A PCR tube starts with 20 copies of a target sequence. If every cycle doubles the number, how many copies are there after 10 cycles?
- Copies = starting copies × 2n, with n = 10.
- 210 = 1024.
- 20 × 1024 = 20 480.
Answer: 20 480 copies.
DNA replication
Biology · D Continuity and change. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The idea
Before a cell divides it copies all its DNA so that each new cell gets the full set of genes. The two strands of the double helix separate, and each one is used as a template for a new strand. Every new molecule therefore has one old strand and one new strand. This is semi-conservative replication.
The copies are exact because of complementary base pairing: A pairs only with T, and C pairs only with G.
PCR uses the same idea in a tube to make millions of copies of a chosen piece of DNA. Gel electrophoresis then separates DNA fragments by size.
Rules to remember
- Helicase unwinds the double helix and separates the strands by breaking the hydrogen bonds between bases.
- DNA polymerase joins nucleotides into a new strand, using the old strand as a template.
- PCR cycle: about 95 °C, strands separate; about 55 °C, primers bind to the ends of the target sequence; about 72 °C, Taq polymerase builds the new strands.
- Taq polymerase comes from a hot-spring bacterium, so it is not denatured at 95 °C.
- Each PCR cycle doubles the DNA: copies = starting copies × 2n after n cycles.
- Gel electrophoresis: DNA is negatively charged and moves towards the positive electrode; small fragments move faster and further.
- DNA profiling: every band in a child's profile must match a band in the mother or the father. It is used in paternity tests and forensic work.
Common mistake
Students say the smallest fragments stay near the wells. It is the opposite: small fragments travel furthest and the largest stay closest to the wells.
Worked example
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