Exact copies by semi-conservative replication

Molecules (Continuity and change) · DNA replication · note 1 of 6

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Exact copies by semi-conservative replicationSpec D1.1.1, D1.1.2

In short

DNA replication is the production of exact copies of DNA with identical base sequences. It is semi-conservative: each new double helix contains one original strand and one new strand. Complementary base pairing (A with T, C with G) means each old strand acts as a template, so the base sequence is copied with a high degree of accuracy.

DNA replication is the production of exact copies of DNA with identical base sequences. It happens before a cell divides, so that each daughter cell receives a full copy of the genome.

Reproduction: DNA must be copied so that it can be passed to offspring, in asexual reproduction and in the production of gametes.
Growth: a multicellular organism grows by mitosis, and every new cell needs a complete copy of the DNA.
Tissue replacement: cells lost through damage or wear (for example skin cells or red blood cell precursors) are replaced by dividing cells, which again need copied DNA.

Semi-conservative replication

The two strands of the double helix separate and each one is used as a template for a new strand. Each of the two DNA molecules produced therefore has one strand from the original molecule (conserved) and one new strand. This is called semi-conservative replication.

Complementary base pairing and accuracy

Free DNA nucleotides are added to the new strand according to complementary base pairing: adenine pairs only with thymine (two hydrogen bonds) and cytosine pairs only with guanine (three hydrogen bonds). Because the base on the template strand decides which base is added, the new strand has exactly the sequence complementary to the template, and the new double helix is identical to the original.

A base that does not pair correctly does not fit or form the right hydrogen bonds, so mismatches are rare. This is why replication copies base sequences with a very high degree of accuracy.

Semi-conservative replication: each daughter molecule keeps one parental strand and gains one new strand.

Common mistake:

Semi-conservative does not mean half of each strand is new. Each daughter molecule has one complete old strand and one complete new strand.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

Why is DNA replication semi-conservative?

DNA replication is semi-conservative because the two strands of the original molecule separate and each acts as a template for a new strand. Each daughter molecule therefore keeps one original strand and gains one new strand. Complementary base pairing makes the new strand match the old partner, so both copies have identical base sequences.

What are the three stages of PCR?

The three stages of PCR are denaturation, annealing and extension. At about 95 °C hydrogen bonds break and the strands separate; at about 55 °C primers bind to the ends of the target sequence; at about 72 °C heat-stable Taq polymerase builds new strands. Each cycle doubles the number of copies.

How does gel electrophoresis separate DNA fragments?

Gel electrophoresis separates DNA fragments by length. DNA is negatively charged because of its phosphate groups, so when a voltage is applied it moves through the gel towards the positive electrode. Smaller fragments move through the gel mesh more easily, so they travel further than larger fragments in the same time.

All 5 questions on DNA replication