Cap, splicing and tail each have their own job
- The idea
- Predict
- See it
- Why
- Check
The idea
“The 5' cap marks where introns get cut, and the signal near the end is one more splice site.”
All three steps happen to the same RNA, often while it's still being made. It's easy to swap their jobs.
The short version
The mistake: The 5' cap marks where introns get cut, and the signal near the end is one more splice site.
What's actually true: The cap guards the 5' end, splicing reads signals at each intron's ends, and a 3' signal directs the cut and tail.
- Capping comes first. Enzymes add a modified G nucleotide to the 5' end. It guards that end and helps the ribosome attach.
- Splicing removes introns and joins the exons. Spliceosomes find each intron by short signals at its ends.
- The 3' end is made by a cut. A short signal near the gene's end directs it, then enzymes add the poly-A tail.
On the exam: Cap on the 5' end, introns found by their own ends, and a 3' signal for the cut and tail.