DNA Replication Questions and Answers

Answer: D

Correct answer: an octamer of four histones without H1 protein

The nucleosome core particle is an octamer made of two copies each of the four core histones H2A, H2B, H3 and H4, with about 147 bp of DNA around it. H1 is not part of it: it binds outside, at the point where the DNA enters and leaves, and is called the linker histone for that reason.

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: A

Correct answer: slowly, so they appear nearer the top of a gel photograph

The gel is a mesh, and a larger fragment threads through it more slowly. Since the samples are loaded at the top and run towards the anode below, the large fragments are left behind near the wells at the top, while small ones run furthest down. That is why a size ladder reads large-to-small from top to bottom.

Answer: A

Correct answer: solenoid

The solenoid is the classical model of the 30 nm chromatin fibre: nucleosomes coiled into a helix with about six per turn, held by H1. It is the second level of packing, above the 10 nm “beads on a string”.

Answer: C

Correct answer: Tm = 4 x (No.of G:C base pair) + 2 x (No.of A:T base pair)

The Wallace rule for short oligonucleotides weights each base pair by the number of hydrogen bonds it makes - three for G:C, two for A:T - giving

in degrees Celsius. The heavier weight goes to G:C, which is the trap in the alternatives. The rule is a rough guide, reliable only for probes of roughly 14-20 bases.

Answer: B

Correct answer: Dispersive theory

The dispersive model held that the parental duplex is broken up and the pieces distributed, so every strand of every daughter molecule is a patchwork of old and new. Meselson and Stahl ruled it out at the second generation: dispersive replication predicts a single band of steadily decreasing density, whereas they observed two distinct bands, one hybrid and one light.

Answer: B

Correct answer: both (a) and (b)

Pol I is unusual in carrying both exonuclease activities. The 3’→5’ activity proofreads, backing up over a mispaired base; the 5’→3’ activity chews forwards, which is what removes RNA primers and makes nick translation possible. Pol II and Pol III have only the proofreading one.

Answer: A

Correct answer: Chromosomes would become progressively shorter

The last RNA primer on the lagging strand leaves a gap at the 3’ end of the template that no polymerase can fill, so each round of replication loses a little sequence - the end-replication problem. Without telomerase the chromosome therefore shortens at every division, until the telomere is too short to protect the end and the cell stops dividing.

Answer: A

Correct answer: DnaA + ATP + HU

Melting the AT-rich 13-mer repeats at oriC is the initiation step, and it needs only three things: DnaA charged with ATP, which binds the 9-mer boxes and bends the DNA, and the histone-like HU protein, which helps that bending. DnaB, the helicase, is loaded afterwards by DnaC to extend the opening, and Pol III arrives later still.

Answer: C

Correct answer: DnaB, gyrase

DnaB is the replicative helicase: by unwinding the duplex at the fork it drives the DNA ahead of it into ever tighter positive supercoils - the overwinding. Gyrase then removes that strain, cutting both strands, passing the duplex through and resealing, so the fork can keep moving.