Which statement about introns is true?

Which statement about introns is true?

• A. Introns code for amino acids
• B. Introns are retained in mature mRNA
• C. Introns are removed by the ribosome
• D. Introns are removed by the spliceosome

Answer: (D)

Introns are noncoding segments within a precursor mRNA that are removed during RNA processing. The machinery that does this is the spliceosome, a large complex of small nuclear RNAs and proteins. It recognizes the splice sites at each intron’s ends and the branch point, cuts out the intron as a loop (lariat), and joins the surrounding exons together to produce a continuous coding sequence for translation. The ribosome only acts after splicing to translate the mature mRNA, so it does not remove introns. While introns themselves don’t code for amino acids in the final message, exons do, and splicing can also generate different mRNA variants through alternative splicing.

Introns are noncoding segments within a precursor mRNA that are removed during RNA processing. The machinery that does this is the spliceosome, a large complex of small nuclear RNAs and proteins. It recognizes the splice sites at each intron’s ends and the branch point, cuts out the intron as a loop (lariat), and joins the surrounding exons together to produce a continuous coding sequence for translation. The ribosome only acts after splicing to translate the mature mRNA, so it does not remove introns. While introns themselves don’t code for amino acids in the final message, exons do, and splicing can also generate different mRNA variants through alternative splicing.