Ultrastructure of cell division in the unicellular red alga Porphyridium purpureum

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Abstract
Mitosis in the unicellular red alga Porphyridium purpureum was studied with the electron microscope. During early prophase two bipartite nucleus associated organelles (NAOs) are seen in a region that will become one of the division poles. The division axis is established by the migration of one NAO. Microbodies are associated with the poles throughout the mitotic cycle. At prometaphase the nuclear envelope (NE) subjacent to each NAO forms a nuclear pocket which breaks down or opens to form a large gap. Concomitant with polar gap formation the large NAO portion proximal to the NE disperses whereas the smaller distal NAO portion remains throughout subsequent mitotic stages. At metaphase a plate arrangement of chromatin is seen and indistinct kinetochores are associated with a single microtubule. Chromatin moves to the poles followed by pronounced interzonal midpiece (IZM) elongation. After IZM abscission the nuclei migrate to opposite ends of the elongating cell. Cytokinesis occurs by means of an ingrowing cleavage furrow. A comparison of mitotic ultrastructural characteristics in Porphyridium with data available on mitosis in other red algae suggests that structural diversity may be of taxonomic significance; a comparison with other lower eukaryotes suggests that the overall features of mitosis in red algae do not support the general belief that this group is one of the most primitive of all eukaryotes.