You are studying an individual with very low levels of insulin in her blood. Further analysis indicates that cells of her pancreas are producing normal levels of this protein, but it is accumulating in the cytoplasm rather than being secreted from the cells. Which hypothesis makes the most sense to explain this observation?

Question

A.  A small deletion has removed the nucleotides that code for the signal sequence at the amino terminus of the protein.
B.  A missense mutation has caused premature termination during translation of this protein.
C.  A chromosomal segment that includes the gene for insulin has been inverted.
D.  A two-base deletion near the middle of the gene has altered the reading frame during translation of the protein.
E.  A missense mutation has altered the ribosome-binding sequence at the 5' end of the mRNA.

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?
· What key words does the question contain and what do they mean?

Gather Content
· What do you already know about transcription and translation?

Consider Possibilities
· Consider the different answer options. Which can you rule out?

 Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?

 Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

A.  A small deletion has removed the nucleotides that code for the signal sequence at the amino terminus of the protein.

Clarify Question
· What is the key concept addressed by the question?
        o This question addresses transcription and translation.
· What type of thinking is required?
        o This question is asking you to analyze the information given, using logic, to dissect the problem and determine the answer.
· What key words does the question contain and what do they mean?
        o Insulin is a protein produced by the pancreas, that plays a critical role in blood sugar regulation.
        o If the protein is accumulating in the cytoplasm rather than being secreted, that means it is building up inside the cells instead of being released from the cells.

Gather Content
· What do you already know about transcription and translation?
        o Different mutations will affect transcription and translation differently.
        o First, a protein like insulin is transcribed into mRNA. Then the mRNA is translated.
        o A secreted protein like insulin will be synthesized on the rough ER (RER) so that the protein ends up in the lumen of the ER, ready for release by the cell. A signal sequence on the amino terminus of the polypeptide is responsible for recognition of the peptide by the SRP, and docking onto the RER.

Consider Possibilities
· Consider the different answer options. Which can you rule out?
        o Could an inversion of the gene explain the lack of secretion? No, it’s not apparent why that would affect secretion.
        o Could a mutation in the ribosome-binding sequence explain the lack of secretion? No, because then there wouldn’t be protein produced at all.
        o Could a premature termination or a change in reading frame explain the lack of secretion? No, because then there wouldn’t be full-length protein produced. (Disruption of the reading frame typically creates early STOP codons.)

 Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o What specifically seems to be wrong with the insulin protein in this individual? The protein is being produced fine, just not secreted.
        o What type of mutation could interfere with that specific aspect? A mutation must have disrupted the signal sequence, which helps the growing peptide dock at the ER for later secretion.

 Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
o This question asked what type of mutation could best explain lack of insulin secretion.
o The question required you to analyze the information given, using logic, to dissect the problem and determine the answer.
o Did you recognize that lack of secretion indicates a problem with the signal sequence on the ER?