Intron transcription

Exon Transcript Intron Transcript Exon Transcript... 

Splicing is a processing step of the pre-mRNA to become a mature transcript. This involves the excision of intervening noncoding sequences (introns) from coding sequences (exons) by a multiple protein complex, the spliceosome. After splicing the mRNA molecule is ready for translation, since it contains a continuous sequence that encode an entire protein. 

After transcription, during RNA processing, introns are removed and the exons are ligated together to form the mamre mRNA that appears in the cytoplasm. 

The mechanisms whereby introns are removed from the primary transcript in the nucleus, exons are ligated to form the mRNA molecule, and the mRNA molecule is transported to the cytoplasm are being elucidated. Four different splicing reaction mechanisms have been described. The one most frequently used in eukaryotic cells is described below. Although the sequences of nu-... 

Figure 37-11. The processing of the primary transcript to mRNA. In this hy-potheticai transcript, the 5 (ieft) end of the intron is cut (i) and a lariat forms between the G at the 5 end of the intron and an A near the 3 end, in the consensus sequence UACUAAC. This sequence is caiied the branch site, and it is the 3 most A that forms the S -2 bond with the G. The 3 (right) end of the intron is then cut (-ll-). This reieases the iariat, which is digested, and exon 1 is joined to exon 2atG residues.

The relationship between hnRNA and the corresponding mature mRNA in eukaryotic cells is now apparent. The hnRNA molecules are the primary transcripts plus their early processed products, which, after the addition of caps and poly(A) tails and removal of the portion corresponding to the introns, are transported to the cytoplasm as mature mRNA molecules. 

In addition to affecting the efficiency of promoter utilization, eukaryotic cells employ alternative RNA processing to control gene expression. This can result when alternative promoters, intron-exon splice sites, or polyadenylation sites are used. Occasionally, heterogeneity within a cell results, but more commonly the same primary transcript is processed differendy in different tissues. A few examples of each of these types of regulation are presented below. 

V8. Vasconcelos, O., Sivakumar, K., Dalakas, M. C., Quezado, M., Nagle, J., Leon-Monzon, M., Dubnick, M., Gajdusek, C., and Goldfarb, L. G., Nonsense mutation in the phosphofructokinase muscle subunit gene associated with retention of intron 10 in one of the isolated transcripts in Ashkenzai Jewish patients with Tarui disease. Proc. Natl Acad. Sci. U.S.A. 92, 10322-10326 (1995). 

Insulin mRNA splicing rates were estimated from the rate of disappearance of the insulin preRNA signal from beta-cells treated with actinomycin D to block transcription. The two introns in mouse insulin II are not spliced with the same ef-... 

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