Q.Replication was allowed to take place in the presence of radioactive deoxynucleotide precursors in an E. coli strain that carried a mutation making its DNA ligase non-functional. The newly synthesised radioactive DNA was purified, its two strands were separated by denaturation, and the single strands were resolved by density-gradient centrifugation. In the results below, radioactivity (y-axis) is plotted against a molecular-weight axis that runs from high molecular weight on the left to low molecular weight on the right. Which result would be correct?

The leading strand is synthesised continuously and is very long (high molecular weight). The lagging strand is normally made as short Okazaki fragments that ligase seals together; in a ligase mutant they remain unjoined and short (low molecular weight). After denaturation and density-gradient centrifugation the radioactivity therefore appears in both a high-molecular-weight …

Concept

A replication fork copies the two antiparallel template strands differently because DNA polymerase can add nucleotides only in the 5'→3' direction:

• Leading strand — template read 3'→5', so synthesis runs continuously toward the fork as a single, ever-lengthening molecule. It ends up long / high molecular weight.
• Lagging strand — template runs the "wrong" way, so it is made in short pieces (Okazaki fragments) away from the fork. Each fragment is short / low molecular weight. Normally DNA ligase seals the nicks between successive fragments to build one continuous strand.

Why this experiment separates them

Density-gradient centrifugation separates molecules by size/molecular weight, and the strands were first separated by denaturation so each single strand migrates according to its own length. The radioactivity marks only the newly made DNA.

Steps

1. In the ligase mutant, the nicks between Okazaki fragments on the lagging strand are never sealed → the lagging-strand product stays as a pool of short fragments.
2. The leading strand needs no ligation to be continuous → it is still one long molecule.
3. So the radioactive new DNA is a mixture of two size classes: high MW (leading strand) and low MW (Okazaki fragments). …

Work the options backwards by asking what each one silently assumes about DNA synthesis: options (B) and (C) assume there is only ONE population of new DNA, which would make ligase's absence invisible -- but that contradicts the very existence of separate leading and lagging strands; option (D) assumes nothing escapes as one continuous long molecule, ignoring that the leading strand needs no ligase at all. Only option (A) is consistent with two genuinely different synthesis modes oper …