Making investigations of DNA possible – the PCR thermal cycler

What is a Thermal Cycler?

A PCR thermal cycler is a high tech laboratory instrument that is used for DNA amplification via Polymerase Chain Reaction (PCR). This makes it to one of the centerpieces of a molecular biology lab. If you look at the thermal cycler (also known as PCR machine, thermocycler or DNA amplifier) objectively, it mainly consists of a thermal block with a grid of holes, which holds the reaction tubes with PCR probes. The thermal block can increase or decrease temperature very rapidly, temperature steps and their duration can be programmed.

Why and how was the Thermal Cycler created?

In the beginning of PCR research a modified DNA polymerase from E. coli – the Klenow fragment – was used. As this enzyme was inactivated by high temperatures in the amplification process and hence became useless for further amplification. This made the process of PCR very cumbersome and time consuming because the enzyme had to be replaced after every single step of the process. Even the invention of automated pipettors was only a minor improvement because in this version open tubes had to be used, which increased the risk of evaporation of the small PCR samples.

The utilization of heat-resistant DNA polymerase from Thermus aquaticus made the final breakthrough possible. Instrumental design of PCR thermal cyclers could be simplified. Temperature control was performed by the use of oil baths with constant temperature, more elaborate models were made from silver in order to ensure rapid temperature changes and uniform temperature distribution throughout the complete thermal block.

How does a Thermal Cycler work today?

Modern PCR thermal cyclers feature a thermoelectric heat pump that provides the possibility for reliable temperature control with the advantage that no constant temperature bath is required anymore. Another improvement are heated lids, which fit tightly against the top of the PCR reaction tubes in order to prevent evaporation of water from the PCR samples that would otherwise condensate on the tube caps. This process would increase the salt concentration of the PCR sample dramatically and to avoid this, PCR tubes had to be sealed with mineral oil in the past. Today PCR thermal cyclers with multiple thermal blocks are available, which enable the user to perform several PCR reactions according to different protocols at the same time. High-end thermal cyclers come with a temperature gradient function that can be very useful when the suitability of annealing temperatures for primers has to be tested.

Current PCR thermal cycler are technical masterpieces but we may essentially assume that new improvements will be launched during the next years.