It's a nitrogenous base in Nucleic Acids. In DNA, the nitrogen in the bottom left is bonded to a third carbon instead of a hydrogen. The third carbon is part of a deoxyribose 'molecule'. In RNA it's part of a ribose molecule. Those sugars are then bonded to phosphate groups. Cytosine is hydrogen bonded to guanine through 3 hydrogen bonds so a C-G bond is stronger than an A-T bond where there are only 2 hydrogen bonds.
Okay, so this WAS by @CodeBit but we got into an argument about weather or not it should have music, and also, the backdrop was white, so if you moved the sprite around to much, you could see it. So anyway, I just remixed this to show how much better it is with music. Let this be a lesson to all science nerds out there: science isn't everything. Art makes science better and vis versa. Music is art. Go music! The original project has no sound. It sounds like something is missing. Hence the music "space" by @bubblebee3 Thanks and sorry @CodeBit.