Which nitrogenous bases form complimentary pairs?
In DNA (Deoxtribonucleic acid) there are 4 bases and the pairning rules are as follows: Adenine-Thymine and the other is Guanine-Cytosine However in RNA (Ribonucleic Acid) the bases are different and thus the base pairing-the "complimentary pairs" are Adenine-Uracil and Guanine-Cytosine
If the DNA is cooled slowly, enough time is allowed for the bases on the ssDNA to re-allign and form the proper Watson/Crick base pairs. However, if the denaturation is followed by rapid cooling, there is not enough time for the nitrogenous bases to line up in an ordered fashion. In this rapid cooling case, the bases form random associations with nearby bases and the ssDNA does not re-assume an ordered double helix structure.
Nucleobases (or nucleotide bases/nitrogenous bases/aglycones) provide the nucleotide structure necessary to form base pairs. The primary nucleobases are cytosine, guanine, adenine (DNA and RNA), thymine (DNA) and uracil (RNA), abbreviated as C, G, A, T, and U, respectively. They are usually simply called bases in genetics. Because A, G, C, and T appear in the DNA, these molecules are called DNA-bases; A, G, C, and U are called RNA-bases. From Wikipedia