Question 1

Which experiment provided the first strong evidence that DNA is the genetic material, and what was the key result?

Accepted Answer

Griffith's experiment, which showed that heat-killed S strain bacteria could transform live R strain bacteria, causing disease in mice.

Question 2

In the Hershey-Chase experiment, what was the main finding and how did it disprove competing hypotheses?

Accepted Answer

DNA, not protein, is the genetic material because only radioactive phosphorus (in DNA) entered the bacterial cells.

Question 3

Which bonds are responsible for holding the two strands of DNA together, and what is their purpose?

Accepted Answer

Hydrogen bonds; they allow complementary base pairing and easy strand separation during replication.

Question 4

What is the replication rate in nucleotides per second for E. coli, and how does it compare to Homo sapiens?

Accepted Answer

E. coli: $$\sim1000 nt/sec$$; Homo sapiens: $$\sim50 nt/sec$$; E. coli replicates DNA much faster than humans.

Question 5

Given the human genome size of $$3 	imes 10^9$ base pairs, why are multiple replication origins necessary?

Accepted Answer

Multiple origins allow the entire genome to be replicated quickly within the limited time of S phase.

Question 6

Which enzyme is responsible for synthesizing RNA primers during DNA replication, and what would happen if it were absent?

Accepted Answer

Primase; without it, DNA polymerase could not initiate synthesis, and replication would not start.

Question 7

Contrast the synthesis of the leading and lagging strands during DNA replication.

Accepted Answer

The leading strand is synthesized continuously in the 5' to 3' direction, while the lagging strand is synthesized discontinuously as Okazaki fragments.

Question 8

What feature of DNA replication was proven by the Meselson and Stahl experiment?

Accepted Answer

DNA replication is semi-conservative, with each daughter molecule containing one old and one new strand.

Question 9

During DNA replication, which enzyme relieves supercoiling ahead of the replication fork?

Accepted Answer

DNA gyrase (topoisomerase); it cuts and reseals DNA to relieve tension.

Question 10

If a cell is labeled with radioactive thymidine and then undergoes two rounds of mitosis, what will be the distribution of labeled DNA strands in the chromatids?

Accepted Answer

Each chromatid will contain either one labeled and one unlabeled strand, or both unlabeled strands, depending on the segregation during replication.

Question 11

Which statement best describes the role of DNA ligase during replication?

Accepted Answer

DNA ligase seals the nicks between Okazaki fragments by forming phosphodiester bonds.

Question 12

What is the function of single-stranded binding proteins during DNA replication?

Accepted Answer

They stabilize unwound DNA and prevent it from re-annealing.

Question 13

Which of the following correctly shows the complementary strand for the DNA sequence 5'-TGAGGATCGGGGTTAGTCTTAGACTCCCTTATGCGAARACTT-3'?

Accepted Answer

3'-ACTCCTAGCCCCAATCAGAATCTGAGGGAATACGCTTTTGAA-5'

Question 14

What would happen if DNA polymerase I was nonfunctional during replication?

Accepted Answer

RNA primers would not be removed and replaced with DNA, resulting in incomplete lagging strand synthesis.

Question 15

Why does the lagging strand require multiple RNA primers during DNA replication?

Accepted Answer

Because it is synthesized discontinuously in short fragments (Okazaki fragments) as the replication fork opens.

Question 16

Which statement about DNA packaging is supported by advances in microscopy and technology?

Accepted Answer

DNA is compacted in multiple layers, with nucleosomes forming coils and higher-order structures.

Question 17

Which process is responsible for the shortening of linear chromosomes during successive rounds of replication, and why?

Accepted Answer

The inability of DNA polymerase to fully replicate the 3' ends of linear DNA, leading to telomere shortening.

Question 18

In a pulse-chase experiment with $$[^{3}H]$$thymidine, what does the presence of both labeled and unlabeled DNA strands in chromatids after two cell divisions indicate?

Accepted Answer

Semi-conservative replication, where each new DNA molecule contains one old (labeled) and one new (unlabeled) strand.

Question 19

Which enzyme is responsible for unwinding the DNA double helix at the replication fork?

Accepted Answer

Helicase

DNA Structure, Replication, and Packaging: Study Notes for Cell Biology

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