Genome Plasticity and Chromosome Evolution

Abstract

All life as we know it traces back to a single common ancestor. The diversity of living organisms could not have been achieved in the 3–4 billion years they have existed on earth without a high level of genome plasticity. Here we will explore some of the aspects of this plasticity that are most relevant to understanding the behavior of human chromosomes. The common origin of all life forms was strongly supported by Charles Darwin, whose theory of evolution by natural selection provided a powerful explanation for the enormous diversity of living organisms. Genetics and molecular biology have confirmed this unity, demonstrating that all organisms store their genetic information in DNA or RNA and use a virtually universal genetic code for translating this information into protein sequences. Genes, too, show a remarkable degree of conservatism. As an example, over 70 human genes are already known that can function in yeast, substituting for the corresponding defective yeast gene (National Center for Biotechnology Information, http://www.ncbi.nim.nih.gov/Bassett/cerevisiae/index.html). A recent study in Drosophila used saturation mutagenesis of a 67-kb region to identify 12 new expressed genes. Nearly all these genes had close relatives in the human and round worm (Caenorhabditis elecjans) databases. Half were present in the yeast (Sacchiromyces cerevisiae) database, and a few were even present in the bacterial databases (Maleszka et al., 1998). This level of sequence conservation is remarkable, especially since warm-blooded birds and mammals have evolved a rather different genome organization, marked by increased heterogeneity in base composition, with highly GC-rich and GC-poor isochores and attendant changes in codon usage (Bernardi, 1995, see also Chapter 7).