Both strands of DNA made of nucleotides come together and start making a helix which makes the bases pair up while the DNA strands are being twisted around like the helix.
Purines form hydrogen bonds to pyrimidines. Adenine pairs up with Thymine (in DNA) and Uracil (in RNA) and Guanine pairs with Cytosine. A-T pairing has 2 hydrogen bonds and C-G pairing has 3 hydrogen...