The polymerase chain reaction (PCR) is a technique used to make millions of copies of a specific section of DNA in a short time. It is carried out in a test tube (in vitro), not in a living cell.PCR is useful because DNA samples are often tiny. A single hair root, a drop of blood or a trace of saliva at a crime scene contains far too little DNA to analyse. PCR amplifies the DNA so there is enough for DNA profiling, sequencing, detecting pathogens or testing for genetic disorders.A PCR reaction mixture contains:
• the DNA sample to be copied (the template)
• primers: short, single-stranded pieces of DNA with base sequences complementary to the start of the region to be copied. Two different primers are needed, one for each strand
• free DNA nucleotides containing the bases A, T, C and G
• Taq polymerase: a DNA polymerase enzyme from the bacterium Thermus aquaticus, which lives in hot springs. Its optimum temperature is about 72°C and it is not denatured at the high temperatures used in PCR
• a buffer solution to keep the pH constant.The mixture is placed in a thermal cycler, a machine that changes the temperature automatically and accurately. Each cycle of temperature changes doubles the amount of the target DNA, so the number of copies increases exponentially.
The primers decide which section of DNA is copied, so PCR can amplify one particular gene or region out of a whole genome.
• the DNA sample to be copied (the template)
• primers: short, single-stranded pieces of DNA with base sequences complementary to the start of the region to be copied. Two different primers are needed, one for each strand
• free DNA nucleotides containing the bases A, T, C and G
• Taq polymerase: a DNA polymerase enzyme from the bacterium Thermus aquaticus, which lives in hot springs. Its optimum temperature is about 72°C and it is not denatured at the high temperatures used in PCR
• a buffer solution to keep the pH constant.The mixture is placed in a thermal cycler, a machine that changes the temperature automatically and accurately. Each cycle of temperature changes doubles the amount of the target DNA, so the number of copies increases exponentially.
The primers decide which section of DNA is copied, so PCR can amplify one particular gene or region out of a whole genome.