RNA polymerase performs two proofreading functions on that growing transcript

pyrophosphorolytic editing

hydrolytic editing

TLS Online TPP Program

#Id: 7948

#Unit 2. Cellular Organization

Biosynthesis of all N-linked oligosaccharides begins in the rough ER with a preformed oligosaccharide precursor containing 14 residues.

TLS Online TPP Program

#Id: 7949

#Unit 2. Cellular Organization

A branched oligosaccharide containing

3 glucose (Glc)

9 mannose (Man)

2 N-acetylglucosamine (GlcNAc)

Glc3Man9(GlcNAc)2

TLS Online TPP Program

#Id: 7950

#Unit 2. Cellular Organization

Prior to transfer to a nascent chain in the lumen of the  ER, the oligosaccharide precursor is assembled on amembrane- attached anchor called dolichol phosphate, a longchain polyisoprenoid lipid

TLS Online TPP Program

#Id: 7951

#Unit 2. Cellular Organization

First sugar residue is attached to dolichol by a high-energy pyrophosphate linkage & the other sugars are added by glycosidic bonds

TLS Online TPP Program

#Id: 7952

#Unit 2. Cellular Organization

Two N- acetylglucosamine (GlcNAc) and five mannose residues are added one at a time to a dolichol phosphate on the cytosolic face of the ER membrane

TLS Online TPP Program

#Id: 7953

#Unit 2. Cellular Organization

Tunicamycin

blocks the first enzyme in this pathway, inhibits the synthesis of all N-linked oligosaccharides in cells.