Apply the exon rules before you count mRNAs
- The idea
- Predict
- See it
- Why
- Check
The idea
“Every exon is kept or skipped, so I multiply by 2 for each exon.”
Alternative splicing sounds like a free mix of exons, so doubling for each one seems fair. The data limits which mixes exist.
The short version
The mistake: Every exon is kept or skipped, so I multiply by 2 for each exon.
What's actually true: Only real choices multiply. An exon in every mRNA adds no choice, and a mutually exclusive group is one choice.
- Splicing cuts out introns and joins exons. In alternative splicing, different mRNAs from one gene keep different exons, so one gene can code for several polypeptides.
- An exon found in every mRNA has only one option. It never changes the count.
- In a mutually exclusive group, each mRNA carries exactly one exon from the group. The group is one choice, with as many options as it has exons.
- Multiply the options at each real choice. With independent choices, that's the most mRNAs the gene can make.
On the exam: Drop the always-kept exons, treat each either-or group as one choice, then multiply.