Different RNA sequences can be formed from a single mRNA transcript because ______.

• A. Different RNA editing enzymes alter nucleotides after transcription
• B. Different combinations of exons can be spliced together during RNA processing
• C. Introns are never removed in some transcripts
• D. The final mRNA is produced by copying DNA with more accuracy during transcription

Answer: (B)

Alternative splicing is the process that allows a single gene to produce multiple RNA transcripts. After transcription, the initial RNA contains both exons and introns. The spliceosome can join different sets of exons together—skipping some, including others, or using alternative splice sites—so several mature mRNA variants can come from one pre-mRNA. This creates RNA sequences that differ in which exons are included, and often leads to different protein products. The other options don’t explain why multiple transcripts arise: RNA editing changes bases but doesn’t broadly produce multiple distinct transcripts from one pre-mRNA; introns are normally removed (and never removed is incorrect); copying DNA more accurately during transcription would tend to produce a single transcript, not multiple variants.