During processing of mRNA, which parts are removed and which are spliced together?

• A. Exons are excised, introns are spliced together.
• B. Introns are excised, exons are spliced together.
• C. Both introns and exons are removed.
• D. Introns are kept, exons are removed.

Answer: (B)

During mRNA processing, the initial transcript contains exons (coding sequences) and introns (noncoding intervening sequences). The introns are excised by the spliceosome, and the exons are joined (spliced) together to form a continuous, mature mRNA sequence that can be translated. This mature mRNA also gets a 5′ cap and a 3′ poly-A tail to stabilize it and assist in translation. Sometimes, exons can be variably included or skipped in a process called alternative splicing, producing different protein isoforms from the same gene. So, introns are removed and exons are spliced together.