dinucleotides

Dinucleotide refers to a molecule consisting of two nucleotides linked by a phosphodiester bond. A dinucleotide can be any pair of the four standard bases—adenine (A), cytosine (C), guanine (G), and thymine (T) in DNA or uracil (U) in RNA—arranged in a defined 5' to 3' orientation on a single strand. In double-stranded DNA, dinucleotides describe adjacent base units on one strand, known as dinucleotide steps, whose identities contribute to the overall stability of the helix through base stacking and hydrogen bonding rules.

A notable class is the CpG dinucleotide, where a cytosine is followed by a guanine on the same strand. CpG sites are major targets for DNA methylation, a regulatory modification that can repress gene expression. In vertebrates, methylated cytosines frequently deaminate to thymines, leading to a suppressed frequency of CpG over evolutionary time and the formation of CpG-rich islands near many promoter regions.

Dinucleotide repeats, such as CA or GT repeats, form microsatellites used as genetic markers. These tandem repeats display high mutation rates due to polymerase slippage and contribute to genetic diversity and forensic typing.

In practice, dinucleotide composition and context influence analyses of genomes and transcripts. They affect mutational patterns, regulatory elements, and RNA stability motifs, and are key considerations in studies of evolution, genomics, and molecular biology.