FUNCTIONAL-ASPECTS OF THE LONGITUDINAL DIFFERENTIATION OF CHROMOSOMES

The discovery of chromosome banding techniques over 20 years ago has r evealed extensive longitudinal differentiation of chromosomes. This lo ngitudinal differentiation can be classified into four types: heteroch romatin, euchromatic bands, nucleolar organisers (NORs) and kinetochor es. The telomeres, at the ends of chromosomes, cannot be detected by b anding methods, but are clearly shown by in situ hybridisation. The fu nctions of nucleolar organisers, kinetochores, and telomeres are reaso nably well known, but the reasons for the differentiation of the great er part of the chromatin into heterochromatin and euchromatic segments remains uncertain. The function of heterochromatin may be sought in i ts centrometric location, where part of it is associated with the kine tochores, and another part appears to hold the sister chromatids toget her until anaphase. It appears that highly conserved nucleotide sequen ces are not required for these functions, but highly repeated sequence s may be necessary. Nevertheless, these functions cannot explain the w hole of heterochromatin. G-banding and other methods for euchromatic b anding have shown that the euchromatic parts of chromosomes are divide d into two major compartments, one gene-rich and the other gene-poor, which also differ in many other properties. The reason for this, which seems to be a fundamental property of chromosome organisation in euka ryotes, is totally obscure. Nevertheless, the observations that the gr eatest concentrations of genes tend to be found near the ends of chrom osomes, and that the telomeres are often located at the nuclear envelo pe, suggest that a mechanism may have evolved to ensure that active ge nes are close to the cytoplasm. The gene-poor segments of euchromatin might then function largerly as packing, to ensure an adequate nuclear volume, thereby producing sufficient nuclear surface area to accommod ate the active genes.