Develops Into New Macronucleus

â– â– TACRONUCLEUS DISINTEGRATES

The micronuclei of Paramecium are considered the germline. A typical Paramecium contains two micronuclei which go through meiosis when the cells start mating. Meiosis produces eight haploid micronuclei, seven of which disintegrate. The remaining haploid micronucleus performs one mitotic division to form two haploid micronuclei in each cell. Reciprocal exchange of the micronuclei occurs through a bridge between the two mating partners. Each cell then has one original micronuclei and one micronuclei from its mate, which fuse to give a diploid micronucleus. After the mating pair separate, the diploid micronucleus divides by mitosis. This triggers the original macronucleus to disintegrate. One of the mitotic descendents of the micronucleus then develops into a new macronucleus.

Macronucleus destined segment dsDNA of micronucleus ^

MDS IES MDS IES MDS IES t

Internal eliminated sequence

FIGURE 19.10 Internal Eliminated Segments of Ciliate DNA

Internal eliminated segments must be removed from the micronucleus DNA before the micronucleus converts into a macronucleus. Once these segments are spliced from the DNA, a fully functional gene is reconstructed.

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