According to HowStuffWorks, cytosine always pairs with guanine. The other two bases that pair together are thymine and adenine. These four bases make up nucleotides and are the building blocks of nucleic acids like DNA.
Cytosine is a nitrogenous base called a pyrimidine, states HowStuffWorks. Of the four bases, two are pyrimidines, single-ringed structures, and two are purines, double-ringed structures. When they bond, one pyrimidine always pairs with one purine.
HowStuffWorks goes on to explain that each nucleotide has a nitrogenous base, like cytosine, a sugar and a phosphate group. One nitrogenous base is paired with another that has its own phosphate and sugar. Then the phosphate molecule in each nucleotide binds to the sugar in the next nucleotide, which creates a long strand.
The National Center for Biotechnology Information says that cytosine has the molecular formula of C4H5N3O. Cytosine was first discovered in 1894 by Albrecht Kossel, according to Wikipedia. It is an important part of DNA and RNA, but it is also a part of converting adenosine diphosphate to adenosine triphosphate. The cytosine gives a phosphate to ADP for the conversion to ATP, a process that is an important component of energy cycle in cells, according to Brooklyn College.