Ultrastructure of Mammalian Chromosome Aberrations

This chapter discusses the ultrastructure of mammalian chromosome aberrations. Chromosome aberrations are induced by a variety of agents, including radiation, chemicals, viruses, temperature changes, and mycoplasms. Morphological evaluation of chromosome damage has been largely confined to light microscope observations. Although this approach has been effective in identifying and classifying the various types of aberrations expressed at metaphase, the limitations of resolution by light optics are obvious. Structures associated with damaged regions smaller than 0.1-0.2 μm are invisible. Since most clastogens interact directly or indirectly with DNA or nucleoprotein to bring about breaks and rearrangements which are later expressed in metaphase chromosomes, much greater resolution is necessary to properly evaluate damage. Progress made in recent years now permits direct observation of chromosome aberrations by both transmission and scanning electron microscopy. With the improved resolution afforded by these instruments, it should be theoretically possible to evaluate the molecular basis of chromosome damage and provide a smoother correlation of damage at the DNA and protein level with aberrations visible in the metaphase chromosome. The chapter reviews the progress made in the analysis of chromosome damage by electron microscopy, and attempts to correlate these findings with information available from light microscope studies at one end of the spectrum and molecular level of organization at the other.