DNA Termini in Ciliate Macronuclei

Abstract

The ciliated protozoa are characterized by two differentiated types of nuclei found within the single cell that comprises each organism. A germinal diploid micronucleus functions both to maintain genetic continuity between generations and to carry out meiosis in the sexual cycle, conjugation. During conjugation, the micronucleus produces haploid products of meiosis, which undergo cross-fertilization between the pairs of conjugating cells. The zygotic nucleus so formed divides mitotically, and the macronucleus or somatic nucleus develops from one of the resulting mitotic daughter nuclei. In each sexual cycle, the old macronucleus is destroyed and is replaced by this newly developed macronucleus. During its development, the macronucleus becomes DNA-rich and transcriptionally active, in contrast to the transcriptionally inactive micronucleus. The macronucleus is thus responsible for gene expression and is necessary for vegetative cell growth.