DNA replication copies a cell's entire genome before it divides, and it runs into a genuinely awkward structural problem while doing it: DNA's two strands run in opposite chemical directions, antiparallel, while the enzyme that actually builds new DNA, DNA polymerase, can only construct a new strand in one specific direction. The result is that one new strand gets built smoothly and continuously following the enzyme's natural direction, while the other has to be built backward in short, separate fragments that get stitched together afterward.
One strand copies continuously, the other copies in short, separate pieces
Because DNA polymerase can only synthesise a new strand in the five-prime-to-three-prime direction, the strand running in a direction that matches this, the leading strand, gets copied in one smooth, continuous motion as the replication machinery moves along. The other strand, the lagging strand, runs the wrong way for continuous synthesis, so it instead gets copied in short separate fragments, Okazaki fragments, each one synthesised in the correct direction over a short stretch before the enzyme has to jump back and start the next fragment, with a separate enzyme, DNA ligase, later joining all those fragments into one continuous strand.
Built-in proofreading keeps replication's error rate remarkably low despite its scale
DNA polymerase carries its own built-in proofreading ability, checking each newly added base against its template immediately and correcting most mismatches on the spot during copying itself, and any errors that slip past that immediate proofreading get caught by separate repair systems working afterward. That layered error-catching is exactly why the final mutation rate per base pair copied stays remarkably low, given that a human cell's replication machinery copies roughly three billion base pairs essentially every time it divides.
What we're still unsure about
That DNA's antiparallel structure forces leading and lagging strand synthesis to work differently, and that built-in proofreading keeps replication's error rate low, are well established, extensively confirmed molecular biology. What's genuinely still an active research question is that replication isn't perfectly error-free even with proofreading and later repair working together, and exactly how much of a given cell type's ongoing mutation rate reflects irreducible chemical limits on how well proofreading can ever work, versus how much reflects genuinely avoidable gaps in a specific repair pathway, remains actively studied, and matters directly for understanding how cancer-causing mutations first arise.
This sits inside DNA Replication, one of eight topics in Genetics, one of six domains in Biology, one of seventeen subjects the app can quiz you on.