An Intron is a non-protein-coding sequence of DNA that is initially copied into RNA but is cut out of the final RNA transcript (mature mRNA). They are found in most multicellular eukaryotes, but are rare in housekeeping genes and RNA-coding genes. They are also rare or absent in most unicellular eukaryotes and prokaryotes, but present in the DNA or RNA of viruses that infect eukaryotic cells. In contrast to introns, the protein-coding segments of DNA are called exons. The presence of introns in many genes can cause problems with protein purification when bacteria are used to propagate a protein, as they lack the machinery to excise introns from genes. The presence of these foreign structures causes the bacteria to recognize and degrade the protein, in a specific reaction that may be part of the bacterial defense against bacteriophage infection. The gene containing the protein to be purified must therefore have its introns excised before being subcloned into a suitable plasmid. This is often done using a cDNA library.
Some introns may be included or excluded from the final RNA transcript, producing alternative versions of the protein. These variations (alternative splicing) may depend on the organ, development stage, environment and other similar factors. In humans, most of the genes (95%) that have introns also have the alternative splicing.
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- Pan, Q; Shai O; Lee LJ; Frey BJ; Blencowe BJ (Dec 2008). Deep surveying of alternative splicing complexity in the human transcriptome by high-throughput sequencing. Nature Genetics. 40 (12): 1413–1415. Free abstract available at http://www.nature.com/ng/journal/v40/n12/full/ng.259.html