What happens to introns during RNA processing?

• A. They are removed from the pre-mRNA during splicing.
• B. They are retained in mature mRNA.
• C. They are translated into proteins.
• D. They form the poly-A tail.

Answer: (B)

During RNA processing, introns are removed from the pre-mRNA by the spliceosome, and only the exons are joined to form mature mRNA. The introns are spliced out and degraded in the nucleus, so they are not present in the mature transcript that is used for translation. Translation occurs on this mature mRNA, which contains exons only, so introns are not translated. The poly-A tail is added to the 3' end during processing as a separate step, not by introns.