Oligonucleotide synthesis is the chemical synthesis of relatively short fragments of
nucleic acids with defined chemical structure (
sequence). The technique is extremely useful in current laboratory practice because it provides a rapid and inexpensive access to custom-made
oligonucleotides of the desired sequence. Whereas
enzymes synthesize
DNA and
RNA only in a 5' to 3' direction, chemical oligonucleotide synthesis does not have this limitation, although it is, most often, carried out in the opposite, 3' to 5' direction. Currently, the process is implemented as
solid-phase synthesis using
phosphoramidite method and phosphoramidite building blocks derived from protected
2'-deoxynucleosides (
dA,
dC,
dG, and
T),
ribonucleosides (
A,
C,
G, and
U), or chemically modified nucleosides,
e.g. LNA,
BNA.