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Nuclear Dimorphism
As is typical of ciliates, the nuclear apparatus of Tetrahymena
is
composed of two structurally and functionally differentiated types of
nuclei, a phenomenon known as nuclear dimorphism. The
micronucleus
(MIC) is the germline, i.e. the store of genetic information for the
sexual progeny. It is diploid and contains 5 pairs of
chromosomes. No
known genes are expressed in the MIC. Amicronucleate
Tetrahymena cells
(i.e., cells lacking a MIC) are frequently collected in nature, but in
laboratory strains of T. thermophila the loss of the MIC leads to
clone death. Only one viable laboratory-obtained amicronucleate cell
line has been described. At cell division the MIC divides mitotically
with kinetochores and intranuclear mitotic spindle.
The macronucleus (MAC) is the somatic nucleus, i.e. the nucleus
actively expressed during vegetative multiplication. No known MAC DNA
is transmitted to the sexual progeny. The MAC contains 200-300
autonomously replicating species derived from the 5 MIC
chromosomes by
site-specific fragmentation. The bulk of
these DNA species is present at the average level of 45 copies per
MAC. There are no visible structures expected for mitotic distribution
of MAC pieces, such as kinetochores or mitotic spindle. The MAC is
thus said to divide by amitosis. Approximately (but seldom exactly)
half of the MAC DNA is distributed to each daughter MAC at cell
division. Alternative allele copies of a locus segregate at random
during MAC division. Physical methods are available
to preparatively separate and purify MICs and MACs from one another.
Eduardo Orias - 2/12/97
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