Processing the RNA Transcript

A primary transcript corresponding to the full length of the gene is first made. This is then chemically modified, and introns (two in the case of /3-globin) are removed by splicing. The mixture of primary transcripts present in the nucleus is known as heterogeneous nuclear RNA (hnRNA). 

Question What modifications are effected in the primary mRNA transcript  

Polyadenylation results in the addition of a poly (A) tail of 40-200 residues at the 3 end of the transcript. The enzyme responsible for this addition is poly (A) polymerase. The function of the poIy(A) tail is unknown. 

The steps involved in the production of a functional mRNA from a typical eukaryotic gene are shown in Fig. 17-5. For this illustration, we assume that the gene has six introns, which are identified as A-F. Exons are represented as 1-7. Not shown in the diagram is the fact that there is probably a defined order for excision of the individual introns. 

Question What is the reason for the presence of introns in most eukaryotic genes  

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