The main step in PCR (Polymerase Chain Reaction tests) and RT-PCR (Reverse transcription polymerase chain reaction) used in gene analyzes is the process of increasing the initial amount of DNA to have enough quantity for macroscopic observation. The process is done based on the natural ability of DNA to replicate. It has basically 3 to 4 steps: 

• Revers transcription (for RNA)

• Denaturation ~95 DEG C

• Hybridization ~47 DEG C

• Chain growth ~72 DEG C

Each of the stages has specific temperature and enzymes needed to run the process. At one cycle of the above processes the amount of DNA is doubled. Which leads for 20 cycles of ~1 000 000 DNA molecules copy from a single molecule on start.

One important consideration for the overall efficiency of the process is the time needed to reach each temperature level. With thermocycler with speed 4-5 deg/s the process may need 40-50 min.

Handling a serious temperature change in so short time is not an easy task. 

The programmable thermostat follows this requirements.

A good explanation of the process and applications of RT-PCR can be found at ThermoFisher.