The big idea: DNA profiling (also called DNA fingerprinting) reads the parts of a person's DNA that differ the most from one individual to the next, producing a unique pattern.
It works in two main stages:
1. PCR copies the chosen DNA millions of times (because a sample is usually tiny).
2. Gel electrophoresis then separates those copies by size, giving a pattern of bands that is (almost) unique to each person.
- DNA profiling
- A technique that produces a pattern of DNA fragments unique to an individual, used to identify people or test relationships.
- PCR (polymerase chain reaction)
- A laboratory method that makes millions of copies of a chosen piece of DNA — the amplification stage of profiling.
- Amplification
- Making many copies of a piece of DNA so there is enough to analyse.
- Gel electrophoresis
- A technique that separates DNA fragments by size by pulling them through a gel with an electric field.
- Fragment
- A short piece of DNA. People differ in how long their fragments are, which is what profiling detects.
| Stage | What it does | Why it is needed |
|---|---|---|
| 1. PCR (amplification) | Makes millions of copies of the chosen DNA fragments | A crime-scene or test sample is usually far too small to analyse — PCR multiplies it up |
| 2. Gel electrophoresis | Separates the DNA fragments by size | Different people have fragments of different lengths — separating them reveals each person's unique pattern of bands |
| 3. Read the profile | Compares the band pattern between samples | Matching patterns suggest the samples came from the same person (or close relatives) |
Copy first, then separate: The order matters: you amplify first (PCR makes enough DNA to see), then you separate (gel electrophoresis spreads the fragments out by size).
PCR is about quantity; gel electrophoresis is about sorting.
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 by copying 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:
1. Denaturation (~95 °C): the heat splits the DNA double helix into two single strands.
2. Annealing (~55 °C): short primers bind to each strand, marking where copying starts.
3. 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.
DNA carries a negative charge, so all the fragments are pulled toward the anode (the positive + electrode).
Smaller fragments slip through the gel more easily, so they travel further; larger fragments are held back and stay near the wells.
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: On Paper 1 a 1-mark question often asks you to identify which DNA-profiling stage uses the polymerase chain reaction — the answer is the amplification (copying) stage.
On Paper 2 a 4-mark Describe question asks for the steps of PCR — score marks by naming denaturation, annealing and extension, with the role of primers and Taq polymerase.
On Paper 1A a diagram of a gel (with the anode (+) and cathode (−) marked) asks you to 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 |