Abstract

The presence of small polydisperse extrachromosomal circular (spc) DNAs composed entirely of chromosomal sequences seems to reflect the plasticity of eukaryotic genomes. The size distribution and number of spc DNAs is found to vary with development, growth state and mitotic capacity. In particular, spc DNAs are observed to be several fold smaller in established immortal cell lines than in diploid cells with a limited life span. Analysis of cloned spc DNA fragments revealed that: (1) most of the spc DNAs thus far investigated contain repetitive sequences; (2) some of the repetitive sequences are enriched in spc DNA clones relative to their representation in the chromosome; (3) various repetitive sequences of different reiteration frequency were found in spc DNA from immortal cell lines while a particular sequence of retroposon was predominant in primary lymphoid tissue cells and stem cells; (4) most of these spc DNAs appear to represent the extra copies generated by replicon‐misfiring in immortal cell lines or by retrotranscription in stem cells, and intermediates or by‐products produced by abortive rearrangement in diploid cells with a finite replicative capacity.