Each replication bubble consists of
Answer: B
Correct answer: 2 replication forks
A bubble opens at an origin and grows bidirectionally, so it has a fork at each end - two in all - moving in opposite directions. Copying from a single origin in both directions halves the time the chromosome takes to replicate.
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Which of the following enzymes is the principal replication enzyme in E. coli?
Answer: C
Correct answer: DNA polymerase III
Pol III is the replicative enzyme of E. coli: it is fast (about 1000 nucleotides a second), highly processive thanks to its sliding clamp, and proofreads as it goes. Pol I is a repair enzyme whose main replication role is removing RNA primers and filling the gaps; Pol II is chiefly for damage repair.
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The packing ratio of nucleosomes is about
Answer: B
Correct answer: 7
Wrapping about 147 base pairs of DNA - some 50 nm of contour length - around a histone octamer roughly 7 nm across compresses it by a factor of about 6-7. That is only the first level of packing; the 30 nm fibre, loops and higher-order folding together achieve some 10,000-fold compaction in a metaphase chromosome.
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In the study of one experiment it was found that the value of Tm for DNA is = 40° C. If the cell has 20% GC at the above Tm, then what will be value of 'Tm' if the GC% increases to 60%?
Answer: B
Correct answer: Increases
A G-C pair is held by three hydrogen bonds and an A-T pair by only two, so DNA richer in GC needs more heat to separate. Raising the GC content from 20% to 60% therefore raises \(T_m\). The relationship is close enough to linear to be useful: \(T_m \approx 69.3 + 0.41 \times \%GC\) for a given salt concentration.
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Enzymes, responsible for unraveling short segments of DNA is
Answer: B
Correct answer: helicase
Helicase uses ATP to break the hydrogen bonds between paired bases and travel along the duplex, unwinding it. Ligase seals nicks, primase makes primers, and polymerase copies the strands once they have been separated.
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DNA polymerase I also has
Answer: B
Correct answer: both (a) and (b)
Pol I is unusual in carrying both exonuclease activities. The 3’→5’ activity proofreads, backing up over a mispaired base; the 5’→3’ activity chews forwards, which is what removes RNA primers and makes nick translation possible. Pol II and Pol III have only the proofreading one.
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During replication, Okazaki fragments elongate
Answer: D
Correct answer: lagging strand away from the replication fork
DNA polymerase can only add nucleotides to a 3’ end, so it works 5’→3’. On the lagging strand that direction is opposite to the way the fork is moving, so each Okazaki fragment is begun near the fork and extended away from it, until it meets the fragment made before. The leading strand, by contrast, is made continuously towards the fork.
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In DNA, guanine pairs with
Answer: B
Correct answer: Cytosine
G pairs with C, held by three hydrogen bonds - one more than the A-T pair, which is why GC-rich DNA takes a higher temperature to melt. Uracil appears in RNA, where it pairs with adenine.
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One hypothesis of DNA replication suggested that the parental DNA molecules are broken into fragments. Both strands of DNA in each of the daughter molecules are made up of an assortment of parental and new DNA. This statement refers to which hypothesis?
Answer: B
Correct answer: Dispersive theory
The dispersive model held that the parental duplex is broken up and the pieces distributed, so every strand of every daughter molecule is a patchwork of old and new. Meselson and Stahl ruled it out at the second generation: dispersive replication predicts a single band of steadily decreasing density, whereas they observed two distinct bands, one hybrid and one light.
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Nucleosome is
Answer: D
Correct answer: an octamer of four histones without H1 protein
The nucleosome core particle is an octamer made of two copies each of the four core histones H2A, H2B, H3 and H4, with about 147 bp of DNA around it. H1 is not part of it: it binds outside, at the point where the DNA enters and leaves, and is called the linker histone for that reason.
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