Question

Describe the processes which occur during the post-transcriptional modification of eukaryote mRNA and explain why each of these processes occur.

Answer 1

Post transcriptional modifications of RNA is a process by which primary transcript RNA after transcription converted into mature RNA.

There are mainly two types of post transcriptional modifications are present to stabilise the messenger RNA or mRNA.

1. 5’ capping: 5’ end containing nucleotide with phosphategroups. One of the phosphate group leave from the end and form diphophate group at 5’ end. Then this 5’ end attacks a alpha phosphorus atom of GTP inorder to add guanine to form 5’5’ triphosphate linkage. An enzyme which helps to the process SAM (s-adenosyl methionine) methylate the guanine at N-7 position. Thus it form 7 methyl guanisine present at the 5’ end act as cap to the mRNA.
2. Poly adenylation or Poly A tail: This occur only after cleavage of mRNA. The cleavage has to occur only if specific sequences are present on the mRNA.

a. Polyadenylation singnal sequence- AAUAAA

b. The above sequence is followed by another sequence: CA is the cleavage signal.

c. Gu rich region present further downstream.

Two proteins such as cleavge and polyadenylation specificityfactor (CPSF) and cleavage stimulation factor are bind to the sequence elements. In the 3’ end with CA region the cleavage occur and add 200 units to the 3’ end by polyadenylate polymerase.

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