Semi-Conservative DNA Replication and Evidence

DNA helicase unwinds the double helix and breaks hydrogen bonds between complementary bases, separating the two strands. Free DNA nucleotides pair with exposed bases on each template: A with T and C with G. DNA polymerase catalyses joining adjacent new nucleotides through condensation, forming phosphodiester bonds.

Each resulting DNA molecule contains one strand from the original molecule and one newly synthesised strand, hence semi-conservative. The parental backbones are not broken into individual bases as a normal requirement of strand separation. Complementary pairing explains sequence continuity, although occasional replication errors can still arise.

In a conservative model, the original double-stranded molecule remains together and an entirely new double-stranded copy is produced. In a dispersive model, old and new material is mixed in segments within strands. These are different predictions about the physical distribution of original DNA, not just different names for “copying”.

In isotope-labelling experiments, bacteria grown with heavy nitrogen produce denser DNA. Moving them to light nitrogen allows newly synthesised strands to contain the lighter isotope. Density-gradient centrifugation separates DNA molecules by density; the position of a band indicates molecular density, not simply the total amount of DNA present.

After one replication in light nitrogen, semi-conservative replication predicts only intermediate-density DNA, because every molecule has one heavy and one light strand. Conservative replication would predict separate heavy and light bands, so a single intermediate band argues against that model.

After a second replication, semi-conservative replication predicts intermediate and light bands: half the molecules retain a heavy strand and half have two light strands. A dispersive model instead predicts a single band becoming progressively lighter. Thus the second-generation result helps distinguish models that the first-generation result alone cannot separate.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.