Adenine Thymine Guanine Cytosine Adenine and Thymine go together
and Guanine and Cytosine got together on the DNA stand
Nucleic acids are linear polymers of nucleotides. Each nucleotide consists of three components: a nucleobase, a pentose sugar, and a phosphate group . The substructure consisting of a nucleobase plus sugar is termed a nucleoside.
Nucleic acid types differ in the structure of the sugar in their nucleotides - DNA contains 2'-deoxyribose while RNA contains ribose, where the only difference is the presence of a hydroxyl group.
The nucleobases found in the two nucleic acid types are also different: adenine (A), cytosine (C), and guanine (G) are found in both RNA and DNA, while thymine (T) occurs in DNA and uracil (U) occurs in RNA.
The nucleobases are classified into two families on the ground of the type of heterocyclic compounds they contain, that is what kind of aromatic rings where one or more carbon is substituted by other atoms.
In particular the purines (A and G) present a structure containing connected five- and six-membered heterocyclic compounds, while the pyrimidines, C, T and U) presents only a six membered ring.
The double strand typical of DNA and of many zones of the secondary and tertiary structure of RNA is based on coupling between complementary nucleobases: in particular the couples A-T and C-G are resent in DNA, while A-U is present in RNA instead of A-T.
Non-standard nucleosides are also found in both RNA and DNA and usually arise from modification of the standard nucleosides. Transfer RNA (tRNA) molecules contain a particularly large number of modified nucleosides.