D1.1 DNA replicationIB Biology HL: Revision notes

DNA replication makes exact copies of DNA with identical base sequences, needed for reproduction, growth and tissue replacement. It is semi-conservative: each new molecule has one original strand and one new strand. Each original strand is a template, and complementary base pairing (A–T, C–G) makes copying highly accurate.

Helicase unwinds the helix and breaks the hydrogen bonds between the strands; DNA polymerase links nucleotides into a new complementary strand.

PCR amplifies a chosen sequence: ~95 °C separates strands, ~55 °C lets primers bind either side of the target, ~72 °C lets heat-stable Taq polymerase extend them. Each cycle doubles the copies (2ⁿ after n cycles).

Gel electrophoresis: DNA is negatively charged, moves to the positive electrode, and shorter fragments move further.

Applications include DNA profiling (forensics, paternity), diagnosing infections and identifying species in food. NOS: using more markers reduces the probability of a false match.

Each strand has a 5' end (phosphate on carbon 5 of the deoxyribose) and a 3' end (—OH on carbon 3). The two strands are antiparallel.

DNA polymerases join the 5' end of a new nucleotide to the 3' end of the growing strand, so all new DNA is made 5' to 3'. A polymerase cannot start a strand from nothing; it needs a primer with a free 3' end.

Because of directionality, the two new strands at a fork are made differently:

• Leading strand — made continuously towards the fork; needs an RNA primer only once.
• Lagging strand — made discontinuously away from the fork as Okazaki fragments (about 1000–2000 nucleotides in prokaryotes); needs a new RNA primer for every fragment.

• Helicase — unwinds DNA, breaks hydrogen bonds.
• DNA primase — makes short RNA primers.
• DNA polymerase III — adds DNA nucleotides to the 3' end of the primer; makes most of both strands.
• DNA polymerase I — removes RNA primers and replaces them with DNA.
• DNA ligase — joins Okazaki fragments by sealing the sugar-phosphate backbone.

If DNA polymerase III adds a nucleotide whose base is mismatched with the template, it removes that nucleotide from the 3' terminal and replaces it with a correctly matched one. This lowers the error rate by about 100-fold, so replication is far more accurate than base pairing alone. Without proofreading, mismatches are copied at the next replication and become permanent mutations.