Figure 29.2:Tailing process in m RNA

INTRONS SPLICING: Introns are noncoding  nucleotide sequence within a gene that don’t code for protein and don’t appear in the final mRNA molecule and are remove by the splicing.  Protein-coding sequence  of a gene (known as exons), which are interrupted by introns. The vast majority of eukaryotic genes are interrupted with non-coding a region (introns), which needs to be spliced out; however, histone protein coding genes in vertebrate is one among few exceptions. The occurrence of introns varies in eukaryotic species, some yeast species lack introns and many genes in eukaryotes carry a dozen of them. Few bacterial and Archeal genes also have introns. Introns can vary in length from 50 to 20000 nucleotides. In higher animals as humans, introns are more than exons. There are four classes of introns: (1) Group I, (2) Group II; both are self-splicing introns and does not involve any protein enzymes.(3) Spliceosomal introns- they are not self-splicing. (4) Introns that require ATP for splicing.

Splicing mechanism of group I and Group II introns: Splicing mechanism of both Group I and Group II involve similar steps of two trans-esterificastion reactions in which a ribose 2’ or 3’ hydroxyl group makes a nucleophilic attack on phosphorus and a new phosphodiester bond is formed at the expense of the old. Mechanisms of these groups differ in nucleophile which is used. Group I uses 3’ hydroxyl group of guanine nucleotide as nucleophile. Group I introns are found in some nuclear, mitochondrial, and chloroplast that code for rRNAs, mRNAs, and tRNAs. Group I and Group II  splicing mechanism are described below in figure 29.3(a) and in 2.3(b) respectively.