Which PCR step separates the DNA strands?

Correct answer: Denaturation

The direct answer is A, Denaturation. PCR copies a chosen DNA region through repeated temperature cycles. DNA normally has two complementary strands held together by weak hydrogen bonds. In the denaturation step, the mixture is heated, and this heat breaks those weak attractions, so the two strands separate. Next, during annealing, short primers attach to the single strands. During extension, DNA polymerase adds nucleotides and makes new strands. Option A is correct because denaturation specifically means separation of the DNA strands. Option B, ligation, joins DNA pieces with DNA ligase; it does not separate the strands. Option C, transformation, means introducing DNA into a cell, not a PCR temperature step. Option D, selection, means identifying cells with a desired genetic feature and is not the strand-separation step. Remember: in PCR, heat first separates, primers then attach, and polymerase then extends.