Chromomeres represent discrete structures in chromosomes that are visible under the light microscope in mitotic or meiotic prophase (see Chromatid). Electron microscopy and staining indicate that chromomeres represent regions of move complex, higher order structure than the 30-nm chromatin fiber (1). It has been suggested that they represent sites of the preferential chromatin compaction that occurs early in the prophase, coincident with the separation of sister chromatids (2). However, electron microscopic images of native chromatin in the interphase nuclei at physiological ionic strengths reveal that chromomeres may be identical to globular clusters of nucleosomes [(superbeads)] that probably represent local coiling of the 30-nm fiber. It is also possible to detect bead-like discontinuities in the chromatin fiber in sectioned nuclei. Under certain conditions, these superbeads are remarkably uniform and contain between 8 and 48 nucleosomes. This suggests that chromomeres represent discrete structural units (3). The failure to find a ubiquitous organization of chromatin into chromomeres or superbeads, however, indicates that all chromatin does not adopt this form of higher order structure in interphase nuclei in vivo. This irregularity may in fact represent the true state of affairs within the nucleus, because, clearly, eukaryotic DNA is not packaged into structures of a crystalline order and stability.
Chromomeres are useful for discriminating between different chromosomes. They are arranged in a specific and consistent pattern along each chromosome. In lampbrush chromosomes, chromomeres occur where there are long regions of inactive chromatin that are consequently compacted into higher order structures.
