Replication
▶︎ Watch it animatedinteractive step-through · ~3 min · optionalBefore a cell divides, it has to hand each daughter a complete, faithful copy of its genome — and Topic 6.2 is where you learn how that copy is made. DNA replication is semiconservative: the double helix unwinds, and each of the two parent strands serves as a template for a new complementary strand. The result is two daughter molecules, and every one of them keeps one original strand paired with one newly synthesized strand. Nothing is copied from scratch and nothing is copied whole — the old is conserved inside the new, which is exactly what "semiconservative" names.
The chemistry sets a hard constraint: DNA polymerase can only add nucleotides to the 3′ end, so it builds every new strand in the 5′→3′ direction. Because the two template strands run antiparallel, that single rule splits the work at the replication fork. One new strand — the leading strand — is synthesized continuously, chasing the fork as it opens. The other — the lagging strand — must be built away from the fork in short Okazaki fragments that are later stitched together. Same enzyme, same 5′→3′ rule, but two very different-looking strands.
Unwind the helix and watch replication run at the fork: each parent strand templates a new complement, the leading strand extends continuously toward the fork, and the lagging strand fills in as Okazaki fragments — every new nucleotide added 5′→3′.
Replication · Open the full sandbox →The mistakes here cluster around three failure modes. One is getting semiconservative replication wrong — picturing daughter molecules as two all-old and two all-new strands, or the copy built from scratch, instead of one old strand paired with one new in every product. The second is ignoring the 5′→3′ rule — imagining polymerase adding to the 5′ end or building a strand in whichever direction is convenient. The third is blurring the leading and lagging strands — forgetting that antiparallel templates force one strand to run continuously and the other to be assembled in Okazaki fragments. Every scenario in this topic asks you to keep the copy semiconservative, the direction 5′→3′, and the two new strands distinct.
The work
3 ways in · any order
Lesson
Replication
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DNA replication is semiconservative — each parent strand templates a new complement, so every daughter keeps one old strand and one new — and because polymerase only builds 5′→3′, the leading strand runs continuously while the lagging strand is stitched from Okazaki fragments. The lesson walks the ways students misread that: treating the copy as all-new or all-old, adding to the wrong end, and blurring the two new strands. It closes with a ten-scenario applet that asks you to keep the copy semiconservative, the direction right, and the strands distinct.
Diagnostic
10-item topic check
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Ten items on DNA replication — that replication is semiconservative, so each daughter molecule keeps one parent strand and one new strand rather than being all-old or all-new (U6-BIO5); that polymerase only extends 5′→3′, adding to the 3′ end (U6-BIO1); and that antiparallel templates split the fork into a continuous leading strand and a lagging strand built from Okazaki fragments (U6-BIO4). Take it cold to surface which of these are still tangled, or after the lesson to confirm they hold.
Targeted Practice
Drill a single misconception
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Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears it for now and moves you to the next. Two in a row is a checkpoint, not proof: if the error resurfaces later, the misconception comes back.