C (cytosine): C stands for cytosine, a DNA nucleotide that which is one member of the base pair in DNA consisting of guanine and cytosine. This base pair is conventionally abbreviated G-C (or GC).
The other base pair in DNA is adenine and thymine which is conventionally abbreviated A-T (or AT).
Each base pair forms a "rung of the DNA ladder." A DNA nucleotide is made of a molecule of sugar, a molecule of phosphoric acid, and a molecule called a base. The bases are the "letters" that spell out the genetic code. In DNA, the code letters are A, T, G, and C, which stand for the chemicals adenine, thymine, guanine, and cytosine, respectively. In base pairing, adenine always pairs with thymine, and guanine always pairs with cytosine.