UNUSUAL PATTERN OF MITOSIS IN THE FREE-LIVING FLAGELLATE DIMASTIGELLA-MIMOSA (KINETOPLASTIDA)

The three-dimensional ultrastructural organization of the mitotic appa ratus of Dimastigella mimosa was studied by computer aided, serial-sec tion reconstruction. The nuclear envelope remains intact during nuclea r division. During mitosis, chromosomes do not condense, whereas intra nuclear microtubules are found in close association with six pairs of kinetochores. No discrete microtubule-organizing centers, except kinet ochore pairs, could be found within the nucleus. The intranuclear micr otubules form six separate bundles oriented at different angles to eac h other. Each bundle contains up to 8 tightly packed microtubules whic h push the daughter kinetochores apart. At late anaphase only, midzone s of these bundles align along an extended interzonal spindle within t he narrow isthmus between segregating progeny nuclei. The nuclear divi sion in D. mimosa can be described as closed intranuclear mitosis with acentric and separate microtubular bundles and weakly condensed chrom osomes.