Past exam of the biochemistry course of the University of Cambridge / 2024 / Paper 5 / Section 1 / a / Solution

The viral DNA is copied by semiconservative replication and its labelling kinetics broadly follow those of host DNA during viral latency. At the earliest time the newly labelled material is mainly hybrid: an old light strand has acquired a heavy complementary strand. Fully heavy duplexes accumulate later, after heavy strands themselves serve as templates. Conservative copying would instead produce a separate fully heavy population from the first copying round, while dispersive copying would produce progressively shifting intermediate densities rather than these three discrete classes.

For an ideal synchronous population starting entirely light, one complete round gives only hybrid duplexes; after $n\ge 1$ rounds the hybrid fraction is $2^{1-n}$ and the fully heavy fraction is $1-2^{1-n}$. The actual culture is asynchronous, so these integer-generation proportions cannot simply be imposed at $t/23$ hours. The similar viral and cellular bars support replication coordinated with cell proliferation rather than rapid autonomous amplification. They do not establish that every episome initiates exactly once in every individual biological cell, or prove an initiation time within S phase. Persistent light material also warns against treating the population as a perfectly uniform set of cycling biological cells. The strand-accounting principle is the one tested in ^[ref].