DNA can be sequenced by the
Answer: C
Correct answer: both (a) and (b)
Two classical methods: the Maxam-Gilbert chemical method, which cleaves end-labelled DNA at specific bases, and Sanger’s chain-termination method, which uses dideoxynucleotides to stop synthesis at each position. Sanger sequencing proved far easier to automate and became the standard.
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What is the IUPAC code for an A, C, T ambiguity in a nucleic acid sequence?
Answer: B
Correct answer: H
The IUPAC ambiguity codes for three bases are named by which base is excluded, and they run B, D, H, V after A, C, G, T: B is not A, D is not C, H is not G, V is not T. So A, C or T - everything but G - is H. (S is a two-base code, for C or G.)
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The 3’ - 5’ phosphodiester linkage joins
Answer: B
Correct answer: two nucleotides
A phosphodiester bond joins the 3’-OH of one nucleotide to the 5’-phosphate of the next, and it is what makes the sugar-phosphate backbone of a single strand. The two strands are held to each other by hydrogen bonds between the bases, not by phosphodiester bonds; and a base joined to a sugar makes a nucleoside, through a glycosidic bond.
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The chromosomal DNA complexes with
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.
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E.coli DNA polymerases II and III lack
Answer: A
Correct answer: 5' → 3' exonuclease activity
Only Pol I carries a 5’→3’ exonuclease, which is what lets it remove RNA primers. Pol II and Pol III lack it; they have the 3’→5’ exonuclease used for proofreading, but nothing that can chew forwards.
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Which of these drugs binds to DNA and cuts it, producing hydroxyl radicals?
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.
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What is the main damaging effect of UV radiation on DNA?
Answer: B
Correct answer: Formation of thymine dimers
Ultraviolet light is absorbed by the bases and makes adjacent pyrimidines - most often two thymines - join through a cyclobutane ring. The dimer kinks the helix and blocks both replication and transcription until it is excised. Depurination, by contrast, happens spontaneously through hydrolysis rather than through UV.
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Semi-conservative DNA replication was first demonstrated in ____________
Answer: A
Correct answer: Escherichia coli
Meselson and Stahl demonstrated semi-conservative replication in E. coli in 1958, using density-gradient centrifugation of DNA labelled with 15N. Taylor, Woods and Hughes had shown the same thing in the broad bean Vicia faba using autoradiography, but the E. coli experiment is the one usually called the demonstration.
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Overwinding or overtightening of DNA during replication is caused and removed by
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.
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Eukaryotes differ from prokaryote in mechanism of DNA replication due to ____________
Answer: C
Correct answer: Different enzyme for synthesis of lagging and leading strand
In eukaryotes the two strands are made by different polymerases - Pol ε on the leading strand and Pol δ on the lagging strand, with Pol α/primase laying down the primers. In E. coli a single Pol III holoenzyme handles both. The other options are false: eukaryotes also use RNA primers, and their replication is semi-discontinuous and bidirectional, just as in bacteria.
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