TBR Bio Question

[happyfellow]

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

A. the same size in the 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

I think the answer is A. The questions says that with or without a cell present we still get our hormone translated--so I'm thinking that the cell is completely translated in the cytosol and doesn't contain an signal peptide sequence to direct it to the ER for secretion, etc. Therefore, the size would be the same regardless of a cell being present or not. For the record, TBR says the answer is B.

any help would be much appreciated.

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[StIGMA]

You say with or without the cell (more specific to this question - the microsome), the hormone is translated. That does not speak to whether it has a signal peptide sequence or not.

The point of the question is that endocrine cells will secrete their peptide hormones (insulin, growth hormone, etc.). Thus, they must go through the ER-Golgi system. The mRNA will translate proteins with a signal sequence that will direct translation into the ER. In the process of translation/translocation, the signal sequence will be cleaved (the translocated protein will be shorter). Thus, the proteins without the ER (microsomes) present will be longer than the proteins with the microsomes present (ie: longer in the absence of microsomes).

 

[MLT2MT2DO]

My take is without the cell the introns aren't being cleaved.

TBR usually gives really good explanations to their questions, is "with or without the cell the hormones are still being produced" the only thing it says?

 

[RogueUnicorn]

My take is without the cell the introns aren't being cleaved.

TBR usually gives really good explanations to their questions, is "with or without the cell the hormones are still being produced" the only thing it says?

mRNA is the fully processed molecules with introns cut out.

 

[loveoforganic]

Can you clarify? mRNA can be in different stages of post-transcriptional modification, which can include further splicing and whatnot.

 

[RogueUnicorn]

strictly speaking, mRNA should only refer to the processed cytoplasmic form; if it's still being processed in the nucleus, it would be hnRNA; at least this is how i understand it.

 

[loveoforganic]

Gotcha, sounds like you just learned a more precise set of definitions, same concepts. In our biochem class, we referred to it as mRNA immediately after transcription.

 

[RogueUnicorn]

that wouldn't really be wrong either... but also going by the context of the question, if an RNA is translated, it's the fully processed mRNA unless you're given an indication otherwise

 

[TFS]

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

A. the same size in the 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

I think the answer is A. The questions says that with or without a cell present we still get our hormone translated--so I'm thinking that the cell is completely translated in the cytosol and doesn't contain an signal peptide sequence to direct it to the ER for secretion, etc. Therefore, the size would be the same regardless of a cell being present or not. For the record, TBR says the answer is B.

any help would be much appreciated.

oh i remember doing this one and getting it wrong!

the signal sequence gets cleaved in the ER. it's only meant to target the protein into the ER - once it's in, it's no longer needed. that's why it should be shorter. i believe there is a signal peptidase that hangs out in the ER that does this.