The Discovery of PCR

In 1983 a scientist by the name of Kary Mullis was driving along a Californian mountain road late one night and formulated a revolutionary way to make laboratory copies of DNA molecules (Saiki et al. 1985, Mullis 1990). The resulting polymerase chain reaction, or PCR for short, probably first became known to the general American public in its forensics use during the 1995 O.J. Simpson trial. In general PCR is sort of like making a genetic photocopy of a section of a chromosome.

This laboratory technique is modeled after a living cell’s natural ability to replicate (make copies of) DNA during normal cell cycles. Every living cell makes a duplicate copy of each chromosome during the early stage of cell division known as interphase.

In the following mitotic stages, one copy of each chromosome is pulled toward a respective pole when 2 cells are formed (see lesson Mitosis and Meiosis and the Cell Cycle). This replication process also occurs similarly in the beginning stages of gamete formation during meiosis. The general idea of DNA replication is that the double-stranded DNA molecule unwinds and partially splits into 2 single strands. Various proteins interact to stabilize it and catalyze the formation of a complementary strand for each of the original single strands. A short RNA primer sequence is made first, followed by the enzyme DNA polymerase connecting new nucleotides to this primer, until the entire chromosome has been replicated. The details will be explained shortly. Figure: DNA replication illustrates the process in a cell.

DNA replication: As the 2 DNA strands of a chromosome pull apart, RNA primers bind. Then DNA polymerase completes the copying process.