What is Intron splicing?

Intron splicing- In case of eukaryotes, 2 sequences are found during transcription.
1.Coding sequences-(Exons)expressing sequences
2.Non-coding sequences -(Introns)intervening sequences

(BOTH INVOLVES 2 TRANSESTERIFICATION REACTIONS )

GROUP I INTRONS-

• Found in some nuclear ,mitochondrial and chloroplast genes.
• Code for rRNAs,mRNAs and t RNAs.

MECHANISM - 

Ribose 2'OH or 3'-OH group makes a nucleophillic attack on phosphorous and a new phosphodiester bond is formed in the expense of the old. Guanosine cofacter is required .

GROUP II INTRONS-

• In primary transcripts of mitochondrial chloroplast mRNAs in fungi,algae and plants .
• In this case nucleophilic attack in 2'-OH group of A residue within an intron.
• As an intermediate a branch lariat structure is formed .

SPLICEOSOMAL INTRONS -
(largest class of introns)

• Found in nuclear m-RNA primary transcript .
• Spliceosome is a large protein complex.(composed of Rna-protein complexes and snRNPs)
• They are not self -splicing .

MECHANISM -

• Spliceosome complex contains many proteins and 5 r-RNAs which names as U1,U2,U4,U5,U6.
• U1-5' splice side is recognize by U1.
• U2 enter the spliceosome complex and attach to the pre-mRNA.
• Binding of U4(catalytic activity )/U6(a ribozyme) AND U5 to the pre m-RNA with the displacement of U1.
• U4 and U6 base paired with one another.
• U4 stripped away from the duplex and the portion of U6 which were paired with U4 become base pair to U2.

IN GENERAL SPLICING PROCESS -

• The non-coding sequences have a split structure in which the coding sequences are separated from it.
• The entire gene is transcribed to yield a long RNA molecule and the introns are then removed by splicing. So, only exons are joined in the m-RNA.
• The process of excision of non- coding sequences in m-RNA and joining of the coding sequences is called as intron splicing.
• Types of introns(in eukaryotic genes) - 
• 1.5'GU-AG 3'(5'GU is the splice site or donor site )and AG3' site is the splice site or acceptor site.
• 2.AU-AC(very rare class of intron as 5'GU-AG 3' are replaced by AU and AC.)A pyrimidine rich region  is present near 3'end.

( Pu = A or G; Py = C or U)

                                                   source credit-Image http://www.web-books.com/MoBio/Free/Ch5A4.htm

   

               Image source credit- http://en.citizendium.org/images/0/0d/DNA_splicing_introns_off_to_make_proteins.jpg
1.Introns which are self-splicing and do not require any cofactor for their splicing, are present in the primary transcripts of:
a. miRNA
b. Nuclear mRNA
c. tRNA
d. Mitochondrial mRNA
Ans.a

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