TBR Section 6 signal peptide

[jparsenal87]

Dealing with a passage about signal peptides directing ribosomes to the rough ER. In this experiment the ER has been fragmented into microsomes in which the ribosomes are found on the outside face.

The basic pattern is "Ribosome translates signal peptide > signal peptide binds to signal recognition particle (SRP) > Binding of signal peptide + SRP causes arrest in protein synthesis until SRP binds to receptor on the ER > SRP moves to microsome membrane and binds to receptor > translational arrest is lifted, the protein is synthesized into the ER, and the signal peptide is cleaved off".

Ok, with that background:

"The mRNA of a hormone normally produced by an endocrine cell is translated by a free ribosome in the presence and absence of microsomes. In this cell free system, the hormone is found to be:

A) same size in presence and absence of microsomes
B) longer in the absence of microsomes
C) shorter in the absence of microsomes
D) longer in the presence of microsomes

First thing: I should have been able to cross off C and D right away, because they are equivalent! But ignoring that because I'm dumb.

I chose C. My reasoning: Without microsomes, the signal peptide could bind to SRP, which would cause translational arrest. This would yield a shorter peptide than one in which the SRP could then bind to a receptor and continue translation.

The correct answer is B because the signal peptide gets cleaved off in the microsomes, shortening the peptide.

The correct answer just ignores the translational arrest part of the passage. How should I know that the mRNA will be FULLY translated and that translational arrest won't affect the length of the resultant peptide?

THANKS

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[5words]

The translational arrest is needed to move the SRP-ribosome complex to the RER. So it doesn't totally stop translation.

 

[Pono_0001]

well. you're assuming in the absence of microsome, there's hormone produced after the halt. if the translation process is halted, then I'd assume no hormone is produced. But the question says the hormone can be translated in the presence or absence of microsome, so it looks like the halt is eventually lifted in the absence of microsome. So you should probably assume that the halt is not permanent in the absence of microsome, and in this case only to consider the cleavage part