Classification Intron

1 classification

1.1 spliceosomal introns
1.2 trna introns
1.3 group , group ii introns

classification

splicing of intron-containing rna molecules superficially similar, described above. however, different types of introns identified through examination of intron structure dna sequence analysis, genetic , biochemical analysis of rna splicing reactions.

at least 4 distinct classes of introns have been identified:

introns in nuclear protein-coding genes removed spliceosomes (spliceosomal introns)
introns in nuclear , archaeal transfer rna genes removed proteins (trna introns)
self-splicing group introns removed rna catalysis
self-splicing group ii introns removed rna catalysis

group iii introns proposed fifth family, little known biochemical apparatus mediates splicing. appear related group ii introns, , possibly spliceosomal introns.

spliceosomal introns

nuclear pre-mrna introns (spliceosomal introns) characterized specific intron sequences located @ boundaries between introns , exons. these sequences recognized spliceosomal rna molecules when splicing reactions initiated. in addition, contain branch point, particular nucleotide sequence near 3 end of intron becomes covalently linked 5 end of intron during splicing process, generating branched (lariat) intron. apart these 3 short conserved elements, nuclear pre-mrna intron sequences highly variable. nuclear pre-mrna introns longer surrounding exons.

trna introns

transfer rna introns depend upon proteins removal occur @ specific location within anticodon loop of unspliced trna precursors, , removed trna splicing endonuclease. exons linked second protein, trna splicing ligase. note self-splicing introns found within trna genes.

group , group ii introns

group , group ii introns found in genes encoding proteins (messenger rna), transfer rna , ribosomal rna in wide range of living organisms., following transcription rna, group , group ii introns make extensive internal interactions allow them fold specific, complex three-dimensional architecture. these complex architectures allow group , group ii introns self-splicing, is, intron-containing rna molecule can rearrange own covalent structure precisely remove intron , link exons in correct order. in cases, particular intron-binding proteins involved in splicing, acting in such way assist intron in folding three-dimensional structure necessary self-splicing activity. group , group ii introns distinguished different sets of internal conserved sequences , folded structures, , fact splicing of rna molecules containing group ii introns generates branched introns (like of spliceosomal rnas), while group introns use non-encoded guanosine nucleotide (typically gtp) initiate splicing, adding on 5 -end of excised intron.