogen, is an isotope of nitrogen that is heavier than the isotope that is typically found in nature, N-14. Conducting
chemical reactions in the presence of different isotopes of nitrogen allow a scientist to follow nitrogen atoms in a metabolic pathway. In a classic experiment, Meselson and Stahl allowed parent DNA (containing N-15) to replicate in the presence of N-14.
After two rounds of DNA replication, which of the following results would support the statement “DNA replication is semiconservative”?
(A) All DNA molecules have one strand containing N-14 and one
strand containing N-15.
(B) 50% of the DNA molecules only contain N-14, and 50% of the
DNA molecules only contain N-15.
(C) All DNA molecules only contain N-15.
(D) 50% of the DNA molecules contain only N-14, and 50% of the
DNA molecules have one strand containing only N-15 and one strand containing only N-14.

▶️Answer/Explanation

Ans:

(D) As shown in Figure 15.3, after two rounds of DNA replication,
50% of the DNA molecules would contain strands with only N-14. The
other 50% of the DNA molecules would contain one strand with only
N-15 and one strand with only N-14. Choice (A) describes the result of
only one round of DNA replication if DNA replication was
semiconservative, not the result of two rounds of DNA replication. So
(A) is incorrect. Conservative DNA replication would produce the result described in choice (B), so (B) is incorrect. Choice (C) is
incorrect because it describes the result if no replication was happening
at all.

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