Post transcriptional modification, Study notes of Cellular and Molecular Biology

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4 . Molecular Biology and Biotechnology AL camping - add 7-methylguanylate (m’G) to the 5’ end. 2. Polyadenylation - add a poly-A tail to the 3’ end. 3. Splicing - remove introns and join exons. 5’-Capping . Capping occurs shortly after transcription begins. The chemical structure of the “cap” is shown in the following figure, where m’G is linked to the first nucleotide by a special 5’-5' triphosphate linkage. In most organisms, the first nucleotide is methylated at the 2’-hydroxyl of the ribose. In vertebrates, the second nucleotide is also methylated (Fig. 7.7)- 7-methyiguanylate | Nt SENS (me) \t 6 3 "oe oO 9 | R376 N “O-F=0 y KN 5 ° ‘ 5-5 1 B triphosphate ° feo “o—P=0 linka “4 I * o-f=0 HC c 9 ° \ / I | HC — at “O—P=0 « “ = | | o-f=0 Oo OH ° ° » dy, 1st base | “o—P=0 |-°s] | \ f ? 5’ cappi | HO 6H 2 capping 5 CH, 1st base | | | -° J Oo (OH ao“ i HC c “o—P=0 f { HC — CH ; i o— gi cH, ° 2nd base 0 I I- AI ~h=o Ho c \ 9 3° HC — CH ! ! | ° OH “O—P=0 | | >? = ' Fig. 7.7. Modifications at the 5’ end of newly.tormed RNA. 7-Polyadenyiation A stretch of adenylate residues are added to the 3” end. The poly-A tail contains ~ 2°?! a + m iF poly tail contams fesidues in mammals, and ~ 100 in yeasts (Fig. 7.8). Scanned by CamScanner