The Organization of genomic DNA in mitotic chromosomes: A novel view

In 1878, W. Flemming discovered a nuclear substance that was visible on staining under the light microscope and named it 'chromatin', which is the basic unit of genomic DNA organisation. During cell division, chromatin forms into microscopic bodies, called 'chromosomes', which ensures the transmission of the duplicated genomic DNA. The term 'chromosome' is derived from the Greek for 'coloured body', reflecting the observation that a condensed chromosome is clearly visible with dyes. Long before the discovery of DNA as the genetic material, the mitotic chromosome has fascinated biologists as being a candidate structure involved in heredity. The basic mitotic chromosome structure is therefore expected to be well conserved among eukaryotes, although some minor differences may be found between different groups of organisms. In this chapter, we provide a new insight into mitotic chromosome structure with an historical background: The available evidence suggests that mitotic chromosomes essentially consist of irregularly-folded nucleosome fibers (beads-on-a-string) without a 30-nm chromatin fiber. We also discuss structural differences between plant and mammalian chromatin.