Describe the three modifications necessary during eukaryotic

Describe the three modifications necessary during eukaryotic transcription for an mRNA to be exported from the nucleus?

Solution

mRNA Processing

Eukaryotic mRNAs are synthesized by Pol II as precursors (pre-mRNAs) that undergoes extensive processing before it is ready to be transported from the nucleus for translation. The three most important steps of pre-mRNA processing are the addition of stabilizing and signaling factors at the 5\' and 3\' ends ( addition of a 5\' cap and addition of a 3\' poly-adenylation tail )of the molecule, and the removal of intervening sequences ( exons ) that do not specify the appropriate amino acids.

5\' End Capping

Post-transcriptional processing of the 5\' end of the RNA product of DNA transcription comes in the form of a process called the 5\' cap. It involves the addition of 7-methylguanosine (m⁷G) to the 5\' end. Terminal 5\' phosphate is removed by phosphatase enzyme. Guanosyl transferase produces the diphosphate 5\' end. 5\' diphosphate then attacks the alpha phosphorus of a GTP to add the guanine residue in a 5\' to 5\' triphosphate link. The guanine-N⁷-methyltransferase transfers a methyl group from S-adenosyl methionine to the guanine ring. This is called capping. Cap protects the nascent mRNA from ribonuclease attack. In addition, initiation factors involved in protein synthesis recognize the cap to help initiate translation by ribosomes.
3\' Poly-A Tail

The pre-mRNA is cleaved by an endonuclease-containing protein complex and then polyadenylated at 3\' end. About 250 adenine residues to form a poly A tail. Polyadenylation signal consensus sequence,5\'- AAUAAA-3\', is found near the 3\' end, which is followed by a GU rich sequence. An enzyme called poly A polymerase (PAP) is part of the same protein complex that cleaves the pre-mRNA with the help of another two protein cleavage and polyadenylation specificity factor (CPSF) and cleavage stimulation factor (CStF). Cleavage site marked by the 5\'-CA-3\' sequences. PAP immediately adds a string of approximately 200 – 250 A nucleotides using ATP as a precursor. It is called the poly (A) tail. The poly A tail protects the mRNA from nuclease mediated degradation, aids in the export of the mature mRNA to the cytoplasm.

Splicing

Splicing is a process by which noncoding portions of mRNA are removed and coding portions are joined to form a continuous chain of protein coding regions. Noncoding parts are called introns and coding parts are called exons. Splicing reaction is catalyzed by a large protein complex called the spliceosome. snRNP or small nuclear ribonucleoparticles, and pronounced \"snurps\",molecules also involved to recognize splice sites.