splicing Flashcards
in eukaryotes, where is is translation usually initiated at?
often close to the boundary between the first and second exons
in eukaryotes, where is the termination site usually located?
just after the boundary between the last two exons
list the patterns of splicing
- exon skipping/inclusion
- alternative 3’ splice sites
- alternative 5’ splice sites
- mutually exclusive exons
- intron retention
what is a cassette exon?
an optional exon
why is it necessary to have different patterns of splicing for a gene?
splicing can vary with the tissue, development, signalling or between individuals
how can you determine if premature termination codons are being picked up deliberately?
degree of conservation
-if the termination codon is conserved, its likely intentional
what is the purpose of premature termination codons?
premature termination can lead to unstable mRNA and early decay, and is a method for regulation of level of protein present
how can drosophila make 30,000 mRNAs from a single gene?
it has many copies of exons 4,6, and 9
when transcribed, the splicing pattern will pick up only one of each
why is it suggested that drosophila can make so many different transcripts of the same gene?
these proteins are thought to mark the cell for involvement in cell-cell adhesions as a way of ensuring that axons in the nervous system do not contact each other
what conserved sequences are found at splice sites?
introns almost always begin with a GU, and end with an AG
what is meant by the polypyrimidine track?
a sequence at the 3’ splice site that is rich in pyrimidines (Us, Cs)
why is there little conservation amongst splice sites?
tightly constrained patterns do not permit alternative splicing, there must be variability in order to permit the production of multiple protein isoforms
outline formation of lariats
- at the branch site, there an A nt with an activated OH at 2’.
- after removal of the H by a base, it attacks the phosphodiester link on 5’ splice site
- this pushes off the 5’ exon and joins the A to the AG at the start of the intron
- 5’ exon is pushed back, and the proton is pulled off the OH, this then attacks to join two exons and push off the intron
how does U1-snRNA bind to the splice site?
the 5’ end of U1 binds to the 5’ splice site
what is ‘special’ about the binding of U2-snRNP to the branch site?
makes the A bulge out of the helix into a pocket of surrounding proteins which allow it to be recognised
