[Ultrastructure of human chromosomes].

The structural elements of chromosomes are chromatin fibrils of about 100 A diameter (see the preceeding paper by NOLL). The chromosomes of most eukaryontic species are single stranded. Each chromatid consists of only one chromatin fibril and thus only of one DNA-double helix. In interphase the chromatin fibril is loosely and irregularly folded. With the onset of mitosis it becomes more densely but still irregularly packed to form the long and thin prophase chromosome. Metaphase chromosomes are formed by a coiling of the thin prophase chromatids (major coils). Chromosome banding can be produced with the aid of specific fluorescent dyes or by special pretreatments before staining. The banding is due to many factors but an important one is the different AT- ad GC-content of bandlike regions. In very thin chromosomes such as premature condensed chromosomes the bands are very fine and can be correlated in size to the chromomeres of polytene giant chromosomes.