Which enzyme class joins DNA fragments during replication?

Ligases, which seal the breaks in the sugar-phosphate backbone. They are necessary because DNA replication is fundamentally lopsided: the two strands run in opposite directions, and the copying machinery can only work one way, so one strand is copied continuously and the other in short backward pieces that then have to be stitched together.

Those pieces are named after Reiji and Tsuneko Okazaki, a married pair of researchers in Nagoya who worked out the mechanism in the 1960s. Reiji died at 44 of leukaemia, attributed to the radiation he was exposed to as a child in Hiroshima; Tsuneko completed the work, raised two children, and spent the rest of her career fighting the Japanese academic system's assumption that a widow with a family was finished.

Ligase turned out to be an industrial tool. Cutting DNA with restriction enzymes and rejoining it with ligase is the whole of basic genetic engineering - the operation that first produced recombinant DNA in 1973 and that makes insulin production possible - and a heat-stable version underpins one of the standard tests for single-letter genetic differences. An enzyme whose job is tidying up after a design flaw became the glue for the entire field.