Intron - RNA sequences between exons that are removed by splicing
Introns have been found in eukaryotic mRNA, tRNA and rRNA, as well as chloroplast, mitochondrial and a phage of E. coli. Eubacteria are the only species in which introns have not been found. In general, genes that are related by evolution have related organizations with conservation of the position at least some introns. Furthermore, conservation of introns is also detected between genes in related species.
The amount and size of introns varies greatly. The mammalian DHFR has 6 exons that total about 2000 bases, yet the gene is 31,000 bases. Likewise, the alpha-collagen has 50 exons that range from 45-249 bases and the gene is about 40,000 bases. Clearly two genes of the same size can have different number of introns, and introns that vary in size.
Some species will have an intron in a gene, but another species may not have an intron in the same gene. An example is the cytochrome oxidase subunit II gene of plant mitochondria where some plant species have an intron in this gene and others do not.
Features of Nuclear Splicing Junctions
EXON | INTRON | EXON | | \_/ \_/ A G G T A........C A G 64 73 100 100 62 65 100 100 Percent occurrence
Splicing of hnRNAs
U1 binds to the 5' splice site. The RNA is complementary to 4-6 nucleotides of the 5' end, but RNA cannot bind alone, it requires the proteins constituent of the particle. An important question is whether U1 is required. If the sequence of the splice site is mutated, binding to the left junction will not occur. But if the U1 RNA is altered to be complimentary to the mutation in the left junction, binding is restored.
Other snRNPs recognized other regions of RNA involved in splicing. The following is a summary table.
Copyright © 1998. Phillip McClean