Electron microscopic analysis of replicating DNA of sea urchin embryos

DNA was extracted from Paracentrotus lividus embryos at the third S phase after fertilization and analyzed with the electron microscope. The most relevant structures observed in this actively replicating DNA are clusters of short, closely spaced microbubbles (about 0.1 μm long on the average), partially or entirely single-stranded molecules and few linear forks. Unexpectedly, no long eye forms were observed. The analysis of DNA purified from gastrulae and from adult somatic tissues has revealed the same structures, although at a low frequency. A quantitative analysis has been carried out to determine the size distribution and spacing of microbubbles. A number of control experiments have been performed to characterize these structures better. Various possibilities are discussed to account for the presence of the observed forms and the absence of larger eyes.