MadSci Network: Microbiology |
Kimmie, Great question! Paramecium is a ciliate, a group of single-celled animal-like creatures (protists) that have a rather unusual way of reproducing. Most protists (and cells in animals and plants) have a single nucleus which, once all the chromosomes have been duplicated, splits in half, along with the rest of the cell. Ciliates, however, are different: they have at least two nuclei: a large macronucleus and a tiny micronucleus. So, both nuclei must divide. But Paramecium is such a complicated cell that it can't just split in half like an amoeba; it has a mouth and an anus too, and its cilia are arranged in a complex, intricate pattern (which can be very nicely seen with a microscope if you mix some paramecium cells with india ink, carefully spread it on a slide in a thin layer, and let it air dry.) Before a Paramecium reproduces, it must first grow a second mouth, below the original one!! Then the cell divides in half, with one Paramecium above the other, like a short totem pole. Paramecium also reproduce sexually, and this is really weird. Two compatable cells join "belly to belly" and part of their cytoplasm fuses (kind of like a siamese twin). For sex, they do not need their big macronuclues: it is slowly destroyed. Their micronuclues goes through a long process where its chromosomes are reduced in number by half (a process called meiosis) and then the post-meiosis nucleus replicates, so each of the mating cells has an extra nucleus. Then, each cell trades the extra nucleus with each other! Finally, they two cells separate; both now have a pair of "his and hers" nuclei, which fuse into one, mixing the chromosomes together. Then the one nucleus divides, and the extra one grows into the macronucleus. Hard to understand, but weird and wonderful! It's actually not hard to see wild Paramecium and other ciliates (from a pond sample to which rice or barley has been added) mating; look for two similar cells stuck together side by side. By the way, most protists simply fuse when they join in sexual union (like a sperm and an egg) but ciliates have such a complicated external stucture that they had to develope this odd way of mating without totally fusing to become one cell. Pretty clever. Cheers, Dean Jacobson, protist watcher
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