DNA Base Pairing & Semi-Conservative Replication — NEET Biology

Correct answer: D — S1, S4 and S5 are correct

Let's evaluate each statement: S1: Correct. James Watson and Francis Crick proposed the Double Helix model for DNA based on X-ray diffraction data provided by Maurice Wilkins and Rosalind Franklin. S2: Incorrect. Friedrich Meischer identified DNA as ‘Nuclein’ in the nucleus in 1869. However, its role as the genetic material, including the transforming principle, was elucidated much later by Griffith (discovery of transformation) and then by Avery, MacLeod, and McCarty (biochemical nature of transforming principle). S3: Incorrect. Hershey and Chase used radioactive phosphorus (32P) to label DNA and radioactive sulfur (35S) to label proteins. They observed that radioactive phosphorus (DNA) entered the bacterial cells, indicating DNA as the genetic material. If radioactive sulfur (35S) was found inside the bacterial cells, it would have implied that proteins, not DNA, were the genetic material. S4: Correct. Meselson and Stahl's experiment demonstrated the semi-conservative replication of DNA using the heavy isotope of nitrogen (15N), which could be distinguished from normal nitrogen (14N) by density gradient centrifugation. S5: Correct. Avery, MacLeod, and McCarty purified DNA, RNA, and proteins from heat-killed S cells. They found that only DNA was capable of transforming live R cells into S cells, thereby concluding that DNA is the transforming principle. Therefore, statements S1, S4, and S5 are correct. The core concept tested here is the historical progression of understanding DNA as the genetic material and its structure, along with key experimental details.