Thermal cycle takes place in which of the following technique?

Gel electrophoresis is the technique used for separation of different nucleic acid molecules on the basis of their size. The gel is composed of polyacrylamide (for smaller DNA fragments) or agarose. An electric field is applied to gel that makes the negatively charged molecules through a matrix of gel. The molecules move according to their size. 
PCR - technique amplify the gene of interest in three steps namely denaturation of target DNA (thermal cycle to separate the DNA strands), annealing of primers to the ssDNA and polymerization. 
Centrifugation is the technique used for separation of components of a mixture based on their density and mass. It is carried out by subjecting particles in suspension to centrifugal forces. 
Southern blotting is used for transfer of DNA from agarose gels to the membrane to analyze their composition. For the purpose, the agarose gel is mounted on the filter-paper wick and the hybridization membrane is sandwiched between the gel and an absorbent material, which draw the transfer buffer through the gel by capillary action. The buffer flow draws the DNA molecules which are then immobilized on the membrane.