PCR

DNA replication can take place outside cells. Providing enzyme (polymerase DNA), nucleotide and primer supply is necessary. Polymerase chain reaction (PCR) takes advantage of this fact. PCR enables making millions of copies from little amount of DNA. Heating separates strands of the helix because providing energy results in breakage of hydrogen bonds between bases. Then cooling takes place and primers and enzymes are added. Chosen fragment of DNA is copied. The starting point of synthesis of the new DNA chain is determined by the starter. Repeating the whole process leads to origin of desired amount of DNA.

PCR was elaborated by Dr Kary Mullis from Cetus Corporation. He was awarded with Noble Prize in 1993 for this achievement. PCR technique is being improved to get the best results. For example it was invented to use polymerase DNA obtained from thermophilous bacteria (living in hot springs) that is not destroyed during heating. It is not necessary then to add it again after each cycle. Thermophilous bacteria (so among other their proteins) are not sensitive to temperature.

Extensive use of PCR method is made for example in genetic diseases diagnosis. It is now routine. Copying DNA from sample of foetus's cells enables reliable prenatal diagnosis. Similar situation is in forensic medicine where DNA must be copied millions of times for a little amount of blood or sperm to serve as credible proof.