Cytosine always pairs with guanine.
Cytosine is one of the five nucleotides that serve as the monomers (building blocks) for nucleic acids, such as DNA and RNA. Nucleotides are composed of a nitrogenous base, a five-carbon sugar, and at least one phosphate group.
One way in which DNA and RNA nucleotides differ has to do with the sugars that they contain. DNA stands for "deoxyribose nucleic acid" because its nucleotides contain the sugar deoxyribose. Similarly, RNA stands for "ribose nucleic acid" because its nucleotides contain the sugar ribose.
DNA and RNA nucleotides also differ in their complementary nitrogenous base pairs. DNA contains adenine, guanine, cytosine, and thymine. In DNA adenine pairs with thymine while guanine pairs with cytosine. RNA contains adenine, guanine, cytosine, and uracil. In RNA cytosine still pairs with guanine, but adenine pairs with uracil.
Cytosine is one of the four nucleotide bases used in DNA to encode heritable genetic information. Cytosine can also be found in RNA, with a number of other bases, as a means of utilizing DNA instructions to produce proteins.
Cytosine always pairs with guanine. This refers to the bonding structure of double-stranded DNA. Most of the DNA molecule is composed of a phosphate and sugar "backbone" with a variable nucleotide base attached to each phosphate and sugar. A single nucleotide, phosphate and sugar constitute a nucleotide, and a series of nucleotides creates a nucleic acid. Nucleic acids can be single or double-stranded; in the case of a double-stranded form, two nucleic acid strands will align so that their bases are in contact with one another, according to chemical bonding rules that optimize the bonding relationship. It is these rules that determine that cytosine and adenine are the best fit for one another.
Thus, whenever a double-stranded form of DNA or RNA is arranged, a cytosine base should be found opposite any adenine base found on either strand, and vice versa. Since DNA is only read from one of the strands, this decreases the chance that the DNA will be altered by being left "open" as well as providing a backup means of knowing that either a cytosine or adenine is required in the corresponding location should one be damaged or lost.