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 mRNA and tRNA). Purines always bond with pyrimidines via hydrogen bonds. The purine Adenine always hydrogen bonds with Thymine and the purine Guanine always hydrogen bonds with Cytosine. In mRNA and tRNA, during transcription and translation Thymine is substituted with Uracil meaning Adenine bonds with Uracil instead.