Paramecium vegetative cell divisions and chromosomal structure of MIC and MAC. (A) Paramecium tetraurelia showing two generative MICs and one vegetative MAC. Cell division involves mitotic separation of condensed MIC chromosomes and amitotic separation of uncondensed MAC chromosomes. While MICs and MAC divide, the nuclear envelope remains at both nuclei. (Figure courtesy of Jens Boenigk and Martin Simon.) (B) Chromosomes of the diploid MIC are large and contain centromeres and telomeres similar to canonical eukaryotic chromosomes. In addition, they consist of about 45,000 internal eliminated sequence (IES) elements and repeats (transposons, minisatellites). During macronuclear development after sexual reproduction (not shown here), telomeres, centromeres, repeats, and IESs become eliminated by different mechanisms. Although IESs are precisely excised, elimination of repeats and, presumably, centromeres is imprecise, resulting in fragmentation into heterogenous macronuclear chromosomes (with rare fusion of fragments). All macronuclear fragments show de novo telomere addition and amplification to 800n (created with BioRender [https://biorender.com]).
