The Polymerase Chain Reaction (PCR) (College Board AP® Biology): Study Guide

The polymerase chain reaction (PCR)

• The polymerase chain reaction (PCR) is a common technique used in most applications of gene technology, e.g. to produce enough DNA for:

  • DNA profiling when identifying criminals or determining paternity

  • phylogenetic analysis

  • transforming bacteria during genetic modification

• PCR is used to produce many identical copies of a DNA fragment from a small initial sample

  • The DNA is said to be amplified

• The PCR process involves three key stages per cycle:

  • denaturation

    • The double-stranded DNA is heated to break the hydrogen bonds that bond the two DNA strands together

  • annealing

    • The temperature is decreased so that DNA primers can anneal to the ends of the single strands of DNA

  • elongation/extension

    • The temperature is increased to the optimum temperature for DNA polymerase to build the complementary strands of DNA

    • This produces the new identical double-stranded DNA molecules

• The three stages are undertaken in a PCR machine, or thermal cycler, which automatically provides the optimal temperature for each stage and controls the length of time spent at each stage

• In each cycle the mass of DNA is doubled, so amplification occurs very quickly