ABSTRACT
Extrachromosomal gene amplification has been conventionally associated with the karyotypic abnormality represented by double minute chromosomes (DMs). DMs generally appear as paired, spherical chromatin structures in metaphase spreads, although they may also occur as unpaired elements. The extrachromosomal submicroscopic elements bearing amplified genes mentioned above have come to occupy an important role in studies of gene amplification in eukaryotic cells. An interesting finding was that in the control cultures maintained in the presence of MTX, there was no detectable loss of amplisomes over the 240-day period in which the experiment was carried out. No direct evidence concerning the structure of mammalian cell amplisomes is presently available, although there is a consensus favoring a circular structure. The instability of the amplisomes in the absence of selection indicates the lack of functional centromeric sequences.
