Chromosome chromatid rearrangements resulting from the mitotic crossing-over between sister-chromatids in ring chromosomes of Creptis capillaris

Summary All radiation-induced aberrations in dry seeds of Crepis capillaris are chromosome rearrangements. The main types of chromosome rearrangements in the above tests were asymmetrical and symmetrical exchanges, ring chromosomes and ring deletions. The majority of ring chromosomes is of a chromosomes type which brings about paired rings. Fig. 1 presents the mechanism of the production of the paired rings. In a number of cases the structure of rings proved to be quite unexpected. Among middle size rings single rings proved to occur in 18.8%, among microrings-1.9% cases. Somewhat fewer are presented by pairs of rings one inside the other. The large rings present complex figures made by tangled chromatids. Two rings make one due to mitotic crossing-over between sister-chromatids (Fig. 5). Double crossing-over would lead to the exchange of part of material between two independent rings or to one ring being thrust into the other due to different strand positions in two points of the exchange. Large rings is the provision of complicated exchanges.     

Cytological demonstration of mitotic crossing-over in man.

Quadriradial (QR) configurations from four different human lymphocyte metaphase samples have been analyzed: a patient with Fanconi's anemia; normal female cells X-irradiated with 150 or 200 R in S or G2; spontaneous QRs occurring in 13,584 metaphases; and cells from two sibs with Bloom's syndrome. That mitotic chiasmata are caused by crossing-over rather than by random breakage and reunion was concluded from the following observations: (1) In the spontaneous sample, mitotic chiasmata are about as frequent as all other QRs together. (2) The frequencies of mitotic chiasmata and of other QRs are not correlated in the different samples. (3) The break points in other QRs are situated at random relative to chromosome length, whereas the distribution of chiasmata is highly nonrandom. (4) Although the centromeres of chromatid translocations occur in alternate and adjacent positions with approximately equal frequencies, there are very few adjacent counterparts to mitotic chiasmata. These can best be interpreted as a result of an abnormal U-type rejoining of chromatids in a chiasma. (5) Chiasmata found in heteromorphic chromosome pairs show that crossing-over has, indeed, taken place.     

The origin and behavior of two isodicentric bisatellited chromosomes.

Karyotyping revealed three cell lines in a boy with mental retardation and few other abnormalities. Thirty cells exhibited a normal karyotype, and 54 had an extra acrocentric chromosome of E group size with satellites on the long and short arms. The remaining 20 cells each had, in addition to the first marker (M1), a second tiny bisatellited chromosome (M2). C-banding demonstrated that both markers were dicentric. G-, C-, and Q-banding and satellite association data were consistent with the markers having originated from chromosome 15 material. We propose that M1 was formed from a meiotic breakage and a chromatid fusion in the proximal long arms of an acrocentric pair. This would have produced a symmetrical isodicentric chromosomes, plus one or two acentric fragments. M2 then could have resulted from a dicentric bridge-break-synthesis-reunion phenomenon. This model of abnormal meiotic exchange can be generalized to encompass the formation of other dicentric isochromosome cases of isochromosome X.     

Structure of mitotic chromosomes

1. Download : Download high-res image (194KB)
2. Download : Download full-size image

     

High-resolution HKG-banding in maize mitotic chromosomes

Root-tip maize chromosomes (2n=20) were prepared by high-resolution procedures after ethidium bromide/colchicine synchronization. Using HKG-banding (HCl−KOH-Giemsa), that shows both centromeric and intercalary heterochromatin, the banding pattern of the elongated-chromosomes showed one to nine well-resolved dark bands. Differences of HKG-banding pattern of elongated and compacted chromosomes were performed by image analysis.     

Twinning and mitotic crossing-over: some possibilities and their implications

Mitotic crossing-over does occur in man and is much more frequent and important than generally assumed. Its postzygotic occurrence before an embryo differentiates into MZ twins is theoretically predicted to have disrupting effects on genomic imprinting and cis-acting sequences, with consequences ranging from early lethality to MZ twin discordance. Some predictions are at odds with classical views on twinning and include a high discordance rate of MZ twins for some genetic diseases. A review of MZ twin discordance and an attempt at explaining some of the data lead one to hypothesize both the existence of a sex differences in the rate of mitotic crossing-over and the impossibility for crossed X chromosomes to undergo inactivation. The close interrelationship of twinning and midline malformations further suggests a major role of mitotic crossing-over in the induction of the twinning process itself. The model can be tested with molecular methods and provides a new approach for the gene mapping of so-called multifactorial diseases and of rarer disorders with apparently irregular inheritance.     

Micromechanics of human mitotic chromosomes

Eukaryote cells dramatically reorganize their long chromosomal DNAs to facilitate their physical segregation during mitosis. The internal organization of folded mitotic chromosomes remains a basic mystery of cell biology; its understanding would likely shed light on how chromosomes are separated from one another as well as into chromosome structure between cell divisions. We report biophysical experiments on single mitotic chromosomes from human cells, where we combine micromanipulation, nano-Newton-scale force measurement and biochemical treatments to study chromosome connectivity and topology. Results are in accord with previous experiments on amphibian chromosomes and support the 'chromatin network' model of mitotic chromosome structure. Prospects for studies of chromosome-organizing proteins using siRNA expression knockdowns, as well as for differential studies of chromosomes with and without mutations associated with genetic diseases, are also discussed.     

Organization of human mitotic chromosomes

Summary Examinations of human mitotic chromosomes using an electron microscope since the last review in Humangenetik (Schwarzacher, 1970) were summarized. Three methods were used for preparation: ultrathinnsectioning, spreading- and critical point drying and a method for comparing cells in the light and electron microscope.These three methods showed that fibrils are the main elements of organization of chromosomes. Fibrils with a diameter of 20–40 Å, of 100 Å, of 250 Å and thick fibrils (bundles) of 500–1000 Å thickness were described.A comparison of chromosomes in the light and electron microscope showed, that metaphase chromosomes can be characterized by the number of their primary coils.Examinations of Giemsa-banding techniques with electron microscope showed fibrils as being clearly visible. G bands are coils of thick fibrils (up to 1000 Å).The methods based on these new results were discussed.

---

Zusammenfassung Es wurde der Stand der Untersuchung menschlicher Mitosechromosomen im Elektronenmikroskop seit der letzten in Humangenetik erschienenen zusammenfassenden Arbeit (Schwarzacher, 1970) behandelt. Drei Methoden wurden bei der Präparation angewandt: Ultradünnschnittechnik, Spreitungs- und Kritischer-Punkt-Trocknungstechnik und vergleichende licht- und elektronenmikroskopische Methode.Alle drei Methoden zeigten, daß Fibrillen wesentlich am Bau von Chromosomen beteiligt sind. Es wurden Fibrillen mit einem Durchmesser zwischen 20 und 40 Å, Fibrillen mit ca. 100 Å, Fibrillen mit 250 Å und dicke Fibrillen (Bündel) mit 500–1000 Å Durchmesser beschrieben.Vergleichende licht- und elektronenmikroskopische Techniken zeigten, daß Metaphasechromosomen durch ihre Primärwindungen zu charakterisieren sind.Untersuchungen der Giemsabandentechniken im Elektronenmikroskop ergaben, daß Fibrillen deutlicher zur Darstellung kommen. G-Banden imponieren als Coils aus dicken Fibrillen (bis 1000 Å) aufgebaut.Aus den neuen Befunden resultierende Modelle werden diskutiert.