Temperature-sensitive yeast mutants have been isolated that block each of the enzymatic steps in the synthesis of the dolichol-linked oligosaccharide precursor for n-linked glycosylation. why do mutations that block synthesis of the intermediate with the structure dolichol-pp-(glcnac)2man5 completely prevent addition of n-linked oligosaccharide chains to secretory proteins, whereas mutations that block conversion of this intermediate into the completed precursor—dolichol-pp-(glcnac)2man9 glc3—allow the addition of n-linked oligosaccharide chains to secretory glycoproteins?