Q: Which equation best represents the directionality of DNA synthesis, and what is the implication for primer requirement?

$$\text{DNA synthesis:}~5' \rightarrow 3'$$; DNA polymerase requires a primer with a free 3' OH$

Question 1

Which experiment provided definitive evidence for the semiconservative model of DNA replication, and what was the key result?

Accepted Answer

Meselson-Stahl experiment; after one round of replication, DNA had intermediate density

Question 2

During DNA replication, which enzyme is responsible for relieving supercoiling ahead of the replication fork, and what is the mechanism?

Accepted Answer

Topoisomerase; creates transient breaks in the DNA backbone to allow rotation

Question 3

A patient presents with symptoms of premature aging and is found to lack the WRN protein. Which cellular process is most directly affected, and what is the consequence?

Accepted Answer

Telomere capping; increased chromosome end degradation

Question 4

In eukaryotic cells, DNA replication initiates at multiple origins. What is the term for the DNA region replicated from a single origin, and how does this relate to genome size?

Accepted Answer

Replicon; larger genomes require more replicons for efficient replication

Question 5

Which DNA polymerase is responsible for mitochondrial DNA replication in eukaryotes, and what is its unique feature?

Accepted Answer

DNA polymerase γ; exclusively replicates mitochondrial DNA

Question 6

A cell exposed to UV light develops thymine dimers. Which repair mechanism is most likely to correct this damage, and what is the key enzyme involved?

Accepted Answer

Nucleotide excision repair; excinuclease and helicase

Question 7

During lagging strand synthesis, what is the sequence of events for primer removal and fragment joining in E. coli?

Accepted Answer

Primase synthesizes RNA primer → DNA polymerase III extends fragment → DNA polymerase I removes primer via 5'→3' exonuclease → DNA ligase joins fragments

Question 8

Which equation best represents the directionality of DNA synthesis, and what is the implication for primer requirement?

Accepted Answer

$$	ext{DNA synthesis:}~5' ightarrow 3'$$; DNA polymerase requires a primer with a free 3' OH$

Question 9

A cell line is able to divide indefinitely in culture. Which molecular feature is most likely responsible for this, and what is the associated risk?

Accepted Answer

Active telomerase; increased risk of cancer due to bypassing the Hayflick limit

Question 10

Which repair pathway is most likely to introduce mutations during the repair of double-strand breaks, and why?

Accepted Answer

Nonhomologous end joining (NHEJ); nucleotides can be lost and ends joined inaccurately

Question 11

A mutation in the p53 gene would most likely result in which cellular outcome after DNA damage?

Accepted Answer

Failure to halt cell cycle, leading to propagation of damaged DNA and increased cancer risk

Question 12

Which equation describes the energy source for DNA polymerization, and what is the chemical basis?

Accepted Answer

$$	ext{dNTP} + (DNA)_{n} ightarrow (DNA)_{n+1} + PP_i$$; hydrolysis of dNTP triphosphate drives the reaction

Question 13

Which repair mechanism is most likely to correct a deaminated cytosine (converted to uracil) in DNA, and what is the key enzyme?

Accepted Answer

Base excision repair; uracil DNA glycosylase

Question 14

A cell exposed to proflavin, an intercalating agent, is most likely to experience which type of DNA mutation?

Accepted Answer

Insertions or deletions due to distortion of the DNA helix

Question 15

Which cell cycle phase is most variable in duration among different adult cell types, and what is the biological significance?

Accepted Answer

G1 phase; allows cells to respond to growth signals and environmental cues before DNA replication

Question 16

A cell with a defect in mismatch repair (MMR) is most likely to accumulate which type of mutation, and what is the clinical consequence?

Accepted Answer

Replication errors (base mismatches); increased risk of Lynch syndrome (hereditary colorectal cancer)

Question 17

Which proteins are directly involved in recognizing and repairing mismatched bases in E. coli, and what is the mechanism for strand discrimination?

Accepted Answer

MutS and MutL; recognize mismatches and use DNA methylation to identify the newly synthesized strand

Question 18

A patient with Huntington's disease has an expanded CAG repeat in their genome. What is the molecular mechanism underlying this expansion?

Accepted Answer

Strand slippage during DNA replication in regions of trinucleotide repeats

Question 19

Which checkpoint proteins cooperate to regulate cell cycle progression and prevent propagation of damaged DNA, and what is the consequence of their inactivation?

Accepted Answer

Rb and p53; loss leads to uncontrolled cell division and cancer

DNA Replication, Repair, and Cell Cycle Regulation: Study Notes

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