IMAT 2014 Q39 [Rough Endoplasmic Reticulum]

The diagram shows a section of rough endoplasmic reticulum (RER).


Which answer correctly identifies the substances associated with structures 1 and 2?

A. 1: amino acids; 2: tRNA
B. 1: tRNA; 2: polypeptide
C. 1: proteins; 2: DNA
D. 1: DNA; 2: carbohydrate
E. 1: phospholipids; 2: mRNA

The answer to this question is choice B.

Before we get into rationalizing the answer, let us be oriented with what the diagram is labeled with:

  • Label 1: The question tells you that this diagram is a rough endoplasmic reticulum. If you can recall, the reason they are called “rough” in the first place is due to the attachment of ribosomes on its surface. Thus it makes sense that the label #1 is a ribosome.
  • Label 2: This label is pointing within the organelle itself or the lumen, which is an important piece of information when trying to answer this question.

With those 2 labels established, how do we begin answering this question?

  • You now know that label #1 is a ribosome, which is the main cellular machinery involved in the process of translation or the production of proteins. Thus, eliminate all the other choices that do not have label 1 correctly associated with proteins in any way or form. This leaves you with choices A, B, and C as your remaining considerations. This is because amino acids (choice A) are the monomers that build up proteins (choice C). tRNA on the other hand (Choice B), is the RNA molecule that decodes the mRNA sequence into a protein in the process of translation.
  • With that being said, what now, could label 2 signify? After the mRNA sequence has been translated into a polypeptide chain, where does it go? It goes within the lumen of the RER to be modified! If you recall, the RER is involved in very important post-translational modification for various functions! Thus, it makes sense that the polypeptide chain that was just produced to go to the lumen for the RER to act on it. This can be supported with the fact that it is different from FREE ribosomes, where the protein synthesized by free ribosomes can freely move around within the cell, whereas the polypeptides produced by ribosomes ATTACHED to the endoplasmic reticulum is NOT free to move around, and is subject to post-translational modification before it is sent out of the cell. Thus, it is only right, that out of choices A to C, only choice B is correct with the association of the polypeptide with the RER lumen.

Thus, it now makes sense that only choice B is correct.