The chromosome field

The location of the genes for 28S and 18S ribosomal RNA can be predicted within reasonable limits independently of the phylogenetic position of a species and independently of the length of a chromosome. The position of these genes is related to that of kinetochores and telomeres. The ribosomal cistrons occur in 86.6% of the cases in the short arm, and as the arm length increases they are successively displaced so as to maintain the same relationship with the kinetochore and the telomere. These results lend support to the concept of the chromosome field, according to which there is a system of order in the eukaryotic chromosome whereby genes in a chromosome arm have their positions specified with respect to the two prime organizers, the kinetochore and the telomere. The field concept emphasizes the co-ordinated and integrated character of the chromosome as a whole, based on a mechanism of molecular messages. This concept should be contrasted with the prevailing assumption that chromosomes evolve at random and solely under the influence of natural selection. Experimental evidence supporting the chromosome field is to be found mainly in position effects and in the fusion of human cells with plant protoplasts.