Extrachromosomal DNA of pea ( Pisum sativum ) root-tip cells replicates by strand displacement

In cultured pea roots there is extrachromosomal DNA associated with cells that differentiate from the G(2) phase of the cell cycle that is absent from those that differentiate from the G(1) phase. We examined this extrachromosomal DNA by electron microscopy and found that it consisted of three types: (i) double-stranded linear molecules with single-stranded branches (74%), (ii) double-stranded molecules without branches (26%), and (iii) free single-stranded molecules. The double-stranded molecules with or without branches were similar in length, having a modal length of 10-15 mum. The free single-stranded molecules were shorter and had a mean length of 3.8 mum. The length of the branches attached to the duplex molecules was only slightly less than that of the free form. The duplex molecules with branches were interpreted as configurations reflecting an ongoing strand-displacement process that results in free single-stranded molecules. Finally, measurements on duplex molecules with multiple branches suggested that the extrachromosomal DNA may exist in the form of tandemly repeated sequences.