Which of the following statements is true with respect to a completely processed hnRNA, called mRNA?

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Exons and introns do not appear in the mRNA.

Only introns appear but exons do not appear in this mRNA

Both exons and introns appear in this mRNA

Only exons appear, but introns do not appear in this processed hnRNA

answer is D.

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Detailed Solution

In eukaryotic gene expression, the initial transcript synthesized from a gene is called hnRNA (heterogeneous nuclear RNA) or pre-mRNA. This hnRNA contains both exons (coding sequences) and introns (non-coding sequences).

However, during RNA processing, which includes splicing, the introns are removed from the hnRNA molecule, leaving only the exons to form the mature mRNA. The mature mRNA is the final product that is transported out of the nucleus and serves as the template for protein synthesis during translation. So, in the completely processed mRNA, only exons appear, while introns do not appear because they have been spliced out. This splicing process ensures that the coding regions of the gene are retained in the mature mRNA, while the non-coding regions (introns) are removed.

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