Step I Nbme 6

[firstaid2012]

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Which of the following best explains why proteins synthesized in the rough endoplasmic reticulum and secreted from eukaryotic cells have a shorter length than would be predicted by their respective mRNAs?
A ) Cleavage of 5? segments of nascent polypeptides being synthesized on polyribosomes in the endoplasmic reticulum
B ) Covalent binding of nascent polypeptides to specific membrane proteins during their synthesis
C ) Nonspecific protease activity during passage of these proteins through the endoplasmic reticulum membrane
D ) Shortening of the carboxy terminus because hydrophobic amino acids remain within the membrane

 

[Myxedema]

Think of how proinsulin is converted into insulin: mature mRNA coding for still contains the sequence for C-peptide. After translation, C-peptide is cleaved from the polypeptide chain, which converts proinsulin into insulin.

So the answer should be A.

 

[KillaCam]

All proteins targeted for the RER must have a single recongition sequence in the mRNA/protein in order to get there from the cytosol; these are at the 5' end and it's the only way they get there! Once the ribosome binds this sequence and takes the mRNA/protein it to the RER, the small sequence gets cleaved within the ER and the protein can go on being modified, secreted, whatever...........