Figure 1.

Different layers of DNA and RNA processing in ciliates. A schematic drawing of how germline (micronucleus) information is passed to soma (macronucleus). Genes, or macronuclear destined sequences (MDSs, large white open boxes), in micronuclear DNA are separated by internal eliminated sequences (IESs, thick black lines), flanked by pairs of direct repeats (grey boxes). Intergenic noncoding sequences are indicated by thin lines. After extensive DNA processing, IESs and intergenic noncoding sequences are deleted, MDSs are sewn together with one copy of each direct repeat retained. Telomeres (hatched boxes) are added to the ends and each macronuclear chromosome undergoes different levels of replication. mRNA transcribed from macronuclear chromosomes is capped (solid oval), polyadenylated, and a representative intron (small white box) is deleted.