DB Lecture 25: pre-mRNA processing Flashcards
What is the “cap” at the end of a mRNA?
guanosine nucleotide that has been modified to contain a methyl group at the 7 position of the purine ring structure (linked to the end of the transcript in an unusual 5’ to 5’ phosphodiester bond)
Why are mRNAs capped?
required for translation initiation
recognition site for binding of proteins that recruit a ribosome
protects form degradation by exonuclease (chew in the 5’-to-3’ direction
What are conserved signals?
introns start with GU
introns end with AG
recognized by loose consensus sequences that include exon and intron sequences
What is the “branch point”?
contained in introns near the 3’ splice site
A residue in the context of a loose consensus sequence and a run of pyrimidines (also near the 3’ splice site
What is the spliceosome?
large complex of nearly 200 proteins and several snRNPs
catalyzes splicing
Describe the process of splicing.
consists of two transesterification reactions (replaces one phosphodiester bond with another)
don’t need to know the pathway
What are ESE sequences?
exonic splicing enhancer sequences
RNA boxes bound by SR proteins (rich in serine and arginine)
What are the functions of ESE sequences?
help the splicing machinery recognize splice sites
regulation of alternate splicing
List the kinds of alternative splicing.
alternative first exon alternative last exon cassette exon mutually exclusive alternative 5' splice site alternative 3' splice site intron retention
What are ESS sequences?
exonic splicing silencers
RNA boxes that bind SR proteins to suppress splicing of an exon such that it is not included in the final mRNA product
What is the polyadenylation signal?
AAUAAA sequence at the end of the 3’ UTR
signals for endonuclease cleavage 10-30 nucleotides down stream
What is poly(A) polymerase?
acts on the free 3’ hydroxyl of the polyadenylation signal
adds 100-200 A residues in a template-independent fashion
What is PABP?
poly(A) binding protein
adds the poly(A) tail to the 3’ end of the transcript
What is the function of the poly(A) tail?
protect the transcript from degradation by 3’-to-5’ exonuclease
enhance translation via interaction between the 3’ end and the 5’ end of the message
What are mRNPs?
messenger ribonucleoprotein particles
composed of mRNA plus SR proteins and exon junction complex (EJC) that bind to it during transcription and the processing steps (and sometimes through nuclear export)
