DNA Replication Questions and Answers

Q: The overall conclusion of the Hershey-Chase experiment was that

Correct answer: DNA was responsible for heredityHershey and Chase labelled phage protein with 35S and phage DNA with 32P, then let the phages infect bacteria and sheared off what remained outside. The 32P entered the cells while the 35S stayed outside, so it was the DNA, not the protein coat, that carried the genetic instructions. The A:T ratio result belongs to Chargaff.

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: If one cell has AT contents 40%, what will be the percentage of Guanine residue?

Correct answer: 30%By Chargaff’s rules \(A = T\) and \(G = C\), and the four bases sum to 100%. If A + T is 40%, then\[ G + C = 100\% - 40\% = 60\%, \qquad G = \frac{60\%}{2} = 30\% \]The halving is the step to be careful with: the 60% is shared equally between guanine and cytosine.

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: Which DNA polymerase removes RNA primers in DNA synthesis?

Correct answer: Polymerase IPol I alone has a 5’→3’ exonuclease, and that is what lets it chew away the RNA primer ahead of itself while its polymerase activity fills the gap behind - the process called nick translation. Pol III, which does the bulk of synthesis, has no such activity.

Q: Enzymes, responsible for unraveling short segments of DNA is

Correct answer: helicaseHelicase uses ATP to break the hydrogen bonds between paired bases and travel along the duplex, unwinding it. Ligase seals nicks, primase makes primers, and polymerase copies the strands once they have been separated.

Q: Enzyme, responsible for proofreading base pairing is

Correct answer: DNA polymeraseDNA polymerase proofreads with its own 3’→5’ exonuclease: a mispaired base distorts the active site, the enzyme backs up, excises the wrong nucleotide and tries again. This raises fidelity by roughly a hundredfold over base pairing alone.

Q: Which of the following would not contain DNA?

Correct answer: Glass crystalsYeast, bacteria and moulds are all living organisms and all carry DNA. Glass is an inorganic solid - mostly silica - with no genetic material of any kind.

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: What bands will be observed in a cesium chloride gradient after two rounds of replication?

Correct answer: One light and one medium bandStart with all-heavy DNA. After one round in light medium every molecule is hybrid, giving a single band of intermediate density. After two rounds those hybrids give one hybrid and one fully light molecule each, so the tube shows a light band and a hybrid band in equal amounts - and no heavy band at all, which is what killed the conservative model.

51.

The overall conclusion of the Hershey-Chase experiment was that

A.DNA was responsible for heredity
B.proteins and DNA were responsible for heredity
C.the ratio of Adenine to thymine was always the same
D.phage DNA was similar to bacterial DNA

Answer: A

Correct answer: DNA was responsible for heredity

Hershey and Chase labelled phage protein with ³⁵S and phage DNA with ³²P, then let the phages infect bacteria and sheared off what remained outside. The ³²P entered the cells while the ³⁵S stayed outside, so it was the DNA, not the protein coat, that carried the genetic instructions. The A:T ratio result belongs to Chargaff.

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52.

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

53.

If one cell has AT contents 40%, what will be the percentage of Guanine residue?

A.60%
B.15%
C.30%
D.Guanine residue can not be calculated

Answer: B

Correct answer: 30%

By Chargaff’s rules \(A = T\) and \(G = C\), and the four bases sum to 100%. If A + T is 40%, then

\[ G + C = 100\% - 40\% = 60\%, \qquad G = \frac{60\%}{2} = 30\% \]

The halving is the step to be careful with: the 60% is shared equally between guanine and cytosine.

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54.

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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55.

Which DNA polymerase removes RNA primers in DNA synthesis?

A.Polymerase I
B.Polymerase II
C.Polymerase III
D.none of these

Answer: A

Correct answer: Polymerase I

Pol I alone has a 5’→3’ exonuclease, and that is what lets it chew away the RNA primer ahead of itself while its polymerase activity fills the gap behind - the process called nick translation. Pol III, which does the bulk of synthesis, has no such activity.

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56.

Enzymes, responsible for unraveling short segments of DNA is

A.DNA polymerase
B.helicase
C.DNA ligase
D.primase

Answer: B

Correct answer: helicase

Helicase uses ATP to break the hydrogen bonds between paired bases and travel along the duplex, unwinding it. Ligase seals nicks, primase makes primers, and polymerase copies the strands once they have been separated.

Discuss in Forum

57.

Enzyme, responsible for proofreading base pairing is

A.DNA polymerase
B.Telomerase
C.Primase
D.DNA ligase

Answer: A

Correct answer: DNA polymerase

DNA polymerase proofreads with its own 3’→5’ exonuclease: a mispaired base distorts the active site, the enzyme backs up, excises the wrong nucleotide and tries again. This raises fidelity by roughly a hundredfold over base pairing alone.

Discuss in Forum

58.

Which of the following would not contain DNA?

A.Yeast
B.Bacteria
C.Glass crystals
D.Mold

Answer: C

Correct answer: Glass crystals

Yeast, bacteria and moulds are all living organisms and all carry DNA. Glass is an inorganic solid - mostly silica - with no genetic material of any kind.

Discuss in Forum

59.

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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60.

What bands will be observed in a cesium chloride gradient after two rounds of replication?

A.One light, one medium and one heavy band
B.One light and one medium band
C.One medium band
D.One medium and one heavy band

Answer: B

Correct answer: One light and one medium band

Start with all-heavy DNA. After one round in light medium every molecule is hybrid, giving a single band of intermediate density. After two rounds those hybrids give one hybrid and one fully light molecule each, so the tube shows a light band and a hybrid band in equal amounts - and no heavy band at all, which is what killed the conservative model.

Discuss in Forum

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