DNA Replication Questions and Answers

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: Tus protein

Tus binds the ter sites and blocks the replication fork from one direction only, arresting the DnaB helicase. The ter sites are arranged so that the two forks travelling round the circle are caught in the terminus region opposite the origin.

Answer: C

Correct answer: Watson and Crick

James Watson and Francis Crick published the double-helix model in 1953, building on Rosalind Franklin’s and Maurice Wilkins’s X-ray diffraction data and on Chargaff’s base ratios. Khorana and Nirenberg deciphered the genetic code a decade later; Darwin’s work long predates any knowledge of DNA.

Answer: D

Correct answer: all of these

Sanger sequencing needs all four normal deoxyribonucleoside triphosphates - dATP, dGTP, dCTP and dTTP - so that the chain can be extended, plus a small proportion of dideoxynucleotides to terminate it at random positions. Leaving any of the four out would stop synthesis wherever that base was required.

Answer: D

Correct answer: All of the above

All three describe bacterial replication. It begins at a single origin (oriC); it proceeds bidirectionally, with two forks setting out in opposite directions and meeting at the terminus; and it is semi-conservative, each daughter chromosome keeping one parental strand.

Answer: B

Correct answer: 20 to 30%

On the classical partition of the human genome by reassociation kinetics - the \( C_0 t \) experiments that first revealed the structure - DNA sorts into three fractions. About 10% is highly repetitive satellite DNA, short sequences present in hundreds of thousands of copies, concentrated at centromeres and telomeres. Roughly 20 to 30% is moderately repetitive: the interspersed elements such as Alu and LINE-1 sequences, together with the tandem repeats of the ribosomal RNA, transfer RNA and histone genes. The remaining 60 to 70% is unique or near-unique sequence, which is where the protein-coding genes lie. It is the moderately repetitive fraction the question is asking about, and the answer is 20 to 30%. Modern sequence-based estimates count considerably more repetition than the reassociation experiments could detect - when every degraded transposable element is included, at least half the genome is repetitive in origin, and some analyses put it at two thirds. The figure keyed here is the textbook one and reflects the older method.

Answer: A

Correct answer: DNA polymerase

DNA polymerase is the elongating enzyme: it adds nucleotides one at a time to the 3’ end of a primed strand, choosing each to pair with the template. Helicase unwinds, primase starts the chain, and ligase seals nicks - none of them extends it.

Answer: A

Correct answer: protamines

In maturing sperm the histones are replaced by protamines - very small, extremely arginine-rich proteins that pack the DNA far more tightly than nucleosomes can, into an almost crystalline state. The result is a compact, transcriptionally silent nucleus that protects the paternal genome in transit. Purines and pyrimidines are classes of base, not proteins.

Answer: C

Correct answer: Semi-conservative

Each daughter molecule keeps one parental strand and one newly made strand. Meselson and Stahl proved it in 1958 by growing E. coli in heavy ¹⁵N and shifting it to ¹⁴N: after one generation all the DNA was of intermediate density, ruling out the conservative model, and after two it was half intermediate and half light, ruling out the dispersive one.

Answer: B

Correct answer: hydrogen bonds between the pair of bases

The two strands are held together by hydrogen bonds between the paired bases - two between A and T, three between G and C - helped by base stacking. Individually these bonds are weak, which is what allows helicase to separate the strands; collectively, over thousands of pairs, they hold the duplex firmly. The covalent bonds run along each backbone, not across.