Answer: it was believed that a count of the number of protein-coding genes would provide a count of the number of proteins produced in any given eukaryotic species. This is incorrect, largely due to the discovery of widespread ALTERNATIVE SPLICING.
Explanation: Alternative splicing is a regulatory mechanism by which variations in the incorporation of the exons, or coding regions, into mRNA leads to the production of more than one related protein, or isoform. Constitutive splicing is the process of intron removal and exon ligation of the majority of the exons in the order in which they appear in a gene. Alternative splicing is a deviation from this preferred sequence where certain exons are skipped resulting in various forms of mature mRNA.Alternative splicing of RNA is a crucial process for changing the genomic instructions into functional proteins. It plays a critical role in the regulation of gene expression and protein diversity in a variety of eukaryotes. In humans, approximately 95% of multi-exon genes undergo alternative splicing.
There are several advantages of alternative splicing:
(i) it allows a high sequence flexibility of exonic regulatory sequences that puts no constrains on coding requirements.
(ii) the protein interaction can be influenced by small changes in the concentration of regulatory proteins which allows the alternative usage of exons.