DNA Replication Questions and Answers

Answer: C

Correct answer: It intercalates between bases

Ethidium bromide is a flat aromatic cation that slides between adjacent base pairs, into the stack. Its fluorescence rises sharply once it is in that hydrophobic environment, which is what makes DNA bands glow under UV. Intercalation also unwinds and lengthens the helix slightly - and, by distorting it, makes the dye a mutagen, which is why it is handled with care.

Answer: B

Correct answer: linking number

The linking number, \(Lk\), is the number of times one strand crosses the other in a closed circular molecule. Because the circle is closed, \(Lk\) is a topological invariant - it cannot change without breaking a strand, which is exactly what topoisomerases do. It divides into twist and writhe: \(Lk = Tw + Wr\).

Answer: C

Correct answer: helicase

Helicase breaks the hydrogen bonds holding the base pairs together and travels along the duplex, separating the strands so each can be copied. It uses ATP, since the base pairs collectively hold the helix firmly. There is no enzyme called “strandase”.

Answer: A

Correct answer: Saccharomyces cerevisiae

Budding yeast has no classical linker histone. Its nucleosomes are packed with unusually short linkers and its genome is small, so it manages without one - it carries only the distantly related Hho1p, which does not behave like H1. Sea urchin, Xenopus and Arabidopsis all have conventional H1 variants.

Answer: D

Correct answer: All of the above

All three describe bacterial replication. It begins at a single origin (oriC); it proceeds bidirectionally, with two forks setting out in opposite directions and meeting at the terminus; and it is semi-conservative, each daughter chromosome keeping one parental strand.

Answer: D

Correct answer: comparison with a molecular weight standard such as Hind III digested phage lambda

A gel gives relative mobility, not an absolute size, so the fragment is sized by running a ladder of known fragments alongside it - lambda DNA cut with HindIII is the classic one - and reading the unknown against them. Since mobility varies roughly with the logarithm of length, plotting log(size) against distance gives a usable calibration curve.

Answer: B

Correct answer: Okazaki fragments

Okazaki fragments are the short pieces from which the lagging strand is assembled. Because polymerase can only work 5’→3’, on that strand it must synthesise away from the advancing fork, so it works in short bursts, each begun near the fork and extended backwards until it meets the previous fragment. Ligase then joins them.

Answer: A

Correct answer: Bleomycin

Bleomycin binds DNA and, with an iron ion and oxygen, generates hydroxyl radicals at the site, cutting the backbone - which is why it is used as an anticancer drug. Erythromycin blocks bacterial protein synthesis at the ribosome, and tunicamycin blocks N-linked glycosylation; neither touches DNA.

Answer: D

Correct answer: both (a) and (b)

Primase (the DnaG protein) makes the primers at the fork, once on the leading strand and once for each Okazaki fragment. RNA polymerase also has a part at initiation: its transcription across the origin region helps open the duplex, and in some replicons it makes the primer itself. So both enzymes are involved in generating the RNA that starts replication.

Answer: B

Correct answer: two DNA molecules such that each one contains a strand of the original

This is what semi-conservative means: the parental strands separate, each templates a new partner, and the two products each consist of one old strand and one new one. Neither daughter is wholly new, and neither is wholly old.