The two sides of DNA are held together by hydrogen bonds. These bonds contribute to the characteristic double helix structure first proposed by scientists James Watson and Francis Crick in 1953. Hydrogen bonds are formed between four nitrogenous bases found within DNA.
The nitrogenous bases that contribute to the structure of DNA are adenine, thymine, cytosine and guanine. Each adenine forms two hydrogen bonds with thymine and does not interact or bind with cytosine or guanine under normal circumstances. Similarly, cytosine forms three hydrogen bonds with guanine and does not interact with thymine or adenine. Improper hydrogen bonding between bases can lead to errors that must be recognized and fixed by a process called DNA mismatch repair.