Strandedness of Vicia faba chromosomes as revealed by enzyme digestion studies

Strandedness of Chromosomes From Two Species of Vicia

Recent experiments (1) have revealed subchromatid structure in chromosomes of Vicia faba after trypsin digestion. Although the chromosome number is the same in V. faba and another member of this genus, V. sativa. V. faba has been found to have approximately 5 times the amount of DNA per nucleus as V. sativa (2). Because it is unlikely that V. faba has genetic loci not represented in sativa in numbers sufficient to account for the noted differences in DNA content, it seems probable that the differences are a reflection of redundancy of the DNA in V. faba, either laterally, to produce true multistrandedness, or linearly, to produce a longer singlestranded chromosome. In an effort to investigate this possibility, chromosomes from the two species were studied with both light and electron microscopy to compare the apparent strandedness of the two species both untreated and after trypsin digestion.

On the nature of visible chromosomal gaps and breaks

From the earliest days of chromosomal aberration studies, the distinction, nature and origin of light-microscope observed "gaps" and "breaks" have been topics for debate and controversy. In this paper we survey, briefly, the various ideas that have appeared in the very extensive literature, and attempt to evaluate them in the light of our current understanding of chromosome structure and aberration formation. Attention is drawn to the problems of interpretation caused by G2/S cell imprecision.

Revelling in cytogenetics

A fascination with chromosomes, especially chromosomal aberrations, linked many at the Oak Ridge National Laboratory and the Medical Research Council Radiobiological Research Unit at Harwell, but they disagreed about the correctness of Revell's exchange hypothesis. This is a personal story of how a critical test of this hypothesis was conceived, with glimpses of life in the two centres.

A fractal model of chromosomes and chromosomal DNA replication

With the aim of clarifying topological problems involved in the process of chromosomal DNA replication, a fractal model of chromosomes was built based on the assumption that a part of a chromosome, e.g. a radial loop, is similar in shape to a whole chromosome and each radial loop represents structures in the lower-order organization (an assumption of self-similarity). Several other assumptions used include (i) one continuous DNA fiber makes a whole chromosome (a unineme hypothesis), (ii) in situ DNA exists in the form of a double duplex or a tetraplex which is made of two duplex DNAs, although a duplex DNA may appear transiently in S-phase (multi-strandedness hypothesis) and (iii) torsional stress on a DNA fiber causes the fiber to supercoil and thus stabilizes chromosome structure (torque-based stabilization). This model allowed to calculate of a fractal dimension of a representative metaphase chromosome (e.g. d = 2.34), to predict the mode of replication of double duplex and to furnish a topological basis for the decondensation unit hypothesis. It must also be admitted that all the arguments in this report except for the possible existence of split telomeres hold true without assuming a tetraplex organization of chromosomes. Implications of this model was discussed and the importance of the fractal dimension as a measure of chromatin condensation stressed.

Eukaryotic genome: model considerations

The paper presents a new model of chromosome structure based on the assumption that multiple circular subunits of DNA exist. The essential difference with previously described models is the circular DNA unit forms a central chromosome axis. Chromosome configurations during various phases of the cell cycle depend on the various conformations of this central integrating unit. The described model can be generalized for all haploid set of eukaryotic nucleus. Some aspects of the chromosome structure and their functions have been discussed.

Stereoscopic scanning electron microscopy of the chromosomes in Vicia faba (broad beans)

Metaphase chromosomes of Vicia faba (broad beans) were observed in situ with a scanning electron microscope by cryofracturing the cell. The chromosome is composed of tortuous fibers 500 A in diameter. They may be seen distributed randomly, but sometimes they are seen running parallel in the chromosome. The parallel fibers spiral around the main body of the chromosome, whereas they run longitudinally in the secondary constriction. If a chromosome is composed of a single strand (unineme hypothesis for the chromosome), the parallel fiber arrangement may imply that a single fiber is looping back and forth in the chromosome.

The basis of chromatin fiber assembly within chromosomes studied by histone-DNA crosslinking followed by trypsin digestion

To determine the structural basis of chromatin assembly that leads to chromosome formation in mitosis, crosslinks were introduced by formaldehyde between contiguous components within chromosomes. Crosslinked stable products were then observed by electronmicroscopy after non-crosslinked portions were briefly digested by trypsin to unfold chromosomes.--When the DNA-histone crosslink was the primary product, trypsin readily unfolded the whole chromosome structure while preserving the 250 A unit chromatin fiber intact; only a single unit fiber was tracked within the centromere region connecting the arms of each chromatid. When a histone polymer was formed by a prolonged formaldehyde crosslinking, trypsin digestion gave rise to chromatin fibers interacting with others at certain distances, and the typical chromosome structure remained unchanged. Regardless of the degree of crosslinking, there were neither thick supercoiled unit fibers nor proteinaceous cores.--These results suggest that the fiber connection may represent, to some extent, the interacting sites of folded chromatin fibers in situ within chromosomes, and also that the 250 A unit fibers are the sole, highest structural basis in chromosomes. Since virtually no appreciable histone digestion took place in the crosslinked chromosomes, the observation that even after DNA-histone crosslinking the fiber interacting sites were accessible to trypsin preferentially over other portions, may be consistent with our recent results that the exposed, lysine-rich tails of histones represent such interacting sites.

DNA synthesis during meiosis of eight-spored strains of Chlamydomonas reinhardi

In strain 137F of Chlamydomonas reinhardi, the zygospores undergo one round of nuclear DNA replication followed by three divisions to produce octospores. The third division without replication has been interpreted by Sueoka et al. (1967, 1969) to mean that the gametes and vegetative cells have at least binemic chromosomes. We have repeated their experiments using the same strain. However, the meiotic products were inviable--unable to undergo postmeiotic vegetative growth, DNA replication or division. On the other hand, using a variant of strain 137C which also has three divisions during germination we have shown that meiosis is normal. Zygospores from this strain undergo two rounds of nuclear DNA replication prior to the formation of octospores. These meiotic products are viable and capable of postmeiotic vegetative growth, replication and division. Since the third division without DNA replication subsequent to the two meiotic divisions leads to inviable products, and the strain which has viable products after three divisions does not lack the additional replication, meiosis in Chlamydomonas reinhardi provides no evidence of a bineme chromosome structure.

On the structure of chromosome ends as revealed by trypsin treatment of Locusta migratoria chromosomes

Anaphase chromosomes exhibit two longitudinal structures which are connected at their ends in a U-shaped closed configuration. The U-shaped ends might be part of the chromosome characterized by a relatively dense condensation of chromosomal material or part of the loop-like structure of the chromosome per se.

The strandedness of chromosomes: evidence from chromosomal aberrations

If cells are irradiated late in the mitotic cycle (late G2or early prophase), at the following anaphase they frequently exhibit characteristic chromosomal configurations known as sidearm bridges. These are often interpreted as sub-chromatid aberrations and are taken as evidence that chromosomes are multi-stranded. This interpretation, although recently challenged, is supported by experiments based upon the normal replication that converts chromatids to full chromosomes. The rationale is that aberrations involving only one chromatid of a chromosome are converted by replication to chromosome aberrations involving both chromatids. After replication, therefore, there should be no chromatid aberrations remaining unless the initial aberration involved less than a full chromatid. The results show that chromatid aberrations do appear after chromosomal replication: at the second mitosis after irradiation. Another experiment shows that most such chromatid aberrations are not the result of errors in the replication of previous chromatid aberrations.

The human chromosome. Electron microscopic observations on chromatin fiber organization

Human lymphocytes were grown in short-term tissue culture and were arrested in metaphase with Colcemid. Their chromosomes were prepared by the Langmuir trough-critical point drying technique and were examined under the electron microscope. In addition, some chromosomes were digested with trypsin, Pronase, or DNase. The chromosomes consist entirely of tightly packed, 240 +/- 50-A chromatin fibers. Trypsin and Pronase treatments induce relaxation of fiber packing and reveal certain underlying fiber arrangements. Furthermore, trypsin treatment demonstrates that the chromatin fiber has a 25-50 A trypsin-resistant core surrounded by a trypsin-sensitive sheath. DNase digestion suggests that this core contains DNA.

Chromosome structure--a model based on DNA replication

A model of chromosome structure is proposed which assumes: (i) that DNA replication is accomplished via right-hand (RH) rotation; and (ii), that where replicating DNA segments arc very long RH-rotation will not proceed with absolute freedom. It is expected that inhibition of rotation in daughter molecules will lead to the formation of left-hand-individual (LH-I) coiling systems in the two daughter molecules. It is also expected that inhibition of rotation in the parental molecule will cause the LH-I coiled daughters to be held together in a right-hand-relational (RH-R) association. The interaction between LH-I and RH-R coiling is expected to cause separation of the daughter molecules. Multistranded DNA-containing structures are expected to show, in addition to the LH-I coiling heirarchies formed within individual strands, an RH-R coiling heirarchy formed by the complex as a whole. An LH-I coiling heirarchy was looked for, and found, in Cleveland's drawings of flagellate chromosomes. Evidence for the existence of RH-R coiling was also found. Results of electron-microscope studies on chromosome structure were briefly examined, as were the structures of lampbrush chromosomes, salivary chromosomes and "normal" chromosomes. These studies provided additional, though less direct, evidence in favor of the replication hypothesis and the predictions developed from it.

Nomarski interference contrast resolution of subchromatid structure

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The strandedness of meiotic chromosomes from Oncopeltus

Meiotic chromosomes were isolated from male Oncopeltus fasciatus by dissecting the testes under insect Ringer's solution and spreading the living cells on the Langmuir trough. After being dried by the critical point method, preparations were examined under the electron microscope. Chromosomes at all stages of prophase prove to be multistranded. A significant increase in the number of parallel 250 A fibers in the chromosomes occurs between zygotene and diakinesis. Parallel folding, rather than true multistrandedness, is interpreted as the mechanism responsible for this observed increase in multistrandedness. It has not been possible to determine whether the multistrandedness observed at leptotene represents true multistrandedness or is the result of parallel folding. Apparent multistrandedness is lost at metaphase when the 250 A fibers of the chromosomes become coiled more tightly. In preparations isolated by these methods, no structures other than the 250 A chromosome fibers are visible in the chromomeres, which appear as regionally coiled or folded areas of the fibers along the arm of the chromosome.