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The World of Protozoa, Rotifera, Nematoda and Oligochaeta

Ref ID : 4696

Stefan Radzikowsk; Replication, Division and Mechanisms Controlling the Variable DNA Content in the Heteromeric Macronucleus of Chilodonella steini (Ciliata). Arch.Protistenk 130:381-396, 1985

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In the macronucleus of Chilodonella steini the following regions can be distinguished: 1) the inner zone (paramere) with chromatin exhibiting a fibrillar ultrastructure and medium electron-density, containing a centrally located endosome in the form of a cluster of electron-dense material, and 2) the outer zone (orthomere) with chromatin in the form of electron-dense granules of different size and numerous nucleoli. It was found that there are two stages of DNA replication, the first beginning in the paramere and the second in the orthomere. DNA replication of the orthomere is continued after DNA replication of paramere has ended. A part of the DNA labelled with 3-H-TdR (3H-Thymine-deoxyribose) in one generation in the paramere is found in the orthomere and endosome in the subsequent generation, while DNA labelled in the orthomere remains at the place of its replication without mixing with DNA of the paramere. Some of this DNA, however, moves to the endosome. With an average generation time of 12 hr the duration of the different phases of the cycle is as follows: for the macronucleus, G1=5.5 hr, S=5.5 hr, G2=1 hr, M=0.5 hr, and for micronucleus, G1=1 hr, S=1 hr, G2=10 hr, M=0.5 hr. It was found that differences in DNA contents of sister macronuclei are due to unequal division of the DNA of the orthomere, and also that the DNA of the orthomere is subject to considerable quantitative fluctuation depending on the state of the culture, unlike the DNA of the paramere. The nature of DNA occuring in the orthomere is discussed, and it appears to be rDNA.