Figure 1.

DNA replication and processing during macronuclear development in E. crassus. On the left, the two types of DNA-processing events that produce the macronuclear linear DNA molecules are diagrammed. Precursors to macronuclear sequences are designated as open boxes, where each box corresponds to a gene that will become a linear DNA molecule (0.5–20 kb in size) bearing telomeric repeats. IES are designated by black boxes for the small, unique sequence IES (SU-IES) and double arrowheads for the Tec elements (Tec IES). Junction regions lying between two mac-destined sequences and other eliminated sequences are shown as heavy black lines. The excision of Tec elements that interrupt mac-destined sequences as IES is diagrammed on the left. The extrachromosomal circular forms are designated EC. This contrasts with non-IES Tec elements, shown on the right, in which the element is juxtaposed with other eliminated sequences. These sequences replicate late and become degraded around the time of chromosome fragmentation and telomere addition. At a given chromosomal locus, which is eightfold replicated during the first S phase, not all of the Tec elements are excised, and copies with and without the element are further amplified during the second S phase. Approximately 50% of the Tec element IES are excised in this first round. During the second excision, both Tec elements and SU-IES are removed, and variability in the extent of excision exists from locus to locus. For simplicity, this variability was not included in the figure.