Coiled internal structure of chromonema within chromosomes suggesting hierarchical coil model for chromosome structure

Abstract

Internal structure of chromonema within chromosomes at the second meiotic division in Mecostethus grossus was studied by staining the axial filament of chromonema with silver nitrate. It was found that axial filament is the coiling product of axial fiber, allowing the conclusion that the internal structure of chromonema is coiled. On the grounds of the present observation combined with other available observations on structural details of chromosomes, a new model for chromosome structure is presented. According to the model, three organizational levels above the 30 nm fiber can be identified. The first level is formed by the loop‐interloop organization of the 30 nm fiber, where interloop regions form a seemingly continuous axial fiber housing the axial DNA. The second level of organization, a chromonema, is achieved by the coiling of the 30 nm axial fiber. The coiled axial fiber forms axial filament or the axial system of chromonema. The third level of organization is the mature metaphase chromatid, which is formed by the coiling of chromonema, i. e., the model is based on the principle of hierarchical coilings of axial DNA. On the basis of the model, detailed quantitative account for package of DNA in Drosophila, human, and Lilium chromosomes is given. Since the packing of coils in both coiling cycles evidently shows species specific variation, it is obvious that there is a species specific packing ratio of axial DNA by which the total package of DNA within metaphase chromatid is achieved. Furthermore, it is suggested that in meiotic cells, only part of axial DNA stretches function as zygDNA and participate in the formation of lateral elements of meiotic prophase chromosomes.

Linking Chromatin Fibers to Gene Folding by Hierarchical Looping

“…A general paradigm of hierarchical chromatin folding has long been assumed for DNA compaction (88). However, a specific mechanism of folding has rarely been suggested beyond coiling as implied by 30-nm structures.…”

Calcium ions function as a booster of chromosome condensation

“…However, the organization of metaphase chromosomes is still controversial. There are three major models describing the formation of metaphase chromosomes from chromatin fibres4: (1) a coiled coil model, which is a stepwise progression from nucleosome fibres to 30 nm chromatin fibres to thicker chromatin fibres5; (2) a radial loop model, where loops of chromatin fibres are formed around a chromosome scaffold67; and (3) a folded fibre model, where chromatin fibres folded irregularly into chromosomes8.…”

Chromatid Cores in Meiotic Chromosome Structure and Segregation

“…In grasshopper metaphase II chromosomes, the chromatid cores, as during anaphase I, are internally located inside chromatids appearing either as straight or coiled longitudinal structures (Nokkala 1985;Nokkala and Nokkala 1986;Zhao et al 1994). Thus, anaphase I and metaphase II chromosomes present a mitotic organization identical to that found in metaphase mitotic chromosomes (compare Fig.…”