DNA Replication & Base Pairing — NEET Biology
📚 Practice Concept"It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material" (Watson and Crick, 1953).
Which of the following statements correctly support the semi-conservative mode of DNA replication as established by experimental evidence? S1: In Meselson and Stahl’s experiment, the appearance of DNA with intermediate density after one generation in ¹⁴N medium was key evidence. S2: The experiment by Taylor and colleagues using radioactive thymidine in Vicia faba chromosomes further confirmed semi-conservative replication in eukaryotes. S3: If DNA replication were conservative, a single band of hybrid density would be observed after two generations in a light medium. S4: The discontinuous synthesis of one of the DNA strands necessitates the action of DNA ligase, which is characteristic of semi-conservative replication. S5: The phenomenon of complementary base pairing suggests the possibility of a copying mechanism for genetic material, supporting the semi-conservative model.
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Correct answer: D — S1, S2, S4 and S5 only
The semi-conservative nature of DNA replication implies that each new DNA molecule consists of one parental strand and one newly synthesized strand. S1: This statement is correct. The observation of a single band of hybrid (intermediate) density after the first generation in the Meselson and Stahl experiment was a crucial finding that supported semi-conservative replication and ruled out the conservative model. S2: This statement is correct. Taylor's experiment, using radioactive thymidine in Vicia faba, demonstrated that DNA in eukaryotic chromosomes also replicates semi-conservatively, extending the principle beyond prokaryotes. S3: This statement is incorrect. If DNA replication were conservative, after one generation in a light medium, there would be two distinct bands: one heavy (original parental DNA) and one light (newly synthesized DNA). No hybrid density band would be observed in this model, let alone a single band of hybrid density after two generations. S4: This statement is correct. Due to the anti-parallel nature of DNA and the 5'→3' directionality of DNA polymerase, one strand (lagging strand) is synthesized discontinuously in short fragments (Okazaki fragments). These fragments are then joined by DNA ligase, a process inherent to the semi-conservative mechanism. S5: This statement is correct. Watson and Crick themselves noted that the specific complementary base pairing (A with T, G with C) immediately suggested a possible copying mechanism for the genetic material, which forms the theoretical basis of semi-conservative replication. Therefore, statements S1, S2, S4, and S5 are correct.
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