Ciliates contain two types of nuclei: a micronucleus and a macronucleus. The micronucleus serves as the germ line nucleus but does not express its genes. The macronucleus provides the nuclear RNA for vegetative growth. Mating cells exchange haploid micronuclei, and a new macronucleus develops from a new diploid micronucleus. The old macronucleus is destroyed. This conversion consists of amplification, elimination, fragmentation, and splicing of DNA sequences on a massive scale. Fragmentation produces subchromosomal molecules in Tetrahymena and Paramecium cells and much smaller, gene-sized molecules in hypotrichous ciliates to which telomere sequences are added. These molecules are then amplified, some to higher copy numbers than others. rDNA is differentially amplified to thousands of copies per macronucleus. Eliminated sequences include transposonlike elements and sequences called internal eliminated sequences that interrupt gene coding regions in the micronuclear genome. Some, perhaps all, of these are excised as circular molecules and destroyed. In at least some hypotrichs, segments of some micronuclear genes are scrambled in a nonfunctional order and are recorded during macronuclear development. Vegetatively growing ciliates appear to possess a mechanism for adjusting copy numbers of individual genes, which corrects gene imbalances resulting from random distribution of DNA molecules during amitosis of the macronucleus. Other distinctive features of ciliate DNA include an altered use of the conventional stop codons.
