Question

Describe the 3 steps of pre-mRNA processing for a Eukaryote. For each step, include any relevant enzymes,molecules, and locations within the cell.

Answer 1

Ans. Transcription : it is the process in which formation of pre- mrna from DNA. In eukaryotes , pre-mrna forms first (hnRna) which convert into mature mRna after removing introns (which are not code any protein).

Location: Eukaryotes transcription is carried out in nucleus.

Transcription proceeds into three steps: initiation, elongation or termination . Then form pre - mrna . Pre-mrna processing in Eukaryotes.

pre-mrna processing :

The three most important steps of pre-mrna processing are the addition of stabilizing and signalling factor at the 5' and 3' end of molecule

(A) .Capping : modification of 5' end

Capping at 5' end occurs rapidly after the start of transcription . An unusual nucleotide ( methyl guanosine triphosphate ) is added to 5' end of hnRna . It is catalysed by guanyl transferase. Cap is essential for formation of mrna- ribosome complex. Translation is not possible if cap is lacking because cap is identified by18srRna of ribosome unit.

(B). Tailing: tailing is the addition of adenylate residues about 200-300 at 3' end in a template - independent manner on newly formed hnRna with the help of POLY A POLIMERASE .

(C) splicing : it is the process of removal of interons and joining of exons in a defined order. Interim are removed by small nuclear Rna(snRna) and protein complex called small nuclear ribonucleoprotein or SnRnps . ( Snurps) .

The fully processed hnRna is now called mRna and it is transported out of the nucleus for translation .