D1.1 DNA replicationIB Biology SL: Revision notes

DNA replication produces exact copies of DNA with identical base sequences. It happens before a cell divides, so every daughter cell gets a complete copy of the genome. It is needed for:

• reproduction — asexual reproduction of single-celled organisms, and the production of gametes;
• growth of multicellular organisms by increasing cell numbers;
• tissue replacement, e.g. constant renewal of skin and the gut lining.

Replication is semi-conservative: each new molecule contains one original strand and one new strand. Each separated strand is a template. Free nucleotides pair with the template by complementary base pairing — adenine with thymine, cytosine with guanine — so the new strand has the same sequence as the strand originally paired with the template. Because only the correct pairing fits, copying is highly accurate.

Evidence: Meselson and Stahl grew E. coli in ¹⁵N then moved it to ¹⁴N. After one generation all DNA was of intermediate density; after two, half intermediate and half light — exactly as semi-conservative replication predicts.

Helicase unwinds the double helix and breaks the hydrogen bonds between the bases of the two strands, separating them.

DNA polymerase links free nucleotides together to build a new strand that is complementary to the template.

The polymerase chain reaction (PCR) makes millions of copies of a chosen DNA sequence from a tiny sample. Each cycle has three temperatures:

1. about 95 °C — strands separate (hydrogen bonds break);
2. about 55 °C — primers (short single-stranded DNA complementary to each end of the target) bind to the templates;
3. about 72 °C — Taq polymerase extends the primers with nucleotides.

Taq polymerase comes from Thermus aquaticus, a hot-spring bacterium, so it is heat-stable and survives every 95 °C step. Each cycle doubles the copies: n cycles give 2ⁿ times the starting number.

Gel electrophoresis separates DNA fragments by length. Samples are loaded into wells in a gel and a current is applied. DNA is negatively charged (phosphate groups), so it moves towards the positive electrode. The gel is a mesh: shorter fragments move faster and further. Fragments of known length run alongside act as a scale.

PCR and gel electrophoresis are used in DNA profiling (forensics, paternity testing), diagnosing infections, identifying species and much of genetic research. In profiling, PCR copies several STR markers — regions whose length varies between people — and the gel shows each person's pattern of fragment lengths.

• Forensics: a crime-scene profile is compared with suspects' profiles.
• Paternity: at each marker the child has one fragment from the mother; the other must come from the father. One mismatch excludes a man.

Reliability (NOS): if each marker is shared by 1 in 10 people, a chance match at n independent markers has probability 0.1ⁿ. More markers reduce the probability of a false match, just as more measurements improve reliability in an experiment.