DNA Replication Questions and Answers

Q: Replication in E. coli is initiated by the generation of short RNA primers using

Correct answer: both (a) and (b)Primase (the DnaG protein) makes the primers at the fork, once on the leading strand and once for each Okazaki fragment. RNA polymerase also has a part at initiation: its transcription across the origin region helps open the duplex, and in some replicons it makes the primer itself. So both enzymes are involved in generating the RNA that starts replication.

Q: The number of times the two strands of a circular DNA molecule are intertwined is known as

Correct answer: linking numberThe linking number, \(Lk\), is the number of times one strand crosses the other in a closed circular molecule. Because the circle is closed, \(Lk\) is a topological invariant - it cannot change without breaking a strand, which is exactly what topoisomerases do. It divides into twist and writhe: \(Lk = Tw + Wr\).

Q: What is the approximate size (in kb) of the E. coli genome?

Correct answer: 4500 kilobaseThe E. coli chromosome is about 4.6 million base pairs - roughly 4500-4600 kb - carrying some 4300 genes on a single circular molecule. For comparison, the human genome is about 3.2 billion base pairs, nearly a thousand times larger.

Q: Okazaki fragments occur during

Correct answer: replicationOkazaki fragments are the short stretches in which the lagging strand is built during replication. They exist because polymerase can only work 5’→3’, which on that strand is away from the advancing fork.

Q: Which of the following proteins is not involved in the initiation of replication?

Correct answer: DnaFThere is no DnaF protein. Initiation at oriC involves DnaA, which recognises the origin and melts the AT-rich repeats; DnaB, the helicase, delivered by DnaC; and the single-strand binding protein, which keeps the opened strands apart.

Q: Which of the following enzyme adds complementary bases during replication?

Correct answer: PolymeraseDNA polymerase selects the nucleotide that pairs with the template base and joins it to the 3’ end of the growing chain. Helicase only separates the strands; “synthesase” and “replicase” are not names of enzymes in this pathway.

Q: The nucleosome consists of histone

Correct answer: octamer and 146 bp of DNAA nucleosome core is an octamer - two copies each of H2A, H2B, H3 and H4 - with about 146-147 base pairs of DNA wound around it in roughly 1.65 turns. The linker histone H1 binds outside the core and is not counted as part of it.

Q: A cesium chloride will separate DNA molecules by

Correct answer: densityCaesium chloride spun at high speed forms a density gradient, and each DNA molecule settles where the solution’s density equals its own. This is what let Meselson and Stahl distinguish 15N-heavy, hybrid and light DNA, and what separates plasmid from chromosomal DNA in a classical preparation.

Q: Which of the following is used in DNA replication studies?

Correct answer: Escherichia coliE. coli has been the workhorse of replication studies: it grows fast, has a single circular chromosome with one origin, and is easy to label isotopically - which is exactly what Meselson and Stahl exploited in 1958 to demonstrate semi-conservative replication.

51.

Replication in E. coli is initiated by the generation of short RNA primers using

A.RNA polymerase
B.primase
C.reverse transcriptase
D.both (a) and (b)

Answer: D

Correct answer: both (a) and (b)

Primase (the DnaG protein) makes the primers at the fork, once on the leading strand and once for each Okazaki fragment. RNA polymerase also has a part at initiation: its transcription across the origin region helps open the duplex, and in some replicons it makes the primer itself. So both enzymes are involved in generating the RNA that starts replication.

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52.

The number of times the two strands of a circular DNA molecule are intertwined is known as

A.helix frequency
B.linking number
C.twisting number
D.none of these

Answer: B

Correct answer: linking number

The linking number, \(Lk\), is the number of times one strand crosses the other in a closed circular molecule. Because the circle is closed, \(Lk\) is a topological invariant - it cannot change without breaking a strand, which is exactly what topoisomerases do. It divides into twist and writhe: \(Lk = Tw + Wr\).

Discuss in Forum

53.

What is the approximate size (in kb) of the E. coli genome?

A.3000 kilobase
B.4500 kilobase
C.5500 kilobase
D.6500 kilobase

Answer: B

Correct answer: 4500 kilobase

The E. coli chromosome is about 4.6 million base pairs - roughly 4500-4600 kb - carrying some 4300 genes on a single circular molecule. For comparison, the human genome is about 3.2 billion base pairs, nearly a thousand times larger.

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54.

Taylor, Woods and Hughes labeled Vicia DNA by allowing new DNA synthesis in the presence of radioactive thymine. After DNA replication (S phase of the cell cycle), it was observed that

A.only one chromatid of a chromosome was labeled
B.both chromatids of a chromosome were labeled
C.both (a) and (b)
D.neither chromatid was labeled

Answer: B

Correct answer: both chromatids of a chromosome were labeled

After one round of replication in labelled thymine, every DNA molecule has one old (unlabelled) and one new (labelled) strand - so both sister chromatids carry label. That is exactly what semi-conservative replication predicts; a conservative mechanism would have left one chromatid entirely unlabelled. The asymmetry appears only in the second round.

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55.

Okazaki fragments occur during

A.transformation
B.replication
C.polymerase reaction
D.synthesis

Answer: B

Correct answer: replication

Okazaki fragments are the short stretches in which the lagging strand is built during replication. They exist because polymerase can only work 5’→3’, which on that strand is away from the advancing fork.

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56.

Which of the following proteins is not involved in the initiation of replication?

A.DnaA
B.SSB (Single strand binding protein)
C.DnaB
D.DnaF

Answer: D

Correct answer: DnaF

There is no DnaF protein. Initiation at oriC involves DnaA, which recognises the origin and melts the AT-rich repeats; DnaB, the helicase, delivered by DnaC; and the single-strand binding protein, which keeps the opened strands apart.

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57.

Which of the following enzyme adds complementary bases during replication?

A.Helicase
B.Synthesase
C.Replicase
D.Polymerase

Answer: D

Correct answer: Polymerase

DNA polymerase selects the nucleotide that pairs with the template base and joins it to the 3’ end of the growing chain. Helicase only separates the strands; “synthesase” and “replicase” are not names of enzymes in this pathway.

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58.

The nucleosome consists of histone

A.octamer and 146 bp of DNA
B.tetramer and 146 bp of DNA
C.hexamer and 146 bp of DNA
D.none of the above

Answer: A

Correct answer: octamer and 146 bp of DNA

A nucleosome core is an octamer - two copies each of H2A, H2B, H3 and H4 - with about 146-147 base pairs of DNA wound around it in roughly 1.65 turns. The linker histone H1 binds outside the core and is not counted as part of it.

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59.

A cesium chloride will separate DNA molecules by

A.absorption
B.resorption
C.density
D.adhesion

Answer: C

Correct answer: density

Caesium chloride spun at high speed forms a density gradient, and each DNA molecule settles where the solution’s density equals its own. This is what let Meselson and Stahl distinguish ¹⁵N-heavy, hybrid and light DNA, and what separates plasmid from chromosomal DNA in a classical preparation.

Discuss in Forum

60.

Which of the following is used in DNA replication studies?

A.Neurospora crassa
B.Drosophila melanogaster
C.Escherichia coli
D.Pneumococcus

Answer: C

Correct answer: Escherichia coli

E. coli has been the workhorse of replication studies: it grows fast, has a single circular chromosome with one origin, and is easy to label isotopically - which is exactly what Meselson and Stahl exploited in 1958 to demonstrate semi-conservative replication.

Discuss in Forum

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