The double-stranded DNA (Deoxyribonucleic acid) breaks into two separate strands when the temperature is increased, such as in the denaturation step of the PCR (Polymerase Chain Reaction), because the increase in the temperature makes the hydrogen bonds between the nitrogenous bases to break. The double hydrogen bond between the adenine and thymine, and the triple hydrogen bond between the guanine and cytosine breaks apart when heated causing the two DNA strands to fall apart.
Hence, the answer is the breaking of the hydrogen bond between the nitrogenous bases.
As the temperature increase the covelent bonds between the bases of two strands breaks making two single strands of DNA.
