Functional aspects of the longitudinal differentiation of chromosomes
Sumner, A.T.
European Journal of Histochemistry Ejh 38(2): 91-109
1994
ISSN/ISBN: 1121-760X PMID: 7524815 Document Number: 430312
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. 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 accommodate the active genes.