RNA is processed before it leaves the nucleus
- The idea
- Predict
- See it
- Why
- Check
The idea
“The mRNA leaves the nucleus first, and then it gets capped, given a tail and spliced.”
The cytoplasm is where mRNA gets used, so it seems like the place to get it ready. The cell finishes that work before the mRNA leaves.
The short version
The mistake: The mRNA leaves the nucleus first, and then it gets capped, given a tail and spliced.
What's actually true: A eukaryotic transcript gets its cap, its poly-A tail and its splicing in the nucleus, and only then is it exported.
- In eukaryotes, RNA polymerase makes pre-mRNA in the nucleus. A 5' cap goes on one end and a poly-A tail on the other. Both protect the mRNA and help ribosomes find it.
- Splicing removes the introns and joins the exons. These steps overlap in time, but all of them finish before export.
- Only finished mRNA goes out through the nuclear pores. In the cytoplasm the tail slowly gets shorter, which limits how long the mRNA lasts.
On the exam: Cap, tail and splicing all happen in the nucleus. What leaves is finished mRNA.