EC3: mRNA splicing, genetic code, and tRNA Flashcards
What are the three post-transcriptional modifications of eukaryotic mRNA?
- Capping
- Tailing
- Splicing
At which end is eukaryotic mRNA capped?
5’ end
In chemical terms, what is the cap on eukaryotic mRNA?
7-methylG
(a cationic base therefore positive charge added)
Why is the cap relevant?
It might be a protective device against nucleases
The camp is recognised by specific proteins that enable ribosomes to bind to the mRNA and initiate protein synthesis
At which end of eukaryotic mRNA does tailing occur?
3’ end
What is the tail in chemical terms? How long is it?
poly-A
approx. 200 A residues
What is the relationship between the age of eukaryotic mRNAs and the tail?
As the mRNAs age, the tail gets shorter. The mRNA is inactive when n<10
Why are mature eukaryotic mRNAs typically much shorter than the genes from which they are transcribed, but the primary transcripts are not?
Introns have been spliced out of mature mRNAs but they are still present in the primary transcript
What are introns?
Junk DNA
What are exons?
The fragmented coding sequence
What is an alternative term for the primary transcript?
pre-mRNA
Where does splicing occur?
The nucleus
Define splicing
The removal of introns and the joining together of exons in the same order as in the gene
What are splicosomes?
Congregations of snRNPs (up to 150). They are used up so need to be replaced
What are snRNPs?
Small nuclear RNAs plus proteins