DNA Replication Step by Step (For Biology Students)
May 9, 2026 · 5 min · dna replication · biology · molecular biology
DNA replication looks like a lot of jargon: helicase, primase, polymerase, ligase. The big idea is simple — copy the genetic code, accurately.
The big idea (semi-conservative)
When DNA replicates, the two original strands separate. Each old strand becomes the template for a new partner. Result: two double helices, each with one old and one new strand.
The cast of enzymes
- Helicase unwinds the double helix at the replication fork
- Primase lays down a short RNA primer (DNA polymerase needs something to start from)
- DNA polymerase III adds new nucleotides, base-pairing with the template
- DNA polymerase I removes the RNA primers and replaces them with DNA
- Ligase seals the gaps
The leading and lagging strand
DNA polymerase only works in one direction (5' to 3'). The two template strands run in opposite directions.
- The strand running the right way is the leading strand — copied continuously
- The other one is the lagging strand — copied in short pieces called Okazaki fragments
This is why ligase is needed — to stitch the Okazaki fragments together.
Proofreading
DNA polymerase has a proofreading function. It catches and fixes most errors. Errors that get past proofreading become mutations.
What examiners ask
- Name the enzymes in order
- Why is DNA replication called semi-conservative?
- Distinguish leading vs lagging strand
- What goes wrong if ligase is faulty?
Common pitfalls
- Confusing replication with transcription
- Forgetting the RNA primer step
- Saying DNA polymerase works in both directions (it doesn't)