Q.Which property of DNA double helix led Watson and Crick to hypothesise semi-conservative mode of DNA replication? Explain.

Watson and Crick observed that the two strands of DNA are complementary to each other due to specific base pairing. Adenine always pairs with thymine, and guanine always pairs with cytosine.

This property implies that if the sequence of bases on one strand is known, the sequence on the other strand is automatically determined. They hypothesised that during DNA replication, the two strands would separate, and each parental strand would then serve as a template for the synthesis of a new, complementary strand. This mechanism ensures that each new DNA molecule formed would contain one original (parental) strand and one newly synthesised strand, which is the essence of the semi-conservative …

When James Watson and Francis Crick elucidated the double helical structure of DNA in 1953, their model not only explained how genetic information could be stored but also provided a profound insight into how this information might be accurately copied. The crucial property of the DNA double helix that led them to hypothesise the semi-conservative mode of replication was complementary base pairing.

Let's unpack this. The DNA molecule, as proposed by Watson and Crick, consists of two polynucleotide strands coiled around each other to form a double helix. The backbone of each strand is made of alternating sugar (deoxyribose) and phosphate groups. The nitrogenous bases project inwards from this backbone, forming the "rungs" of the ladder.

The key to their insight lay in the specific way these nitrogenous bases pair up across the two strands:

• Adenine (A) always pairs with Thymine (T).
• Guanine (G) always pairs with Cytosine (C).

These specific pairings are maintained by hydrogen bonds – two between A and T, and three between G and C. This strict pairing rule means that the sequence of bases on one strand dictates the sequence on the other. The two strands are, therefore, complementary to each other.

Watson and Crick immediately recognised the profound implication of this complementarity for DNA replication. They reasoned that if the two strands of the double helix could somehow separate or "unzip," each individual strand would then expose its sequence of bases. Because of the complementary pairing rule, each separated strand could then act as a template or a mould. New nucleotides, floating freely in the cell, would come and align themselves with their complementary bases on the exposed template strand – A with T, and G with C. …

Approach it from the experimenter's point of view rather than the structural one: ask what a scientist would need to be true for a 'copying' mechanism to work at all. Complementary base pairing is exactly that requirement -- it is the only property that lets one separated strand fully sp …