Purines and Pyrimidines

Purines and pyrimidines are the two building blocks of nucleic acids. There are two types of purines: Adenine and Guanine, and three different types of pyrimidines: cytosine, thymine and uracil (which is only used in RNA, mRNA and tRNA). Purines always bond with pyrimidines via hydrogen bonds. The purine Adenine forms two hydrogen bonds with the pyrimidine Thymine and the purine Guanine forms three hydrogen bonds with the pyramidine Cytosine. In RNA, mRNA and tRNA, during transcription and translation, Thymine is substituted with Uracil meaning Adenine bonds with Uracil instead, forming two hydrogen bonds.