Introns Flashcards
What are group I introns?
Self splicers
e.g. Tetrahymena
Found in mitochondria and chloroplasts
How does self splicing occur?
A free G nucleotide cleaves the RNA; newly created 3’ end of the RNA chain then attacks the other side of intron to complete reaction
What are group II introns?
Mitochondria and chloroplasts reverse transcriptase ORF, self splicers, different splice mechanisms from eukaryote pre-mRNAs. Spliceosomal introns, tRNA
How do group II introns splice themselves?
Specially reactive A nucleotide within intron itself cleaves the RNA
What are group III introns?
Twintrons
Introns within introns - e.g. chloroplasts
Can jump, like transposable elements
What are the introns early arguments?
Exons usually compactly folded domains
Can predict intron positions based on GO plots
So introns predate prokaryotic-eukaryotic split
What are the introns late arguments?
Introns do not always separate domains e.g. ADH, globins
Slightly different positions due to ‘sliding’
Introns positions are in preferred insertion sites, are mobile and are recent acquisition
What is the evidence for introns as transposons?
Twintrons
If introns were mobile - then recent otherwise would be found in all lineages
What is the model of intron loss?
Reverse transcription of mRNA recombines into genome
Genomic deletion leaves incomplete intron
Only one example - jingwei gene of D. teissieri
What is the model of intron gain?
Intron transposition Transposon insertion Tandem genomic duplication Intron transfer Self-splicing type II intron
Why are there more introns in higher organism?
Slowly evolving species - i.e. humans - retain more ancestral introns
E.g. mice have faster generation time and few introns
Many examples of inverse relationship so not universal
What is the introns first theory?
Spliceosome found in early eukaryotes - took long time to evolve. Must have been there in earliest ancestors.
Was it lost from prokaryotes?
What is the function of snoRNAs?
Methylate rRNA
Encoded within spliceosomal introns of genes encoding chaperones and ribosomal proteins
Found in Eukaryotes and Archaea
