Genes and non-coding DNA sequences

Many new categories of DNA sequence organization in higher organisms have been discovered in the last 20 years. Most of these are not found in bacterial and each in turn is hailed as a likely candidate for those regulatory elements which are assumed to distinguish higher from lower organisms. This article stresses the similarities between their sequence organizations and the continuing lack of convincing function for any of them. It suggests that these sequences often appear after chance recombinational events and their presence then facilitates further change. Within the constraints of proper function the genome is clearly able to find many organizational solutions of apparently equal advantage to the organism. Finally, an example of rapid genome reorganization resulting from selection for resistance to the drug methotrexate is considered.