A double helix unzips at a travelling replication fork: the two parent strands peel apart into a Y, and behind the fork new complementary strands are synthesized — the leading strand continuous, the lagging strand in dashed Okazaki fragments — with colour-coded A-T / G-C base pairs zipping shut into each daughter duplex; the fork sweeps the molecule on a loop, the pointer scrubs it, scroll sets the winding and a press magnifies the base detail.