DNA replication: copying genetic information — Biology, 14–17
Before a cell divides, it must make a reliable copy of its DNA. Replication explains how a long molecule can be copied, checked and passed to two new cells.
A copy made from two old strands
DNA replication starts when the two strands unzip, rather like opening a zip. Each old strand guides the building of a new partner strand, so every new DNA molecule has one old half and one new half. This helps cells pass the same instructions on when they divide.
Why cells need a copy
A dividing cell produces two daughter cells, but its DNA cannot simply be split in half: each daughter needs the complete set of instructions. Replication solves this supply problem before division. It also creates a chance to check copying errors, because a wrong base can change a later protein.
Following one short sequence
Take one DNA strand with the sequence A–T–G–C. During copying, A pairs with T, T with A, G with C and C with G. The new partner strand is therefore T–A–C–G. The original strand and this new strand then belong to one finished DNA molecule, while the other old strand guides a second copy.
“The cell copies both strands from scratch”
It is reasonable to picture two brand-new DNA molecules being built independently, because the result looks like two complete copies. But the old strands are not discarded: each one acts as a template. This matters when you track mutations, because a copying error first appears in one newly built strand.
Copying in medicine and forensics
DNA replication is part of what lets a wound heal: cells around it divide and give each new cell a genome. Laboratories also exploit copying systems when they make many DNA molecules for testing. The copies do not prove a person caused an event by themselves, but they can provide useful evidence when handled carefully.
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