This piece illustrates the heating and cooling steps of a polymerase chain reaction (PCR), a fundamental technique used for genetic testing and research. Heating (candle) denatures DNA strands and cooling (snow) allows primers and free nucleotides (snowflakes) to attach to the denatured strands. The attachment of primers and free nucleotides, summarized in the cooling portion of this illustration, is actually a 2 step process involving 2 different low temperatures. This reaction is repeated several times to amplify a segment of DNA.