12. Summary

2.12 Summary

Genes and genomes can be mapped by the use of overlapping restriction fragments. Ultimately this can be extended into a sequence. Restriction sites can be used as genetic markers. The existence of polymorphisms (RFLPs) allows linkage maps to be constructed using restriction fragments.

All types of eukaryotic genomes contain interrupted genes. The proportion of interrupted genes is low in yeasts and increases in the lower eukaryotes; few genes are uninterrupted in higher eukaryotes.

Introns are found in all classes of eukaryotic genes. The structure of the interrupted gene is the same in all tissues, exons are joined together in RNA in the same order as their organization in DNA, and the introns usually have no coding function. Introns are removed from RNA by splicing. Some genes are expressed by alternative splicing patterns, in which a particular sequence is removed as an intron in some situations, but retained as an exon in others.

Positions of introns are conserved when the organization of homologous genes is compared between species. Intron sequences vary, and may even be unrelated, although exon sequences remain well related. The conservation of exons can be used to isolate related genes in different species.

The size of a gene is determined primarily by the lengths of its introns. Introns become larger early in the higher eukaryotes, when gene sizes therefore increase significantly. The range of gene sizes in mammals is generally from 1 V100 kb, but it is possible to have even larger genes; the longest known case is dystrophin at 2000 kb.

Some genes share only some of their exons with other genes, suggesting that they have been assembled by addition of exons representing individual modules of the protein. Such modules may have been incorporated into a variety of different proteins. The idea that genes have been assembled by accretion of exons implies that introns were present in genes of primitive organisms. Some of the relationships between homologous genes can be explained by loss of introns from the primordial genes, with different introns being lost in different lines of descent.