Chromomere

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chromomere

[′krō·mō‚mir]
(cell and molecular biology)
Any of the linearly arranged chromatin granules in leptotene and pachytene chromosomes and in polytene nuclei.

Chromomere

 

a thickened, tightly coiled group of deoxyribonucleoprotein threads (chromonemata) that comprise the chromosome; they stain strongly with nuclear dyes. Under a microscope, the chromomere is easily discernible during the prophase of meiosis and mitosis, appearing in the form of dark granules arranged in a specific order along the chromosome thread.

The chromomeres contain up to 95 percent of the deoxyribonucleic acid (DNA) in the chromosome; the remaining 5 percent is contained in the uncoiled areas between the chromomeres. The shape, size, and number of chromomeres are strictly constant for each chromosome and form a pattern of chromomeres that is individualized according to species, tissue, and age. The chromomeres of different organisms range in size from 500 angstroms to 0.5 micron, and their DNA content ranges from 103–106 pairs of nucleotides, respectively. Very large chromomeres, called nodules, have been discovered during the prophase of meiosis in some plants; they serve as precise chromosome markers in cytogenetic research.

When giant polytene chromosomes form, their homologous chromomeres conjugate closely in pairs, forming disks whose pattern, like that of chromomeres, is specific for each chromosome. In many disks of polytene chromosomes, the loci of certain genes have been established by means of cytogenetic analysis. Traditional genetics viewed chromomeres and disks as cytological equivalents of a single gene or of several genes. Most modern cytogeneticists, on the other hand, regard chromomeres as functional units of the chromosome that include structural genes with regulatory areas. According to an opposing hypothesis, chromomeres are inactivated areas of a chromosome that are not identical to any information units.

I. I. KIKNADZE