DNA Replication and the Cell Cycle Flashcards
(8 cards)
How do events in DNA replication correspond to stages of the cell cycle?
G₁ phase: Cell grows; replication machinery components (e.g., DNA polymerases, helicase) are synthesized.
S phase: DNA is actively replicated—origins fire, replication forks progress.
G₂ phase: DNA replication is complete; cell checks for damage and prepares for mitosis.
M phase: No new replication; sister chromatids (duplicated during S) are segregated into daughter cells.
How does the quantity of DNA in a cell change as it progresses through the cell cycle?
G₁: 1× haploid (or 2× diploid) amount of DNA.
S phase: DNA content gradually increases from 1× to 2× (diploid cells go from 2C to 4C).
G₂: DNA content is fully doubled (2× haploid or 4× diploid).
M phase (post‐cytokinesis): Each daughter cell returns to the G₁ amount (1× haploid or 2× diploid).
What is the difference between the leading and lagging strands during DNA replication?
Leading strand: Synthesized continuously in the 5′→3′ direction toward the replication fork, using a single RNA primer.
Lagging strand: Synthesized in short 5′→3′ Okazaki fragments away from the fork; each fragment requires its own RNA primer, then fragments are joined by DNA ligase.
How would altering components of the replication machinery affect DNA replication?
Helicase defect: Forks cannot unwind DNA → replication stalls.
DNA polymerase (proofreading) mutation: Increased replication errors → higher mutation rate.
Primase inhibition: Lagging‐strand synthesis fails (no primers), and leading‐strand initiation is delayed.
Ligase deficiency: Okazaki fragments on lagging strand remain unjoined → chromosome breaks or incompletely replicated DNA.
How do changes in telomerase activity affect cell division?
High telomerase activity (e.g., stem cells, cancer cells): Telomeres are maintained → cells divide indefinitely.
Low/no telomerase activity (e.g., most somatic cells): Telomeres shorten each division → eventual senescence or apoptosis when telomeres become critically short.
How can “heavy” and “light” nitrogen isotopes distinguish between models of DNA replication?
Conservative model prediction: After first replication in ^14N medium, one DNA molecule should be all ^15N and one all ^14N.
Semiconservative model prediction: After one generation in ^14N, all DNA molecules are hybrid (^15N/^14N). After a second, half are hybrid and half are light (^14N/^14N).
Dispersive model prediction: All DNA strands would be mixed ^15N/^14N in both generations without discrete hybrid vs. light bands.
How do you interpret centrifugation data to evaluate DNA replication models?
One band at hybrid density after 1st replication, two bands (hybrid + light) after 2nd: Consistent with semiconservative.
One band at original (heavy) + one at light after 1st: Would indicate conservative (not observed).
Only hybrid bands over multiple generations: Would indicate dispersive (not observed).
What was the 2013 Supreme Court decision regarding Myriad Genetics’ BRCA1 gene testing, and what were the main arguments for and against?
Decision: Naturally occurring DNA sequences (BRCA1/BRCA2 genes) cannot be patented; however, complementary DNA (cDNA) is patent‐eligible.
For patenting (Myriad’s arguments): Claimed that isolating the gene made it a “new and useful composition of matter,” encouraging innovation.
Against patenting (petitioners’ arguments): Naturally occurring genes are products of nature and thus not inventions; patents restricted patient access and research on BRCA testing.
Impact: Opened the market to other diagnostic labs, lowered testing costs, and spurred research.
