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Question
Which of the classes of RNA molecules is never found in the cytosol?
Options
Ribosomal RNA
Transfer RNA
Messenger RNA
Primary mRNA transcript
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Solution
Primary mRNA transcript
Explanation:
The primary mRNA transcript (also known as pre-mRNA) is a premature form of mRNA that is synthesised directly from DNA during transcription inside the nucleus. Before it can enter the cytosol (cytoplasm), it must undergo significant modifications, such as splicing out non-coding introns and adding protective caps and tails. Only after these modifications are complete is it considered “mature” mRNA and exported out of the nucleus. In contrast, Ribosomal RNA (rRNA), transfer RNA (tRNA), and mature Messenger RNA (mRNA) are all essential components of the protein-synthesis machinery located in the cytosol.
