DNA Replication Questions and Answers

Answer: C

Correct answer: semi-conservative theory

The semi-conservative model is the one the evidence supports: each daughter duplex keeps one parental strand. Meselson and Stahl’s density experiment excluded the conservative model at the first generation and the dispersive model at the second.

Answer: B

Correct answer: Cytosine

Base pairing is fixed by hydrogen bonding: G pairs with C through three hydrogen bonds, and A with T through two. So a guanine on the template directs the insertion of a cytosine. Uracil belongs to RNA, not DNA.

Answer: B

Correct answer: Guanine

Telomeric repeats are G-rich on the strand running 5’→3’ towards the chromosome end - TTAGGG in vertebrates. The guanine runs let the single-stranded overhang fold into a G-quadruplex, which is part of how the end is protected from being read as a double-strand break.

Answer: A

Correct answer: Nucleotide

A nucleotide contains all the others: a nitrogenous base joined to a pentose sugar, with one or more phosphate groups attached - and carbon is a single atom within them. Removing the phosphate leaves a nucleoside; removing the sugar as well leaves the base.

Answer: B

Correct answer: five types of histone as H1, H2A, H2B, H3 and H4

There are five. Two copies each of H2A, H2B, H3 and H4 form the octamer core around which about 147 base pairs of DNA are wound, and H1 is the linker histone that binds outside the core and packs the nucleosomes together.

Answer: B

Correct answer: both chromatids of a chromosome were labeled

After one round of replication in labelled thymine, every DNA molecule has one old (unlabelled) and one new (labelled) strand - so both sister chromatids carry label. That is exactly what semi-conservative replication predicts; a conservative mechanism would have left one chromatid entirely unlabelled. The asymmetry appears only in the second round.

Answer: B

Correct answer: hydrogen bonds between the pair of bases

The two strands are held together by hydrogen bonds between the paired bases - two between A and T, three between G and C - helped by base stacking. Individually these bonds are weak, which is what allows helicase to separate the strands; collectively, over thousands of pairs, they hold the duplex firmly. The covalent bonds run along each backbone, not across.

Answer: B

Correct answer: carbon number in sugar

The primes refer to the carbon atoms of the deoxyribose, numbered 1’ to 5’ to distinguish them from the numbering of the base. The phosphate joins the 5’ carbon of one sugar to the 3’ carbon of the next, which is what gives each strand a direction.

Answer: B

Correct answer: semi-conservative replication

Meselson and Stahl grew E. coli in ¹⁵N, shifted it to ¹⁴N, and separated the DNA by density. After one generation all the DNA was of intermediate density - which no conservative mechanism could produce - and after two it was half intermediate and half light, which the dispersive model could not produce. Only semi-conservative replication fits both results.

Answer: B

Correct answer: μl

Molecular biology works with tiny volumes, and the microlitre is the everyday unit - a PCR reaction is typically 20-50 μl, and micropipettes are calibrated in it. Millilitres are for buffers and media; nanolitres are below what a hand pipette can deliver reliably.