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Exons are the coding regions of genes that remain in mRNA after splicing and are translated into proteins.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Post-Transcriptional Modifications - Capping Splicing Polyadenylation
Major class introns removed by spliceosome share highly conserved terminal dinucleotides, GU at 5' end and AG at 3' end, recognized by U1 and U2 snRNPs respectively. These U2-type GU-AG introns constitute vast majority, over 99% of introns in eukaryotic nuclear pre-mRNA, distinguished by consensus sequences including branch point adenine. They reside in protein-coding genes transcribed by RNA polymerase II in nucleus, removed co-transcriptionally. Minor U12-dependent AU-AC introns represent rare variant. GU-AG is absent from tRNA introns or group I/II self-splicing introns found in organelles.
Ref: Steitz and Moore, RNA Splicing, GU-AG Introns in Eukaryotic Nuclear pre-mRNA, NCBI Bookshelf
Most vertebrate genes contain introns spliced by major U2-dependent spliceosome recognizing GU-AG boundaries. However, a small class of genes, notably many interferon family members, histone genes and some GPCR genes, lack intervening sequences, possessing continuous open reading frame from start to stop codon. Human interferon-alpha, beta and gamma genes on chromosome 9 are intronless, their mRNA sequence identical to genomic DNA except for cap and polyA addition. Absence of splicing allows rapid induction during viral infection, bypassing spliceosome assembly and enabling immediate translati
Ref: Nagata et al., Nature 1980, Human Fibroblast Interferon Gene Lacks Introns, Nucleic Acids Res.
Eukaryotic gene expression in bacterial system Escherichia coli faces multiple barriers collectively contributing to failure. Bacterial RNA polymerase cannot splice introns as bacteria lack spliceosome, producing nonfunctional transcripts. Post-translational modification systems for glycosylation, phosphorylation, and formation of correct disulfide bonds in endoplasmic reticulum are absent. Cytoplasmic milieu, chaperone repertoire, and codon usage diverge significantly, causing improper folding, codon bias mediated stalling, and proteolysis. Consequently, most complex eukaryotic proteins aggre
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.
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