Basic DNA structure proposed by Watson and Crick. DNA is made up of two kinds of nitrogenous bases, purines (adenine and guanine) and pyrimidines (thymine and cytosine). Purine bases bind only to their respective pyrimidine bases, i.e., adenine always pairs with thymine, while guanine binds to cytosine .
Zahid et al. Nanoscale Research Letters 2013 8:119 doi:10.1186/1556-276X-8-119