DNA Replication Questions and Answers

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.

Answer: C

Correct answer: Polymerase III

Pol III is the replicative polymerase of bacteria - fast, highly processive, and equipped with proofreading. Pol I is mainly a repair and primer-removal enzyme; Pol II is chiefly for damage bypass and repair.

Answer: C

Correct answer: Thousands and tens of thousands

A mammalian genome is roughly a thousand times the size of a bacterial one, and a single fork would take weeks to copy it. Eukaryotes therefore fire tens of thousands of origins, spaced along every chromosome, so that many replicons are copied at once and an S phase takes hours rather than weeks.

Answer: A

Correct answer: using enzyme called telomerase

Removing the last RNA primer on the lagging strand leaves a gap that no polymerase can fill, so a linear chromosome would shorten at each division. Telomerase solves it: the enzyme carries its own RNA template and extends the 3’ end with repeats, giving the lagging-strand machinery somewhere to prime. Cells without active telomerase do shorten their telomeres, which is one limit on how often they can divide.

Answer: B

Correct answer: 3’ to 5’ exonuclease activity

Proofreading means backing up over the base just added, so the enzyme must chew in the direction opposite to synthesis - 3’→5’. When a mispaired base distorts the geometry at the active site, the mismatched nucleotide is moved into the exonuclease site, excised, and replaced. This raises the fidelity of replication by roughly a hundredfold. (Pol I’s separate 5’→3’ exonuclease is for removing RNA primers, not proofreading.)

Answer: A

Correct answer: Histone H1

H1 is the linker histone: it binds where the DNA enters and leaves the core particle, together with the linker DNA between nucleosomes, sealing the two turns in place and helping fold the fibre into its 30 nm form. The other four are the core histones of the octamer.

Answer: B

Correct answer: can be inhibited with antibiotics

DNA gyrase is the target of the quinolone antibiotics, such as ciprofloxacin, and of novobiocin - a useful target precisely because gyrase is bacterial and has no human counterpart. Note that gyrase introduces negative supercoils, not positive ones; it works throughout the chromosome, not only at oriC; and its job, relieving torsional strain, is quite different from the helicase’s.

Answer: C

Correct answer: Formation of a phosphodiester bond between the 3’-OH of one Okazaki fragment and the 5’-phosphate of the next on the lagging strand

Ligase does not extend a strand; it seals a nick. After the RNA primer of each Okazaki fragment has been removed and the gap filled by polymerase, one phosphodiester bond is still missing, and ligase makes it - using ATP (or NAD⁺ in bacteria) to activate the 5’-phosphate.

Answer: D

Correct answer: Semiconservative

E. coli replicates semi-conservatively: each daughter chromosome carries one parental strand and one new one. This is the organism in which Meselson and Stahl established it, by following the density of ¹⁵N-labelled DNA through successive generations in ¹⁴N medium.

Answer: C

Correct answer: DNA gyrase

Unwinding the helix at the fork forces the DNA ahead of it into ever tighter positive supercoils, which would soon stall the fork. DNA gyrase, a type II topoisomerase unique to bacteria, cuts both strands, passes the duplex through, and reseals - introducing negative supercoils and so relieving the strain. Being bacteria-specific makes it a good antibiotic target.