A base is an organic nitrogenous molecule which, when covalently bonded to a pentose sugar, forms a nucleotide, which is in turn linked to phosphate molecules. There are four types of bases in DNA: adenine, thymine, guanine and cytosine. Generally, purine bases such as adenine and guanine form hydrogen bonds with pyrimidine bases such as thymine and cytosine. According to Watson-Crick principles in DNA, adenine only base pairs with thymine and cytosine only base pairs with guanine. Strong hydrogen bonds form between complementary bases A and T or C and G. A has 2 hydrogen bonds linked to T whilst G has 3 hydrogen bonds with C. These hydrogen bonds are broken during semi-conservative replication so the DNA molecule can unwind. In RNA, thymine is replaced with uracil, complementary base pairing with A. They both have a similar structure apart from uracil lacks the 5' methyl group.