A sample from a crime scene, a cheek swab or a drop of blood usually holds only a tiny amount of DNA — far too little to test.
PCR solves this: PCR copies the chosen DNA over and over. Each round of copying doubles the amount, so after about 30 rounds there are millions of copies.
PCR — three steps, repeated in cycles: Each PCR cycle repeats the same three steps, and the temperature is changed to control each one.
One PCR cycle
Denaturation (~95 °C)
The heat splits the DNA double helix into two single strands.
Annealing (~55 °C)
Short primers bind to each strand, marking where copying starts.
Extension (~72 °C)
Taq DNA polymerase adds nucleotides to build a new complementary strand.
Repeating the cycle doubles the DNA each time — 1 → 2 → 4 → 8 … → millions.
| Step of a PCR cycle | Temperature | What happens |
|---|---|---|
| Denaturation | ~95 °C (hot) | Heat breaks the hydrogen bonds, so the double helix separates into two single strands |
| Annealing | ~55 °C (cooler) | Short primers bind (anneal) to the start of each single strand, marking where copying begins |
| Extension | ~72 °C | Taq DNA polymerase adds free nucleotides to each primer, building a new complementary strand |
Why a special polymerase?: The denaturation step reaches ~95 °C, which would destroy a normal enzyme.
PCR uses Taq polymerase, a heat-stable enzyme from a bacterium that lives in hot springs, so it keeps working cycle after cycle without breaking down.
Once there are enough copies, the DNA is cut into fragments and loaded into a gel for gel electrophoresis, which separates the fragments by size.
Gel electrophoresis — separate by size: The DNA samples are loaded into wells at one end of a gel, and an electric field is switched on.
How the bands form
Pulled to +
DNA carries a negative charge, so all the fragments are pulled toward the anode (the positive + electrode).
Sorted by size
Smaller fragments slip through the gel more easily, so they travel further; larger fragments are held back and stay near the wells.
Pattern
The result is a pattern of bands — a 'ladder' sorted by size — that differs from person to person.
| Fragment | How far it moves | Where its band ends up |
|---|---|---|
| Small (short) | Moves FAR through the gel | Band near the anode (+ end) |
| Large (long) | Moves only a SHORT way | Band stays near the cathode (− end, the wells) |
| All fragments | Move toward the anode (+) | Because DNA carries a negative charge |
PCR (copy)
- Amplifies the DNA (makes copies)
- Cycles of denaturation → annealing → extension
- Uses primers and Taq polymerase
- Each cycle doubles the amount of DNA
Gel electrophoresis (sort)
- Separates the DNA by size
- Driven by an electric field
- DNA is negative → moves to the anode (+)
- Smaller fragments travel further
| Feature | PCR | Gel electrophoresis |
|---|---|---|
| Job | Copies (amplifies) the DNA | Separates the DNA by size |
| What it makes | Millions of identical copies | A pattern of bands on a gel |
| Driven by | Repeated heating and cooling cycles | An electric field across the gel |
| Key fact | Uses primers + Taq polymerase | Smaller fragments travel further |
A memory hook: PCR = Plenty of Copies Rapidly. Gel electrophoresis = a race where the small runners win (smaller fragments travel furthest toward the + end).
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How this is tested: DNA profiling is tested on three papers. Each one asks about a different part of the process.
What each paper asks
- Paper 1 (1 mark): identify which DNA-profiling stage uses the polymerase chain reaction — the answer is the amplification (copying) stage.
- Paper 2 (4-mark Describe): give the steps of PCR — score marks by naming denaturation, annealing and extension, with the role of primers and Taq polymerase.
- Paper 1A (a gel diagram with the anode (+) and cathode (−) marked): deduce a conclusion — the key idea is that smaller fragments travel further toward the positive electrode.
IB-style question — describe the steps of PCR
Describe the steps of the polymerase chain reaction (PCR) used to amplify a sample of DNA. [4]
How to score all four marks
- Denaturation. The DNA is heated to about 95 °C, which breaks the hydrogen bonds and separates the double helix into two single strands.
- Annealing. The mixture is cooled to about 55 °C so that short primers bind (anneal) to the start of each single strand.
- Extension. At about 72 °C, Taq DNA polymerase adds free nucleotides to each primer, building a new complementary strand.
- Cycling. These three steps are repeated many times, and the amount of DNA doubles each cycle, producing millions of copies. (Award 1 mark each for denaturation, annealing, extension, and for repeated cycling / doubling.)
Final answer
Heat (~95 °C) denatures the DNA into single strands; cooling (~55 °C) lets primers anneal; Taq polymerase (~72 °C) extends a new complementary strand; the cycle is repeated, doubling the DNA each time to make millions of copies.
Why this scores full marks: It names all three steps in order (denaturation, annealing, extension) and the repeated-cycling point.
A 4-mark 'describe' needs four distinct points, not the same idea reworded — naming the steps plus the doubling is the safe way to four marks.
| Step of a PCR cycle | Temperature | What happens |
|---|---|---|
| Denaturation | ~95 °C (hot) | Heat breaks the hydrogen bonds, so the double helix separates into two single strands |
| Annealing | ~55 °C (cooler) | Short primers bind (anneal) to the start of each single strand, marking where copying begins |
| Extension | ~72 °C | Taq DNA polymerase adds free nucleotides to each primer, building a new complementary strand |
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The big idea: The genome is the whole of an organism's genetic information — every base of all its DNA added together.
Zoom out step by step
Base
A single base (one letter: A, T, C or G) is part of a gene.
Gene
Many genes lie along one chromosome.
Chromosome
All the chromosomes together make up the genome.
Genome
The outermost, all-containing level — not one gene, and not one chromosome, but the complete set of DNA.
base → gene → chromosome → genome
The nested scale of genetic information: a single base sits inside a gene, a gene inside a chromosome, and all the chromosomes together make up the genome — the whole of an organism's DNA.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
- Genome
- The whole of the genetic information of an organism — all of its DNA, including every gene and the non-coding DNA between genes.
- Gene
- A length of DNA that codes for one product (such as a protein). A genome contains thousands of genes.
- Chromosome
- One long DNA molecule that carries many genes. A human body cell has 46 chromosomes.
- Base
- One of the four DNA letters — adenine (A), thymine (T), cytosine (C) or guanine (G) — the smallest unit of genetic information.
- DNA profiling
- A technique that reads the highly variable repeated regions of the genome to identify an individual or test how closely two individuals are related.
Genome = the WHOLE lot: If a question asks for the term for all of an organism's genetic information, the answer is the genome.
Do not write 'gene' (just one length of DNA) or 'chromosome' (just one DNA molecule) — the genome is everything.
How this is tested: The headline skill here is a 1-mark recall: name the term for the whole of an organism's genetic information — the answer is the genome.
Which cell holds the whole genome: A common 1-mark identify question asks which cell could supply a complete copy of the genome — any nucleated body cell works, because every cell holds the whole genome.
The genome-size table trap: On Paper 1B (data) this topic also appears as a genome-size table: you are given the genome sizes of several very different organisms and asked to deduce whether genome size matches complexity. The trap answer is 'yes' — the correct answer is no.
IB-style question — name the genome and the right cell
(a) State the term used for the whole of the genetic information of an organism. [1]
How to score the mark
- Read what is asked. It wants the single term for all of an organism's DNA — not a gene, not a chromosome.
- Give the term. The answer is the genome. (Award 1 mark for 'genome'.)
Final answer
The genome.
IB-style question — which cell holds a full copy?
(b) A scientist needs a complete copy of a person's genome. Identify a type of cell that could provide one, and explain why. [2]
Model answer
- Name a suitable cell. Any nucleated body cell — for example a white blood cell, a skin cell or a cheek (epithelial) cell.
- Explain why. Every nucleated body cell contains a complete copy of the whole genome, so a single such cell carries all of the person's DNA. (Mark 1: a named nucleated body cell. Mark 2: each cell holds a complete copy of the genome.)
Final answer
A nucleated body cell (e.g. a white blood cell or cheek cell) — because every nucleated body cell contains a complete copy of the whole genome.
Why these score: Part (a) needs the exact term — genome — not a description.
Part (b) scores by naming a nucleated cell (a red blood cell has no nucleus, so it would not work) AND giving the reason: every nucleated cell carries the whole genome.
| Level | What it is | How much of the DNA |
|---|---|---|
| Base | One DNA letter: A, T, C or G | A single position in the sequence |
| Gene | A length of DNA that codes for one product | A small section of one chromosome |
| Chromosome | One long DNA molecule carrying many genes | One of the cell's DNA molecules |
| Genome | ALL of an organism's genetic information | EVERY chromosome — the whole lot |