Figure 1

DNA content and morphology of macronuclear developmental stages. DNA content values were taken from [13]. After separation of the conjugation partners, the macronuclear anlage at an early stage (a₁) is only slightly larger than a micronucleus (m). At this stage the beginning of chromatin decompaction is observed. As macronuclear development progresses (a₂) spiralized chromosomes become visible, which subsequently develop into polytene giant chromosomes (a₃) during the first rounds of DNA amplification. In this stage excision of internal eliminated sequences, unscrambling of macronucleus destined sequences of scrambled genes and excision of transposon-like elements occur. At subsequent DNA elimination stages (e) bulk micronuclear sequences are removed resulting in a DNA-poor stage containing less than 10% of micronuclear sequences. Further rounds of DNA amplification then take place (r), resulting in a mature nanochromosomes-containing macronucleus (M). For details see [12–14].