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Before a cell divides, how does it make a perfect copy of all its DNA?

Describe how DNA is replicated, including the role of the template strand and complementary base pairing

A focused answer to the O-Level outcome on DNA replication. Unwinding the helix, using each strand as a template, complementary base pairing, and why replication is accurate.

Generated by Claude Opus 4.88 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking

This outcome asks you to describe how DNA is copied so that, when a cell divides, each new cell gets a complete and accurate set of instructions. The whole process rests on the base-pairing rule you met in the structure of DNA: it is what makes copying both possible and reliable.

The answer

Why DNA must be copied

Before a cell divides, it must make a complete copy of all its DNA so that each new cell receives a full set of genetic instructions. This copying is called DNA replication.

The steps of replication

Replication can be described in a few clear steps:

  1. Unwinding. The double helix unwinds and the two strands separate as the hydrogen bonds between the bases break.
  2. Templating. Each separated strand acts as a template, a pattern for building a new partner strand.
  3. Base pairing. Free nucleotides in the cell line up against each template by complementary base pairing: A with T and C with G.
  4. Joining. An enzyme (DNA polymerase) joins the lined-up nucleotides into a continuous new strand.

Two identical molecules

The result is two DNA molecules, each made of one original (template) strand and one newly built strand. Because the pairing rule is fixed, each new molecule is identical to the original.

Why it is accurate

The base-pairing rule means only the correct nucleotide can fit at each position on the template. This makes replication highly accurate, so genetic information is passed on faithfully with very few errors.

Examples in context

Example 1. Before cell division. Every time a body cell divides to grow or repair tissue, it first replicates all its DNA so each daughter cell gets a complete copy. Accurate replication is therefore the basis of healthy growth.

Example 2. The principle behind PCR. The polymerase chain reaction, used everywhere in biotechnology, is replication carried out in a tube: strands are separated, primers and free nucleotides pair against templates, and a polymerase builds new strands. Understanding natural replication makes PCR easy to follow.

Try this

Q1. State what happens to the two strands of DNA at the start of replication. [1 mark]

  • Cue. The double helix unwinds and the strands separate as the hydrogen bonds between the bases break.

Q2. Explain the role of each separated strand during replication. [2 marks]

  • Cue. Each acts as a template against which free nucleotides line up by complementary base pairing to build a new strand.

Q3. Describe the make-up of each DNA molecule produced by replication. [2 marks]

  • Cue. Each molecule contains one original (template) strand and one newly built strand, and is identical to the original.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original6 marksDescribe the process of DNA replication, explaining how two identical DNA molecules are produced from one.
Show worked answer →

Examiners want the ordered process and the reason it produces identical copies.

First, the two strands of the DNA double helix unwind and separate as the hydrogen bonds between the bases break. Each separated strand acts as a template.

Free nucleotides in the cell line up against each template by complementary base pairing: adenine with thymine and cytosine with guanine. An enzyme joins these nucleotides together to form a new strand alongside each template.

Because each new molecule has one original strand and one new strand, and the pairing rule is fixed, the two resulting DNA molecules are identical to the original. This is why DNA can be passed on accurately when a cell divides.

What markers reward: unwinding and separation of the strands, each strand acting as a template, complementary base pairing of free nucleotides, joining by an enzyme, and the point that each new molecule has one old and one new strand, giving two identical copies.

Original3 marksExplain why complementary base pairing makes DNA replication accurate.
Show worked answer →

The answer should connect the fixed pairing rule to faithful copying.

Complementary base pairing means each base can only pair with one specific partner: adenine with thymine and cytosine with guanine. So when free nucleotides line up against a template strand, only the correct base can fit at each position.

This ensures the new strand is an exact complement of the template, and therefore the new DNA molecule is identical to the original, with very few errors.

What markers reward: the idea that each base pairs with only one partner, so only the correct nucleotide fits at each position on the template, producing an exact and accurate copy.

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