The most common liquid volumes in molecular biology are measured in
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.
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Enzyme, responsible for proofreading base pairing is
Answer: A
Correct answer: DNA polymerase
DNA polymerase proofreads with its own 3’→5’ exonuclease: a mispaired base distorts the active site, the enzyme backs up, excises the wrong nucleotide and tries again. This raises fidelity by roughly a hundredfold over base pairing alone.
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Which is the largest among the followings?
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.
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How does ethidium bromide interact with DNA?
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.
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Which of the following species lack a 'classical' histone H1?
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.
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DNA replication takes place in which direction?
Answer: B
Correct answer: 5’ to 3’
The new strand always grows 5’→3’, because polymerase can only add to a free 3’-OH. The template is read in the opposite sense, 3’→5’. This single restriction is why the lagging strand has to be built from Okazaki fragments.
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Telomeres are usually rich in which nucleotide?
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.
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DNA replication results in
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.
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Which of the following enzymes separates the two strands of DNA during replication?
Answer: C
Correct answer: Helicase
Helicase is the motor that unwinds the duplex: it moves along the DNA and breaks the hydrogen bonds between the base pairs, using ATP, so that each strand can serve as a template. Gyrase and the other topoisomerases relieve the supercoiling that unwinding creates ahead of the fork, but they do not separate the strands; DNA polymerase copies them once separated.
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Which of the following enzymes is required for connecting Okazaki fragments?
Answer: D
Correct answer: DNA ligase
DNA ligase makes the one phosphodiester bond still missing between consecutive Okazaki fragments, after the RNA primer has been removed and the gap filled in. Helicase unwinds, primase primes, and gyrase relieves supercoiling - none of them can seal a nick.
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