1) The DNA sample is heated at 95 degrees C. This breaks the hydrogen bonds between the bases on each strand. But it doesn't break the bonds between the ribose of one nucleotide and the phosphate on the next - so the DNA molecule is broken into separate strands.
2) Primers are attached to both strands of the DNA - these will tell the enzyme where to start copying later in the process. They also the two DNA strands from joining together again.
3) The DNA and primer mixture are cooled to 40 degrees C so that the primers can fully bind on to the DNA
4) Free DNA nucleotides and the enzyme DNA polymerase are added to the reaction mixture. heated up to 70 degrees C. Free DNA nucleotides pair with their complementary bases. The DNA polymerase attaches the new nucleotides together into a strand, starting at the primers.
5) The cycle starts again. Each Cycle doubles the amount of DNA
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