The Thermal Cycler (also known as a Thermocycler, PCR Machine or DNA Amplifier) is a laboratory apparatus used to amplify segments of DNA via the polymerase chain reaction (PCR) process. The device has a thermal block with holes where tubes holding the PCR reaction mixtures can be inserted. The cycler then raises and lowers the temperature of the block in discrete, pre-programmed steps.

The earliest Thermal Cyclers were designed for use with the Klenow fragment of DNA Polymerase I. Since this enzyme is destroyed during each heating step of the amplification process, new enzyme had to be added every cycle. This led to a cumbersome machine based on an automated pipettor, with open reaction tubes (see photo). Later, the PCR process was adapted to the use of thermostable DNA polymerase from Thermus aquaticus, which greatly simplified the design of the Thermal Cycler. Modern Thermal Cyclers are often equipped with a heated lid, a heated plate that presses against the lids of the reaction tubes. This prevents condensation of water from the reaction mixtures on the insides of the lids and makes it unnecessary to use PCR oil. Some thermal cyclers are equipped with multiple blocks allowing several different PCR reactions to be carried out simultaneously. Also some apparatus have a gradient function, which allows different temperatures in different parts of the block. This is particularly useful when testing suitable annealing temperatures for primers. The prices for thermal cyclers start from USD $2500 (in 2004).

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