DNA duplication: polymerase chain reaction (PCR)
- PCR technique allows rapid replications of DNA as well as laying the foundation of sequencing - sequence is read as DNAs are created.
- One pot chain reaction
- Reagents: DNAs to be replicated, primers, ATCG monomers, polymerase (proteins that stitch nucleotides together)
- Primer: short DNAs with 15 20 bases long that pair with ends of the DNAs to be duplicated. For example, the red lines at the top and bottom are two primers. It’s the reverse complement of the 3’ end (itself starts from 5’).
5' - CTATTCATT
3' - GATAAGTAAGTATGTGGTTCTAT......CTATCTATATCATATCTAACCCAGAC - 5'
5' - CTATTCATTCATACACCAAGATA......GATAGATATAGTATAGATTGGGTCTG - 3'
ACCCAGAC - 5'
- Reaction cycle: 3 stages to duplicate each DNA chain in the system.
- Heating to 94 °C: DNA melts and two helix strands separate
- Annealing to 54 °C: the primers bind with full DNA chains (its harder for 2 full chains to find each other)
- Heating to 72 °C: polymerase fill in the missing part for each primer, the cycle ends.
- Cycle repeats.
- At the end of each cycle, number of full DNA chains is doubled. The system can go back to the first step by heating up again. After 30 cycles, it would produce 2^30 (about 2 billion) from the original one.
(A piece of irrelevant history: apparently the guy who invented PCR - Kary Mullis was tripping on LSD when he came up with the idea.)