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DNA replication, step by step
How a cell copies its DNA with high accuracy before it divides.
DNA replication copies a cell's DNA before it divides. The double helix unzips, each strand acts as a template for a new one, and the result is normally two identical DNA molecules. Each has one old strand and one new strand, so replication is called semi-conservative.
Covers GCSE, AP and A-level. Exam boards differ slightly, so check your course specification.
The steps
GCSE only needs the idea: unzip, use each strand as a template, join up matching bases. A-level and AP add the enzymes.
- 01
Unwinding
HelicaseThe enzyme helicase breaks the hydrogen bonds between base pairs, unzipping the double helix at the replication fork.
- 02
Priming
PrimasePrimase adds a short RNA primer, giving DNA polymerase a place to start.
- 03
Building
DNA polymeraseDNA polymerase adds nucleotides to the new strand, matching each base to the template: A with T, and C with G.
- 04
Leading and lagging
Polymerase and ligaseDNA polymerase builds 5′ to 3′. The leading strand is made continuously towards the replication fork. The lagging strand is made in Okazaki fragments. RNA primers are removed and replaced with DNA before ligase seals the gaps.
Two identical DNA molecules
Try it: build the new strand
This is the template strand. Click the base that pairs with each highlighted base. Remember: A pairs with T, and C pairs with G.
Template strand (5′ → 3′, left to right)
Complementary strand (3′ → 5′, left to right)
Start with the first base.
Why “semi-conservative”?
After replication, each of the two new DNA molecules contains one original strand and one newly made strand. Half of the original molecule is conserved in each copy. This was shown by the Meselson and Stahl experiment in 1958, which used nitrogen isotopes to tell old DNA from new.
Because of complementary base pairing, the new strand is determined entirely by the template, so the copies normally match the original. Proofreading and repair correct most errors, but any that remain can become mutations.
Key terms
- Helicase
- Enzyme that unwinds the double helix by breaking hydrogen bonds between bases.
- DNA polymerase
- Enzyme that adds nucleotides to the growing new strand.
- Replication fork
- The Y-shaped region where the DNA is being unzipped and copied.
- Okazaki fragments
- Short pieces of DNA made on the lagging strand.
- DNA ligase
- Enzyme that joins DNA fragments together.
- Complementary base pairing
- A pairs with T, and C pairs with G (in DNA).
Common mistakes
Replication is semi-conservative. Each new DNA molecule keeps one old strand. Neither copy is fully new.
Replication is not transcription. Replication copies DNA into DNA. Transcription copies DNA into RNA.
Helicase breaks hydrogen bonds, not covalent bonds. The sugar-phosphate backbone stays intact.
Replication happens before division, in the S phase of interphase, not during mitosis itself.
A worked example
This is the kind of answer SnapBio gives you when you snap a question: the answer first, then the steps.
A DNA strand reads 5′-ATGC-3′. What is its complementary strand, aligned beneath it?
How to get there
- 1
Bases pair in a fixed way: A with T, and C with G.
- 2
Pair each base in turn: A→T, T→A, G→C, C→G.
- 3
Reading across, the aligned complementary strand is 3′-TACG-5′. DNA strands run in opposite directions.
Check yourself
Which enzyme unwinds the DNA double helix?
Helicase breaks the hydrogen bonds between base pairs and unzips the helix.
A DNA template reads 3′-ATTGC-5′. What is the complementary sequence written 5′ to 3′?
A pairs with T and C pairs with G, so ATTGC gives TAACG. (UAACG would be RNA, which uses U instead of T.)
Why is DNA replication called semi-conservative?
Each new DNA molecule conserves one of the original strands.
FAQ
When does DNA replication happen?
In the S phase of interphase, before a cell divides by mitosis or meiosis.
What enzyme unzips DNA?
Helicase. It breaks the hydrogen bonds between the base pairs.
What are Okazaki fragments?
Short pieces of DNA made on the lagging strand. DNA polymerase can only build in one direction, so the lagging strand is made in pieces that DNA ligase then joins.
How is replication different from transcription?
Replication copies DNA into DNA before cell division. Transcription copies a gene into RNA. For protein-coding genes, that RNA can be processed into mRNA for translation.
Sources and further reading
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