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?
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.

Question

Quizplus · Accepted Answer

The signal sequence is necessary for directing the protein to the endoplasmic reticulum (ER) where it can be properly processed and secreted. Without the signal sequence, the protein would not be able to enter the ER and would instead accumulate in the cytoplasm. This fits with the observation that the cells are producing normal levels of insulin, but it is not being secreted. The other choices do not provide a clear explanation for why the protein is not being secreted.

You Are Studying an Individual with Very Low Levels of Insulin