The process of N-linked glycosylation starts with the formation of dolichol-linked GlcNAc sugar. Dolichol is a lipid molecule composed of repeating isoprene units. This molecule is found attached to the membrane of the ER. Sugar molecules are attached to the dolichol through a pyrophosphate linkage (one phosphate was originally linked to dolichol, and the second phosphate came from the nucleotide sugar). The oligosaccharide chain is then extended through the addition of various sugar molecules in a stepwise manner to form a precursor oligosaccharide.
The assembly of this precursor oligosaccharide occurs in two phases: Phase I and II. Phase I takes place on the cytoplasmic side of the ER and Phase II takes place on the luminal side of the ER.
The precursor molecule, ready to be transferred to a protein, consists of 2 GlcNAc, 9 mannose, and 3 glucose molecules.