[Variability of the highest levels of chromosome organization in vertebrates].

Biochemical assay of the length of the element of the highest level of compactization of deoxyribonucleoprotein fibril, i.e. the loop, in interphasic chromosomes of the liver of some animals (rat, cat, hen, pigeon, tortoise, frog) reveals significant variation of the magnitude of this value. The smallest length of the loop among the organisms investigated was found in tortoise chromosomes--5--6 X 10(3) nucleotide pairs. The length of the loop structures in mammalian chromosomes (rats, cats) is equal to 45-75 X 10(3) nucleotide pairs. In birds and amphibians, chromosomes exhibit an intermediate size of the loop element as compared to chromosomes from mammals and reptiles. Possible connection between the length of the loop structures and chromosomal diameter is discussed together with the "anomalous" size of chromosomal loops in the frog.