DNA Replication: GCSE Biology Guide

Why must DNA replicate before cell division?

Every time a cell divides, each daughter cell needs a full, working copy of the genome — all the genes that code for the proteins the cell needs to survive and function. Without replication, each division would halve the DNA content: after a few rounds, cells would have too little DNA to function at all.

Replication takes place in the nucleus during interphase — the period of growth and preparation before mitosis begins. The cell must replicate all 46 chromosomes completely before a single chromosome separates.

What is the structure of DNA that makes replication possible?

DNA is a double helix: two polynucleotide strands wound around each other. Each strand is made up of nucleotides, and each nucleotide contains:

• A deoxyribose sugar
• A phosphate group
• One of four bases: adenine (A), thymine (T), cytosine (C) or guanine (G)

The two strands are held together by hydrogen bonds between complementary base pairs:

This strict complementary base pairing — A with T, C with G — is what makes accurate replication possible: each original strand acts as a template for a new, complementary strand.

How does DNA replication work?

1. Unwinding. The enzyme helicase breaks the hydrogen bonds between complementary base pairs and unzips the double helix, producing two separate template strands.
2. Base pairing. Free nucleotides (present in the nucleus) align alongside each template strand according to complementary base pairing rules: A pairs with T, C with G.
3. Joining. The enzyme DNA polymerase joins the new nucleotides together, forming a new complementary strand alongside each original strand.
4. Two double helices formed. The result is two complete DNA molecules, each consisting of one original (parent) strand and one newly synthesised strand.

Each new DNA molecule is identical to the original because the base sequence of the new strand is determined entirely by the base sequence of the template strand.

What does "semi-conservative" mean?

The term semi-conservative describes the fact that each new DNA molecule is made up of:

• One conserved strand — the original parent strand (retained, not destroyed)
• One new strand — synthesised from free nucleotides

So after replication, each of the two new DNA molecules is "half old, half new". This was confirmed in 1958 by the Meselson–Stahl experiment, which used nitrogen isotopes to show that replication is neither conservative (both strands old) nor dispersive (all strands mixed), but strictly semi-conservative.

Why does accurate replication matter?

Any error in base pairing during replication is a mutation — a change to the DNA base sequence. Most mutations are caught and corrected by proofreading enzymes that travel along the new strand checking for mismatched bases and replacing them.

Uncorrected mutations can:

• Have no effect (if they fall outside a gene or the changed codon codes for the same amino acid)
• Alter the protein coded by the gene (possibly changing its shape and function)
• Cause uncontrolled cell division if the mutation affects genes controlling the cell cycle, potentially leading to cancer

The mutation rate in human DNA replication is approximately 1 error per billion base pairs copied — an extraordinary level of accuracy that relies on DNA polymerase's proofreading activity.

How is replication different from transcription?

Students often confuse the two processes:

Frequently asked questions

Why is semi-conservative replication better than making a completely new copy?

Using one original strand as a template means the new strand's sequence is directly checked against the parent — any nucleotide that does not fit its template position is unlikely to be inserted. A completely new copy (conservative replication) would need a separate mechanism to verify accuracy. Semi-conservative replication also means that after millions of cell divisions, each daughter cell still contains at least one strand that traces back to the original DNA.

What happens if DNA polymerase makes an error?

DNA polymerase proofreads as it works, removing and replacing incorrectly paired nucleotides. The raw error rate before proofreading is roughly 1 in 10,000 bases; after proofreading and repair, it drops to approximately 1 in 10⁹ to 10¹⁰. Errors that are not corrected become permanent mutations in daughter cells. Most are harmless, but rare mutations in proto-oncogenes or tumour-suppressor genes can lead to cancer.

Does replication happen for every chromosome simultaneously?

Yes — in human cells, all 46 chromosomes replicate during the same interphase. Replication does not start at one end of each chromosome and work along it; instead, replication begins at thousands of specific sites called origins of replication across each chromosome and proceeds simultaneously in both directions from each origin, greatly speeding up the process. The entire human genome (about 3 billion base pairs) is replicated in roughly 6–8 hours.

How is DNA replication related to cancer?

Mutations in genes that normally control cell division — proto-oncogenes (which promote division) and tumour-suppressor genes (which restrict it) — can cause cells to divide uncontrollably. These mutations can arise from errors during DNA replication, from damage by mutagens (UV radiation, certain chemicals) or spontaneously. Cancer is essentially a disease of uncontrolled mitosis driven by accumulated DNA damage, which is why anything that increases the mutation rate (smoking, excess UV exposure) raises cancer risk.

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