Introns

Effects on genome size - Eukaryotic genomes may be so large, in part, because most eukaryotic genes in higher eukaryotic organisms are interrupted by introns (see here). For example, the ovalbumin gene codes for a protein 386 amino acid residues in length, which could be accommodated by a message 1158 nucleotides long. The total ovalbumin gene is about 7700 base pairs in length, however, containing eight exons interspersed by seven introns. Introns are present in most eukaryotic structural genes and frequently exceed exons in total length. Some of the lower eukaryotes like yeast have many fewer introns, and their genome size is correspondingly smaller. The smaller yeast genome presumably reflects a need for more efficiency in cell replication.

Intron functions - The function of introns is not yet wholly understood. It seems likely that they serve as loci for genetic recombination, allowing functional parts of proteins to be interchanged in evolution. Such loci also allow some eukaryotes to make variants of a protein from a single gene, by splicing different exons together. Alternative splicing is a more efficient way to store information than having a whole gene for each variant of the protein.

See also: Size of the Eukaryotic Genome

INTERNET LINKS:

> 1. Group I Intron Database

> 2. Splice Sites

> 3. Spliceosome