Split gene

An interrupted gene (also called a split gene) is a gene that contains sections of DNA called exons, which are expressed as RNA and protein, interrupted by sections of DNA called introns, which are not expressed.

The DNA sequence in the exon provides instructions for coding proteins. The function of the intron was not understood at first, and they were called noncoding or junk DNA. Split genes were independently discovered by Richard J. Roberts and Phillip A. Sharp in 1977, for which they shared the 1993 Nobel Prize in Physiology or Medicine Their discovery implied the existence of then-unknown machinery for splicing out introns and assembling genes; namely, the spliceosome. It was soon accepted that 94% of human genes were interrupted, and perhaps 50% of hereditary diseases involved errors in splicing introns out of interrupted genes. The best-known example of a disease caused by a splicing error is Beta-thalassemia, in which extra intronic material is erroneously spliced into the gene for making hemoglobin.

Lower eukaryotes, including yeast, have many uninterrupted regions, as they contain long stretches of exons that create the mRNA necessary for the synthesis of proteins. This does not mean, however, that these sections are fully uninterrupted, as tRNA synthesis requires excision of a nucleotide sequence, followed by ligation. Nevertheless, gene interruption is the rule.

Most bacteria have some interruption of some genes. Interrupted genes are universal in eukaryotes; yeasts may display single interruptions of a minority of genes, while in higher organisms most genes are interrupted, some multiple times and with introns that can be longer than exons. Introns are well-conserved across evolutionary history, suggesting their structure has some importance for the organism, and they are longer in advanced organisms (higher plants and animals), whose longer growth and development requires longer sequences of gene activation and down-regulation. Details of the role of introns in the regulation of gene accessibility and transcription have yet to be worked out.

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