A scientist translates an enzyme from pre-processed cell mRNA in a test tube (in vitro). However, the in vitro enzyme is found to be inactive compared to the extracted in vivo enzyme from a live cell culture. Which statement is the LEAST likely explanation for this phenomenon?
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Correct Answer: B. A nonsense mutation occurred in the extracted mRNA.
This is a great example of a comprehension DAT biology question. All of the answer choices are plausible, but only one stands out like a sore thumb. A nonsense mutation leads to a premature stop codon in the mRNA, halting translation and the production of the protein, which would lead to a lack of activity. However, this is the least likely situation, as mutations are already very rare. It is much more likely that one of the other scenarios occurred. Leaving the introns (the non-coding regions of the DNA) in the mRNA would destroy the intended enzyme by not allowing it to properly fold, leading to inactivation. Introns are normally removed as mRNA is processed through the nucleus, directly extracting “pre-processed” mRNA would likely leave the introns in place. This is why scientists often use cDNA, or complementary DNA, which is the intended gene sans introns and can be directly translated. Most enzymes also undergo post-translational modifications to become active, such as phosphorylation or get cleaved at a specific environment to control their activity. This is especially useful in the production of pepsin in the stomach; producing an inactive version of pepsin (pepsinogen) allows us to control the activity of the enzyme so it does not end up eating the cell that produced it! Molecular chaperones are often present in cellular translation to help an enzyme fold properly.
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