Introns & splicing Flashcards
What are Introns?
a nucleotide sequence whcih is REMOVED during mRNA splicing and is not present in mature mRNA
What are Exons?
protein coding nucleotide sequences that are are NOT REMOVED during mRNA splicing and are still present in mature mRNA
where are introns found?
tRNA,mRNA,rRNA
WHat are the 4 groups of Introns?
Group I, Group II, splicesome-dependant Introns, Nuclear tRNA Introns.
What is conserved between all Intron groups?
5’ and 3’ splice site
What is unique to Group 2 and splicesome-dependant Introns?
branch site
How do Group I Introns undergo splicing?
Self-splicing using Co-factor of guainine or similar (G,GMP,GDP,GTP).
What type of enzyme does RNA act as during splicing?
ribozymes
What is the universal method of splicing between the first 3 groups of introns?
Transesterfication - the exchanging of organic R’ groups of alcohol with another organic R’ group of an ester, to form two different alcohol and ester.
What is the mechanism for splicing in Group I introns?
3’-OH of the CO-Factor acts a nucleophile and attacks the 5’ splice site, binding to it and seperating the upstream exon from the intron.
THe 3’-OH of the upstream exon acts a nucleophile attacking the 3’splice site of the downstream , which fuses the two exons.
leaving the Intron.
What occurs to Introns after splicing ?
they fold into a tertiary structure and degrade.
What is the difference in mechanism between Group I and Group II introns?
Group I introns use external Co-Factors , using the 3’-OH of a G molecule.
Group II, does not have a Co-factor, instead uses 2’-OH of adenine on the BRANCH SITE.
What is the mechanism for splicing in Group II Introns?
2’-OH on adenine on the BRANCH SITE acts as a nucleophile and attacks the 5’splice site, forming a LARIAT structure( A has 3 phosphodiester bonds, one being a 2’-5’) and seperating he upstream exon from the intron.
The 3’-OH of the guanine on the upstream exon acts a nucleophile and attakcs the 3’ splice site, fusing the two exons.
leaving the intron in its Lariat form.
What are the conditions for self splicing in Group II introns?
in vitro - high [salt]
otherwise in vivo they require enzymes, Maturase, which some produce.
Group I introns also produce maturase to increase efficiencey of splicing.
What does the presence of Group I and II introns in mitochondria in plants and animals, say about transcription in Bacteria
Mitochndria thought to have bacterial orgin, but are in fact then processed,
HOWEVER, this is self splicing.
