Molecular and Cellular Biology, Lecture 9: RNA Processing

RNA cap

When a 7 methylguanosine is covalently attached to the 5' end of mature mRNA of eukaryotes

poly-A tail

Modified end of the 3' end of an mRNA molecule consisting of the addition of some 50 to 250 adenine nucleotides.

Splicing

the process of removing introns and reconnecting exons in a pre-mRNA, pre-mRNA -> mRNA

Group I intron splicing

self-splicing

Group II intron splicing

self-splicing

Splicesome

a complex of specialized RNA and protein subunits that removes introns from a transcribed pre-mRNA segment.

5' capping

Addition of methyl guanosine to the 5' end of a eukaryotic mRNA during processing.

Capping enzymes

Ride along phosphorylated tail of RNA polymerase II so they are right there when RNA starts

CPSF

cleavage and polyadenylation specificity factor

CstF

cleavage stimulation factor

CPSF and CstF

Enzymes of Polyadenylation (adding a Poly-A tail)

Role of CPSF

cleaves the RNA molecule to make the poly-A tail

role of CstF

cleaves GU-rich segment in order to detach Pol 2 from the DNA.

Roles of Cap and Poly(A) Tail

• protect ends from nucleases

• required for efficient translation

snRNPs

(small nuclear ribonucleoproteins) composed of RNA and protein molecules, recognize the splice sites, join with additional proteins to form a spliceseome

alternative splicing

Splicing of introns in a pre-mRNA that occurs in different ways, leading to different mRNAs that code for different proteins or protein isoforms. Increases the diversity of proteins.

isoforms

proteins generated from alternative splicing that are somewhat different from each other

constitutive exons

an exon that is always found in mRNA following splicing

cassette exons

can be used as exon or spliced out as introns