Functional aspects of the longitudinal differentiation of chromosomes

The discovery of chromosome banding techniques over 20 years ago has revealed extensive longitudinal differentiation of chromosomes. This longitudinal differentiation can be classified into four types: heterochromatin, euchromatic bands, nucleolar organisers (NORs) and kinetochores. The telomeres, at the ends of chromosomes, cannot be detected by banding methods, but are clearly shown by in situ hybridisation. The functions of nucleolar organisers, kinetochores, and telomeres are reasonably well known, but the reasons for the differentiation of the greater part of the chromatin into heterochromatin and euchromatic segments remains uncertain. The function of heterochromatin may be sought in its centrometric location, where part of it is associated with the kinetochores, and another part appears to hold the sister chromatids together until anaphase. It appears that highly conserved nucleotide sequences are not required for these functions, but highly repeated sequences may be necessary. Nevertheless, these functions cannot explain the whole of heterochromatin. G-banding and other methods for euchromatic banding have shown that the euchromatic parts of chromosomes are divided 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 eukaryotes, is totally obscure. Nevertheless, the observations that the greatest concentrations of genes tend to be found near the ends of chromosomes, and that the telomeres are often located at the nuclear envelope, suggest that a mechanism may have evolved to ensure that active genes are close to the cytoplasm.(ABSTRACT TRUNCATED AT 250 WORDS)