Paramecium, like other ciliates, stores its genetic information in two types of nuclei. There is usually one large (macro) nucleus and one or more smaller (micro) nuclei in each cell. The macronucleus is polyploid, containing up to 800 copies of each gene, and is transcriptionally active. The micronucleus is diploid and holds the cell's germline DNA, genes which are exchanged during sexual reproduction.
Paramecium reproduces asexually when food is plentiful, dividing by binary fission to form two identical daughter cells. In this process, macronucleus and micronucleus are replicated and each passed on to the daughter cell. Following prolonged periods of starvation, Paramecium undergoes the process of autogamy, during which the macronucleus breaks down while the micronucleus divides several times, giving rise to new micronuclei. One of the newly formed micronuclei then undergoes extensive genomic rearrangements, including the fragmentation of the DNA and elimination of some DNA sequences (IES: Internal Eliminated Sequences).
Paramecium also reproduce sexually during periods of starvation when two compatible mating types are present. In conjugation, two cells are joined; micronuclei undergo meiosis to produce haploid nuclei, and haploid micronuclei are exchanged between the two joined cells. A 'donor' micronucleus then fuses with a 'recipient' micronucleus in each cell, generating a new diploid micronucleus which then is replicated to produce the cell's new macronucleus and micronucleus.
This page was last edited on 08/21/01 .
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