Identify the enzymes involved in DNA replication for their function :(Enzyme)(Funtion)(P) DNA polymerase I(1) Unzipping the DNA helix(Q) Primase(2) Supercoiling of DNA(R) Helicase(3) Remove primer and close gaps(S) Gyrase(4) Synthesize RNA primer

The correct code is (P)-(3), (Q)-(4), (R)-(1), (S)-(2).DNA polymerase I removes RNA primers with its 5' to 3' exonuclease and fills the gaps with DNA (3).Primase synthesises the short RNA primers that DNA polymerase extends (4).Helicase unwinds, or unzips, the double helix at the replication fork (1).Gyrase, a type II topoisomerase, introduces negative supercoils and relieves the strain ahead of the fork (2).The other codes give helicase or gyrase the wrong role, or assign primer removal to primase.

DNA Replication Questions and Answers

Q: Which one of the following statements about the DNA polymerase III of E. coli is NOT correct?

The statement that is NOT correct is it catalyzes nick translation.Nick translation needs a 5′ → 3′ exonuclease working ahead of the polymerase, removing nucleotides from the 5′ end at a nick while new ones are added at the 3′ end. That combination belongs to DNA polymerase I, which uses it to remove RNA primers. DNA polymerase III has no 5′ → 3′ exonuclease.The other statements are true of polymerase III. It is the main replicative enzyme, so losing it is lethal. It copies a single-stranded template into a complementary strand, extending a primer. Its ε subunit carries 3′ → 5′ exonuclease activity, which proofreads misincorporated bases.

Q: Which protein helps in opening of DNA double helix in front of replication fork ?

Among the options, the protein is DNA gyrase.As helicase unwinds the double helix at the replication fork, the DNA ahead of the fork becomes overwound (positively supercoiled). DNA gyrase, a type II topoisomerase, cuts both strands, passes the DNA through and reseals it, relieving this tension so that the helix ahead of the fork can go on opening. Helicase itself is not offered.DNA ligase joins Okazaki fragments, and DNA polymerases synthesise the new strands; none of them opens the helix.

Q: The lagging strand of DNA is replicated in short pieces because :

The lagging strand is made in pieces because DNA polymerase can synthesize in one direction only.DNA polymerase adds nucleotides only to a free 3'-OH, so it synthesises new DNA in the 5' to 3' direction. The two template strands are antiparallel, so as the fork opens one new strand (the leading strand) can be extended continuously towards the fork, while the other must be made backwards, away from the fork, in short Okazaki fragments, each started from a new RNA primer and later joined by ligase.Space, distortion of the helix and ease of proof-reading are not the reason; the cause is the fixed direction of polymerase.

Q: Which of the statements regarding DNA replication is correct ?

The correct statement is that synthesis of the leading strand requires one RNA primer.Once primase lays down a single RNA primer at the origin, DNA polymerase III can extend the leading strand continuously in the 5' to 3' direction as the fork moves, so no further primers are needed. The lagging strand, by contrast, needs a new primer for every Okazaki fragment.Option A describes the lagging strand, not the leading strand. Polymerase does not cap the 5' end of DNA; capping is a modification of eukaryotic mRNA. The replicative helicase DnaB is a hexamer of six identical subunits, not five.

Q: Which one of the following enzymes can alter the linking number of covalently closed circular form of DNA ?

The enzyme is Gyrase.The linking number of a covalently closed circle can change only if a strand is cut, passed through and resealed. DNA gyrase, a type II topoisomerase, cuts both strands, passes a duplex segment through the break and reseals it, introducing negative supercoils and changing the linking number by 2 in each cycle, using ATP.Primase makes RNA primers and DNA polymerase extends DNA; neither breaks and rejoins a closed circle. Ligase seals nicks, but a covalently closed circle has none to seal.

Q: Proof reading activity of DNA Polymerase is :

Proofreading is the 3' → 5' exonuclease activity of DNA polymerase.When a wrong nucleotide is added, the mismatched 3' end does not pair properly and the primer end moves into the exonuclease site. There the enzyme removes the last nucleotide by working backwards, from the 3' end towards the 5' end, and synthesis then resumes. This improves fidelity about a hundred- to thousand-fold; in E. coli DNA polymerase III it is carried by the epsilon subunit.The 5' → 3' exonuclease of DNA polymerase I removes RNA primers ahead of the enzyme (nick translation), not errors behind it. 5' → 3' polymerase activity is the synthesis itself, and endonuclease activity cuts within a strand, which is not how proofreading works.

Q: (I) The 3' to 5' exonuclease activity of eukaryotic DNA polymerase operates in reverse direction of DNA synthesis and participate in fidelity of replication.(II) The ability of DNA polymerase to extend a primer only in 5' to 3' direction appears to make replication a complicated process, it is necessary for proofreading of duplicated DNA.Choose the correct answer :

Both statements are correct.(I) The 3' to 5' exonuclease of replicative DNA polymerases works in the direction opposite to synthesis. When a wrong base is added, the polymerase backs up, removes it and tries again. This proofreading improves fidelity by about a hundredfold.(II) Synthesis only in the 5' to 3' direction makes replication complicated, requiring Okazaki fragments on the lagging strand. But it is what allows proofreading: the energy for adding each nucleotide comes from the incoming triphosphate, so after a mismatch is removed the 3' end can still be extended. In 3' to 5' synthesis, removing a mismatch would remove the activated end. Since both statements hold, the options calling either one wrong are incorrect.

Q: Which enzyme is directly responsible for introducing negative supercoils in bacterial DNA ?

The enzyme is DNA gyrase.DNA gyrase is a bacterial type II topoisomerase. It uses ATP to cut both strands, pass a segment through the break and reseal it, which introduces negative supercoils. This relieves the positive supercoiling that builds up ahead of the replication fork. It is the target of the quinolone antibiotics, such as ciprofloxacin.Topoisomerase IV is also a type II enzyme, but its main job is to separate (decatenate) daughter chromosomes after replication, not to introduce negative supercoils. DNA polymerase I fills gaps and removes primers, and DNA ligase seals nicks.

Which one of the following statements about the DNA polymerase III of E. coli is NOT correct?

A.It catalyzes nick translation.
B.Its absence is lethal to E. coli.
C.It synthesizes a complementary DNA strand using a single-stranded template.
D.It possesses 3′ → 5′ exonuclease activity.

Exam Year(s)

GATE Life Sciences - 2022,

Answer: A

The statement that is NOT correct is it catalyzes nick translation.

Nick translation needs a 5′ → 3′ exonuclease working ahead of the polymerase, removing nucleotides from the 5′ end at a nick while new ones are added at the 3′ end. That combination belongs to DNA polymerase I, which uses it to remove RNA primers. DNA polymerase III has no 5′ → 3′ exonuclease.

The other statements are true of polymerase III. It is the main replicative enzyme, so losing it is lethal. It copies a single-stranded template into a complementary strand, extending a primer. Its ε subunit carries 3′ → 5′ exonuclease activity, which proofreads misincorporated bases.

Discuss in Forum

Which protein helps in opening of DNA double helix in front of replication fork ?

A.DNA Gyrase
B.DNA Ligase
C.DNA Polymerase
D.DNA Polymerase 1

Exam Year(s)

MPPSC Forest Service - 2022,

Answer: A

Among the options, the protein is DNA gyrase.

As helicase unwinds the double helix at the replication fork, the DNA ahead of the fork becomes overwound (positively supercoiled). DNA gyrase, a type II topoisomerase, cuts both strands, passes the DNA through and reseals it, relieving this tension so that the helix ahead of the fork can go on opening. Helicase itself is not offered.

DNA ligase joins Okazaki fragments, and DNA polymerases synthesise the new strands; none of them opens the helix.

Discuss in Forum

The lagging strand of DNA is replicated in short pieces because :

A.Of limitation of space
B.Otherwise the helix will become distorted
C.DNA polymerase can synthesize in one direction only
D.To make proof-reading of code easier

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: C

The lagging strand is made in pieces because DNA polymerase can synthesize in one direction only.

DNA polymerase adds nucleotides only to a free 3'-OH, so it synthesises new DNA in the 5' to 3' direction. The two template strands are antiparallel, so as the fork opens one new strand (the leading strand) can be extended continuously towards the fork, while the other must be made backwards, away from the fork, in short Okazaki fragments, each started from a new RNA primer and later joined by ligase.

Space, distortion of the helix and ease of proof-reading are not the reason; the cause is the fixed direction of polymerase.

Discuss in Forum

Which of the statements regarding DNA replication is correct ?

A.The leading strand is synthesized discontinuously from multiple primers.
B.The polymerase enzyme caps the 5′ end of the nascent DNA strand
C.Lagging strand helicases are composed of 5 identical sub-units
D.Synthesis of the leading strand requires one RNA primer

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: D

The correct statement is that synthesis of the leading strand requires one RNA primer.

Once primase lays down a single RNA primer at the origin, DNA polymerase III can extend the leading strand continuously in the 5' to 3' direction as the fork moves, so no further primers are needed. The lagging strand, by contrast, needs a new primer for every Okazaki fragment.

Option A describes the lagging strand, not the leading strand. Polymerase does not cap the 5' end of DNA; capping is a modification of eukaryotic mRNA. The replicative helicase DnaB is a hexamer of six identical subunits, not five.

Discuss in Forum

Which one of the following enzymes can alter the linking number of covalently closed circular form of DNA ?

A.Primase
B.DNA polymerase
C.Ligase
D.Gyrase

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: D

The enzyme is Gyrase.

The linking number of a covalently closed circle can change only if a strand is cut, passed through and resealed. DNA gyrase, a type II topoisomerase, cuts both strands, passes a duplex segment through the break and reseals it, introducing negative supercoils and changing the linking number by 2 in each cycle, using ATP.

Primase makes RNA primers and DNA polymerase extends DNA; neither breaks and rejoins a closed circle. Ligase seals nicks, but a covalently closed circle has none to seal.

Discuss in Forum

Proof reading activity of DNA Polymerase is :

A.3' → 5' exonuclease activity
B.5' → 3' exonuclease activity
C.Endonuclease activity
D.5' → 3' polymerase activity

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: A

Proofreading is the 3' → 5' exonuclease activity of DNA polymerase.

When a wrong nucleotide is added, the mismatched 3' end does not pair properly and the primer end moves into the exonuclease site. There the enzyme removes the last nucleotide by working backwards, from the 3' end towards the 5' end, and synthesis then resumes. This improves fidelity about a hundred- to thousand-fold; in E. coli DNA polymerase III it is carried by the epsilon subunit.

The 5' → 3' exonuclease of DNA polymerase I removes RNA primers ahead of the enzyme (nick translation), not errors behind it. 5' → 3' polymerase activity is the synthesis itself, and endonuclease activity cuts within a strand, which is not how proofreading works.

Discuss in Forum

Identify the enzymes involved in DNA replication for their function :

(Enzyme)	(Funtion)
(P) DNA polymerase I	(1) Unzipping the DNA helix
(Q) Primase	(2) Supercoiling of DNA
(R) Helicase	(3) Remove primer and close gaps
(S) Gyrase	(4) Synthesize RNA primer

A.(P)-(2),(Q)-(3),(R)-(4),(S)-(1)
B.(P)-(3),(Q)-(4),(R)-(1),(S)-(2)
C.(P)-(1),(Q)-(4),(R)-(2),(S)-(3)
D.(P)-(3),(Q)-(1),(R)-(2),(S)-(4)

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: B

The correct code is (P)-(3), (Q)-(4), (R)-(1), (S)-(2).

DNA polymerase I removes RNA primers with its 5' to 3' exonuclease and fills the gaps with DNA (3).
Primase synthesises the short RNA primers that DNA polymerase extends (4).
Helicase unwinds, or unzips, the double helix at the replication fork (1).
Gyrase, a type II topoisomerase, introduces negative supercoils and relieves the strain ahead of the fork (2).

The other codes give helicase or gyrase the wrong role, or assign primer removal to primase.

Discuss in Forum

(I) The 3' to 5' exonuclease activity of eukaryotic DNA polymerase operates in reverse direction of DNA synthesis and participate in fidelity of replication.
(II) The ability of DNA polymerase to extend a primer only in 5' to 3' direction appears to make replication a complicated process, it is necessary for proofreading of duplicated DNA.

Choose the correct answer :

A.(I) is correct, (II) is wrong
B.(II) is correct, (I) is wrong
C.Both (I) and (II) are correct
D.Both (I) and (II) are wrong

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: C

Both statements are correct.

(I) The 3' to 5' exonuclease of replicative DNA polymerases works in the direction opposite to synthesis. When a wrong base is added, the polymerase backs up, removes it and tries again. This proofreading improves fidelity by about a hundredfold.

(II) Synthesis only in the 5' to 3' direction makes replication complicated, requiring Okazaki fragments on the lagging strand. But it is what allows proofreading: the energy for adding each nucleotide comes from the incoming triphosphate, so after a mismatch is removed the 3' end can still be extended. In 3' to 5' synthesis, removing a mismatch would remove the activated end. Since both statements hold, the options calling either one wrong are incorrect.

Discuss in Forum

Which enzyme is directly responsible for introducing negative supercoils in bacterial DNA ?

A.DNA polymerase I
B.DNA ligase
C.Topoisomerase IV
D.DNA gyrase

Exam Year(s)

Tripura Public Service Commission - 2026,

Answer: D

The enzyme is DNA gyrase.

DNA gyrase is a bacterial type II topoisomerase. It uses ATP to cut both strands, pass a segment through the break and reseal it, which introduces negative supercoils. This relieves the positive supercoiling that builds up ahead of the replication fork. It is the target of the quinolone antibiotics, such as ciprofloxacin.

Topoisomerase IV is also a type II enzyme, but its main job is to separate (decatenate) daughter chromosomes after replication, not to introduce negative supercoils. DNA polymerase I fills gaps and removes primers, and DNA ligase seals nicks.

Discuss in Forum

Loading…

Topics

Other Subjects

Tags: DNA Replication Questions and Answers || DNA Replication MCQ Questions and Answers || DNA Replication GK Questions and Answers || DNA Replication GK MCQ Questions || DNA Replication Multiple Choice Questions and Answers || DNA Replication GK || GK on DNA Replication || Molecular Biology Questions and Answers || Molecular Biology MCQ Questions and Answers || Molecular Biology GK Questions and Answers || GK on Molecular Biology

DNA Replication Questions and Answers

Topics

Other Subjects

About Us

Stay informed about all important Dates & Notifications