Steps in RNA processing: (1) Add a cap to the 5’ end, (2) Add a poly-A tail to the 3’ end, (3) splice out introns
Capping: In eukaryotic cells, m RNA is inherently unstable  at the ends.  So needs to modified  the end to protect it againsed ribonucleases, mRNA is capped so that it is protected from ribonucleases as well as it is important in binding of m-RNA to ribosome for translation; it uses certain cap binding protein complexes. Capping reaction starts soon after transcription has started. As soon as 20-30 nucleotides are formed, capping occurs. At the 5’ end  capping process occurs, a slightly modified guanine (7-methyl G) is attached “backwards”, by a 5’ to 5’ linkage, to the triphosphates of the first transcribed base. Capping reaction includes condensation of GTP with triphosphate at 5’ end followed by methylation of guanine at N-7. Further methylation occurs at 2’hydroxyls of 2nd and 3rd nucleotide adjacent to the cap which described below in Figure 29.1.

Figure 29.1: Capping process in m RNA

Tailing: Eukaryotic mRNA has series of adenosine residues ranging from 80 to 250 in number forming a poly (A) tail at 3’ end of the primary transcript. This poly (A) tail has several uses- 1) it can export mature mRNA out of the nucleus. 2) It increases stability of mRNA. 3) It serves as recognition signal for binding of translational factors during ignition of translation. The process requires template-independent RNA polymerase called as poly (A) polymerase. Tailing is described below in different steps in Figure 29.2