A

In molecular genetics an "A" usually represents adenine a purine and one of the bases that make up DNA and RNA.

In a DNA sequence the nitrogenous base adenine is covalently bonded to a deoxyribose sugar which is bound to a phosphate group (the phosphate group in turn is bound to the next sugar in the chain forming a deoxyribose-phosphate backbone). Adenine pairs with thymine on the opposite anti-parallel strand forming two hydrogen bonds and an AT basepair.

In an RNA sequence the nitrogenous base adenine is covalently bonded to a ribose sugar which is bound to a phosphate group (the phosphate group in turn is bound to the next sugar in the chain forming a ribose-phosphate backbone). In RNA adenine pairs with uracil forming two hydrogen bonds.