Many scientists have argued that the null hypothesis should be splicing noise, putting the burden of proof on those who claim biologically relevant alternative splicing. According to those scientists, the claim of function must be accompanied by convincing evidence that multiple functional products are produced from the same gene. Biological functions and evolution
While introns do not encode protein products, they are integral to gene expression regulation. Some introns themselves encode functional RNAs through further processing after splicing to generate noncoding RNA molecules. Alternative splicing is widely used to generate multiple proteins from a single gene. Furthermore, some introns play essential roles in a wide range of gene expression regulatory functions such as nonsense-mediated decay and mRNA export.The biological origins of introns are obscure. After the initial discovery of introns in protein-coding genes of the eukaryotic nucleus, there was significant debate as to whether introns in modern-day organisms were inherited from a common ancient ancestor (termed the introns-early hypothesis), or whether they appeared in genes rather recently in the evolutionary process (termed the introns-late hypothesis). Another theory is that the spliceosome and the intron-exon structure of genes is a relic of the RNA world (the introns-first hypothesis). There is still considerable debate about the extent to which of these hypotheses is most correct. The popular consensus at the moment is that introns arose within the eukaryote lineage as selfish elements.Early studies of genomic DNA sequences from a wide range of organisms show that the intron-exon structure of homologous genes in different organisms can vary widely.
Tandem genomic duplication is the only proposed mechanism with supporting in vivo experimental evidence: a short intragenic tandem duplication can insert a novel intron into a protein-coding gene, leaving the corresponding peptide sequence unchanged. This mechanism also has extensive indirect evidence lending support to the idea that tandem genomic duplication is a prevalent mechanism for intron gain. The testing of other proposed mechanisms in vivo, particularly intron gain during DSBR, intron transfer, and intronization, is possible, although these mechanisms must be demonstrated in vivo to solidify them as actual mechanisms of intron gain. Further genomic analyses, especially when executed at the population level, may then quantify the relative contribution of each mechanism, possibly identifying species-specific biases that may shed light on varied rates of intron gain amongst different species. See also
Structure:

Exon
mRNA
Eukaryotic chromosome fine structure
Small t intronSplicing:

Alternative splicing
Exitron
Minor spliceosome
OutronFunction

MicroRNAOthers:

Exon shuffling
Intein
Interrupted gene
Noncoding DNA
Noncoding RNA
Selfish DNA
Twintron
Exon-intron database

References
External links

A search engine for exon/intron sequences defined by NCBI
Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter Molecular Biology of the Cell, 2007, ISBN 978-0-8153-4105-5. Fourth edition is available online through the NCBI Bookshelf: link
Jeremy M Berg, John L Tymoczko, and Lubert Stryer, Biochemistry 5th edition, 2002, W H Freeman. Available online through the NCBI Bookshelf: link
Intron finding tool for plant genomic sequences
Exon-intron graphic maker