extrachromosomal DNA circles

Iuval clejan clejan at mindspring.com
Wed Apr 25 14:16:10 EST 2001

I haven't read it yet. My guess is these are from proliferating cells,
as I don't know any other process besides DNA replication which can give
rise to them. But they are definitely telomeric sequences.
Here is the abstract and reference:
Oncogene 1998 Dec 31;17(26):3455-61

Telomeric repeats on small polydisperse circular DNA (spcDNA) and
genomic instability.
 Regev A, Cohen S, Cohen E, Bar-Am I, Lavi S.

 Department of Cell Research and Immunology, The George S. Wise Faculty
of Life Sciences, Tel Aviv University, Israel.

                       Small polydisperse circular DNA (spcDNA) is a
heterogeneous population of extrachromosomal circular molecules present
in a large
                       variety of eukaryotic cells. Elevated amounts of
total spcDNA are related to endogenous and induced genomic instability
in rodent and
                       human cells. We suggested spcDNA as a novel
marker for genomic instability, and speculated that spcDNA might serve
as a mutator.
                       In this study, we examine the presence of
telomeric sequences on spcDNA. We report for the first time the
appearance of telomeric
                       repeats in spcDNA molecules (tel-spcDNA) in
rodent and human cells. Restriction enzyme analysis indicates that
                       molecules harbor mostly, if not exclusively,
telomeric repeats. In rodent cells, tel-spcDNA levels are higher in
transformed than in
                       normal cells and are enhanced by treatment with
carcinogen. Tel-spcDNA is also detected in some human tumors and cell
lines, but not
                       in others. We suggest, that its levels in human
cells may be primarily related to the amount of the chromosomal
telomeric sequences.
                       Tel-spcDNA may serve as a unique mutator, through
specific mechanisms related to the telomeric repeats, which distinguish
it from the
                       total heterogeneous spcDNA population. It may
affect telomere dynamics and genomic instability by clastogenic events,
alterations of
                       telomere size and sequestration of telomeric

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