DNA Replication

Q1: The synthesis of DNA by DNA polymerase occurs in the

A 3' to 5' direction

B 5' to 5' direction

C 5' to 3' direction

D 3' to 3' direction

ANS:C - 5' to 3' direction

This is because the DNA polymerase can add a nucleotide to the chain to the 3rd carbon of deoxyribose sugar. The nucleotide added to the strand in the polymerization is in the form of deoxynucleotide triphosphate.

This bond that is made between deoxyribose sugar and the nucleotide triphosphate is known as a phosphodiester bond.

Q2: The replication of chromosomes by eukaryotes occurs in a relatively short period of time because

A the eukaryotes have more amount of DNA for replication

B the eukaryotic replication machinery is 1000 times faster than the prokaryotes

C each chromosome contains multiple replicons

D eukaryotic DNA is always single stranded

ANS:C - each chromosome contains multiple replicons

Eukaryotes have multiple origins of replication in the chromosome.

Q3: Which of the following lacks 3'-5' exonuclease activity?

A Short fragment of DNA polymerase I

B Taq DNA polymerase

C T4 DNA ligase

D All of the above

ANS:D - All of the above

Both DNA polymerase 1 and T4 DNA ligase possess 3'-5' exonuclease activityi .e, proofreading activity. So the correct answer is B.

Q4: The enzyme that catalyzes the synthesis of DNA is called

A DNA polymerase

B DNA gyrase

C DNA ligase

D helicase

ANS:A - DNA polymerase

Thanks for explaining.

Q5: DNA gyrase is inhibited by

A tetracycline

B nalidixic acid

C both (a) and (b)

D cephalosporin

ANS:B - nalidixic acid

It's a quinoline derivatives which is DNA gyrase inhibitor or other fluoroquinolone inhibitor.

Q6: Which of the following repairs nicked DNA by forming a phosphodiester bond between adjacent nucleotides?

A Helicase

B DNA gyrase

C Topoisomerases

D DNA ligase

ANS:D - DNA ligase

DNA ligase helps in the formation of a phosphodiester bond between adjacent 3' hydroxyl and 5' phosphate termini in DNA.

Q7: During which of the following process a new copy of a DNA molecule is precisely synthesized?

A Trasformation

B Transcription

C Translation

D Replication

ANS:D - Replication

During replication the genetic information in the form of nitrogen bases gets replicated, thus making a copy of another DNA molecule.

Q8: Proofreading activity to maintain the fidelity of DNA synthesis

A occurs after the synthesis has been completed

B is a function of the 3'-5' exonuclease activity of the DNA polymerases

C requires the presence of an enzyme separate from the DNA polymerases

D occurs in prokaryotes but not eukaryotes

ANS:B - is a function of the 3'-5' exonuclease activity of the DNA polymerases

It means that this process occurs after the formation of the replication fork.

Q9: Which of the following statements on replication in E.coli is correct?

A It occurs in a unidirectional manner

B It occurs in bidirectional manner

C Always uses T7 DNA polymerase when infected by T7 phage

D Occurs only when λ phase has infected E. coli

ANS:B - It occurs in bidirectional manner

It is because the DNA is antiparallel. It can be synthesised both in 3'-5' direction and the 5'-3' direction by the DNA polymerase enzyme.

Q10: Both strands of DNA serve as templates concurrently in

A replication

B excision repair

C mismatch repair

D none of these

ANS:A - replication

Both DNA strand particate during DNA replication because the leading strand is replicated continuosly and lagging strand also at the same time being replicated discontinuosly.

Q11: During DNA replication in prokaryotes, synthesis begins on the circular chromosome

A always at the same place

B at any stretch of DNA that is high in AT pairs

C randomly on the chromosome

D at the promoter

ANS:A - always at the same place

Yes, it always start at origin of replication.

Q12: Which of the following enzymes unwind short stretches of DNA helix immediately ahead of a replication fork?

A DNA polymerases

B Helicases

C Single-stranded binding proteins

D Topoisomerases

ANS:B - Helicases

Why not Topoisomerases?

Please explain.

Q13: DNA replication rates in prokaryotes are approximately of the order of

A 10 bases per second

B 1,00 bases per second

C 1,000 bases per second

D 10,000 bases per second

ANS:C - 1,000 bases per second

No answer description is available.

Q14: In E.coli, which enzyme synthesizes the RNA primer for Okazaki fragments?

A DnaA

B DnaC

C DnaG

D all of these

ANS:C - DnaG

All known DNA polymerases require a pre-existing "primer" from which to begin replication. E. Coli uses an RNA primer which is synthesized by an enzyme called DnaG (primase). DnaG makes RNA primers (about 10 nucleotides long) that are used by DNA pol III holoenzyme to start DNA synthesis. DnaG acts distributively (does not remain associated with DNA). It drops off DNA after primer synthesis, then reloads onto DNA a second or so later by protein-protein interactions with DnaB (helicase) to synthesize the next primer on the lagging strand. There are many places along the template where RNA primers can be made by primase.

Q15: Which of the following possesses both 5'-3' and 3'-5' exonuclease activity?

A Kornberg enzyme

B DNA polymerase III

C Taq DNA polymerase

D None of these

ANS:A - Kornberg enzyme

DNA pol 1.

5' to 3' exonuclease --> removal of primer.
5' to 3' polymerization ----> add nucleotides in place of primer(removed).
3' to 5'exonculease----> proofreading of this newly synthesized segment.

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