Which of the following would not contain DNA?
Answer: C
Correct answer: Glass crystals
Yeast, bacteria and moulds are all living organisms and all carry DNA. Glass is an inorganic solid - mostly silica - with no genetic material of any kind.
Please log in or register to report this question.
Klenow fragment without free nucleotides exhibits
Answer: A
Correct answer: exonuclease activity
The Klenow fragment is Pol I with the 5’→3’ exonuclease domain removed, so it retains polymerase and 3’→5’ exonuclease activity. Without free dNTPs it cannot polymerise, so only the exonuclease is left, and it will chew back a 3’ end. This is exactly how Klenow is used to blunt a 3’ overhang.
Please log in or register to report this question.
Association of histones H1 with nucleosome shows
Answer: D
Correct answer: the condensation of DNA into chromatin fibre
H1 is the linker histone. It binds where the DNA enters and leaves the nucleosome core, seals the two turns in place, and draws neighbouring nucleosomes together - folding the “beads on a string” into the compact 30 nm chromatin fibre. Its presence therefore marks condensation, not the opening that transcription or replication requires.
Please log in or register to report this question.
Which of the following are short nucleotide strands made by DNA polymerase working in the opposite direction of the replication fork?
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.
Please log in or register to report this question.
In DNA, there are
Answer: B
Correct answer: four bases known as adenine, guanine, thymine and cytosine
DNA uses four bases: the purines adenine and guanine, and the pyrimidines thymine and cytosine. RNA replaces thymine with uracil. Tryptophan is an amino acid, not a base.
Please log in or register to report this question.
How many replicons are found in E. coli?
Answer: C
Correct answer: Single replicon
A replicon is the stretch of DNA served by one origin. The E. coli chromosome is a single circle with a single origin, oriC, so the whole chromosome is one replicon - two forks set out from oriC in opposite directions and meet at the terminus. Eukaryotic chromosomes, being far longer, need many origins and so have many replicons.
Please log in or register to report this question.
Which of the following enzymes remove supercoiling in replicating DNA ahead of the replication fork?
Answer: D
Correct answer: Topoisomerases
Unwinding at the fork drives the DNA ahead of it into positive supercoils. Topoisomerases relieve that strain by cutting one strand (type I) or both (type II), letting the DNA turn or pass through, and resealing. Helicase creates the problem rather than solving it; polymerases and primases play no part in it.
Please log in or register to report this question.
The rapid appearance of HIV-1 strains that are resistant to AIDS drugs is due in part to which of the following property of its reverse transcriptase?
Answer: C
Correct answer: It lacks a proofreading exonuclease
HIV-1 reverse transcriptase has no 3’→5’ exonuclease, so a misincorporated base is never corrected. The resulting error rate - roughly one per genome per replication cycle - means an infected person carries a swarm of variants, and any drug-resistant one already present is simply selected for as soon as the drug is applied. Combination therapy is the answer, since resistance to three drugs at once is far less likely.
Please log in or register to report this question.
The accepted hypothesis for DNA replication is
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.
Please log in or register to report this question.
Which of the following would relax negatively supercoiled DNA?
Answer: B
Correct answer: Type I topoisomerase
Type I topoisomerase nicks one strand, lets the DNA swivel to release one supercoil, and reseals - needing no ATP, because relaxing a supercoil is energetically downhill. DNA gyrase does the opposite: it uses ATP to introduce negative supercoils. Collagen and elastin are structural proteins with no role here.
Please log in or register to report this question.
