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3. Beskrive forekomsten af introns i gener
Devlin, s.74-75, fig. 2.54
Stryer, s.137
 

Introns - non-coding intervening sequences found in eukaryotic genes.

• The average gene has 3-4 introns and a molecular weight of ca. 50 kDa.

• The intron length varies from 50 to 10.000 nucleotides.

The introns are removed during processing of the RNA transcript by splicing.

The biological role of introns is not clear. Their presence in eukaryotes may represent a stage in evolution of genes, since introns are rare in prokaryotes.

Introns in prokaryotes were lost in the evolution because there organisms became optimized for very rapid growth.

Introns in eukaryotes may have arisen relatively recently in evolution as a result of migration of mobile DNA elements from other parts of the genome and their insertion into protein coding genes. Through mutation, these inserts may have subsequently lost their mobility.

One of the hypotheses about the function of introns is that they helped in the mixing and matching exons under the evolution, so that new protein coding genes should be created.  

Some intron examples:

1. the collagen gene contains more then 50 different introns, all in all, 90% of the gene.

2. the dystrophin gene, the biggest in the human genome, has 79 introns with average length of 30 kb. 99% of the total length of the gene is made up of introns. 

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