at constant temperatures between which samples are moved with a
robotic arm 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. 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. While in some old machines the block is submerged in an oil bath to control temperature, in modern PCR machines a
Peltier element is commonly used. Quality thermal cyclers often contain
silver blocks to achieve fast temperature changes and uniform temperature throughout the block. Other cyclers have multiple blocks with high heat capacity, each of which is kept at a constant temperature, and the reaction tubes are moved between them by means of an automated process. Miniaturized thermal cyclers have been created in which the reaction mixture moves via channel through hot and cold zones on a
microfluidic chip. Thermal cyclers designed for
quantitative PCR have optical systems which enable fluorescence to be monitored during reaction cycling. ==Modern innovation==