DNA Replication Questions and Answers

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.

Answer: B

Correct answer: can be inhibited with antibiotics

DNA gyrase is the target of the quinolone antibiotics, such as ciprofloxacin, and of novobiocin - a useful target precisely because gyrase is bacterial and has no human counterpart. Note that gyrase introduces negative supercoils, not positive ones; it works throughout the chromosome, not only at oriC; and its job, relieving torsional strain, is quite different from the helicase’s.

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.

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.

Answer: D

Correct answer: None of the above

All three statements are correct: the strands are held together through their bases, A pairs with T, and G pairs with C. Since none of them is incorrect, the answer to “which is incorrect” is none of the above.

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.

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.

Answer: B

Correct answer: 4500 kilobase

The E. coli chromosome is about 4.6 million base pairs - roughly 4500-4600 kb - carrying some 4300 genes on a single circular molecule. For comparison, the human genome is about 3.2 billion base pairs, nearly a thousand times larger.

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.

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.