30 nm wide helix with five or six nucleosomes per helix is known as
Answer: A
Correct answer: solenoid
The solenoid is the classical model of the 30 nm chromatin fibre: nucleosomes coiled into a helix with about six per turn, held by H1. It is the second level of packing, above the 10 nm “beads on a string”.
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The base sequence of each parental strand is used as the template for synthesising a new complementary strand.
Answer: A
Correct answer: True
That is precisely what makes replication possible. Because A pairs only with T and G only with C, each parental strand specifies the sequence of its partner - so separating the two strands yields two complete templates, and copying each gives two identical duplexes.
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The higher order structure of DNA shows symmetry, whereas the higher order structures of most proteins do not. Why isn't protein shape more regular like DNA?
Answer: D
Correct answer: All of the above
All three statements bear on it. DNA is essentially one molecule doing one job - storing sequence - so a single regular structure suffices, while proteins do thousands of different jobs. And the reason they can is chemical: the twenty different R groups, some charged, some hydrophobic, some (like proline) forcing a bend, let the chain fold into an enormous variety of shapes. DNA’s four bases, all of similar shape and all paired the same way, allow no such variation.
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What is the range of melting point temperatures (Tm) for most DNA molecules?
Answer: D
Correct answer: 80 to l00°C
Most DNA melts between about 80 and 100°C under standard conditions, with the exact value set by GC content and by the salt concentration of the buffer. This is why PCR denaturation steps are run at 94-95°C.
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If one cell has AT contents 40%, what will be the percentage of Guanine residue?
Answer: B
Correct answer: 30%
By Chargaff’s rules \(A = T\) and \(G = C\), and the four bases sum to 100%. If A + T is 40%, then
\[ G + C = 100\% - 40\% = 60\%, \qquad G = \frac{60\%}{2} = 30\% \]The halving is the step to be careful with: the 60% is shared equally between guanine and cytosine.
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What is the only common methylation in the DNA of eukaryotes?
Answer: C
Correct answer: Cytosine in CpG dinucleotides
The one common modification in eukaryotic DNA is 5-methylcytosine, and it occurs almost entirely at cytosines followed immediately by a guanine - the CpG dinucleotide. Because the pattern is symmetrical on the two strands, it is copied faithfully after replication, which is what makes DNA methylation heritable and central to gene silencing and imprinting.
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Without telomerase, what would happen to the ends of chromosomes after each round of replication?
Answer: A
Correct answer: Chromosomes would become progressively shorter
The last RNA primer on the lagging strand leaves a gap at the 3’ end of the template that no polymerase can fill, so each round of replication loses a little sequence - the end-replication problem. Without telomerase the chromosome therefore shortens at every division, until the telomere is too short to protect the end and the cell stops dividing.
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The enzyme responsible for breaking the hydrogen bonds, and thus separating the DNA strands during DNA synthesis is
Answer: C
Correct answer: helicase
Helicase breaks the hydrogen bonds holding the base pairs together and travels along the duplex, separating the strands so each can be copied. It uses ATP, since the base pairs collectively hold the helix firmly. There is no enzyme called “strandase”.
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DNAs when charged, migrate in a gel towards the
Answer: A
Correct answer: positive pole
Every phosphate in the backbone carries a negative charge at the pH of the buffer, so DNA is uniformly negative regardless of sequence. In an electric field it therefore migrates towards the anode - the positive pole. The uniform charge-to-mass ratio is also why the gel separates fragments purely by size.
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DNA helicase is used to
Answer: A
Correct answer: unwind the double helix
Helicase separates the two strands by breaking the hydrogen bonds between the paired bases, using the energy of ATP. It does not touch the covalent phosphodiester backbone - cutting that is the work of nucleases and topoisomerases.
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