Publisher Summary The reproduction of biological systems may be referred to the reproduction of cells Subcellular entities such as viruses cannot maintain themselves indefinitely without parallel reproduction of the cells in which they live The asexual reproduction of organisms such as flatworms is limited by the production of new cells, and all cases of sexual reproduction imply the generation of new cells after the mixing of genetic material from the parent cells An evolutionary consequence of cellularity is implicit in the designation of multicellular organisms as higher organisms Single cells can develop a high level of variety and complexity of structure and function In the development of a multicellular organism, cell division, beginning with the egg, is the essential step toward differentiation If mitosis is a device for the distribution of sister genes to sister cells, the chromosomes may be viewed as a system in which the numerous genes are packaged into a small number of units Mitosis is accompanied by some changes in the physical state or texture of the cytoplasm, which have been assessed as viscosity changes

Mitosis and the Physiology of Cell Division

Publisher Summary This chapter discusses chromosome aberrations induced by ionizing radiations. Chromosome aberrations are used as general indices of cellular damage or as cell markers in studying cell-population kinetics in irradiated or tumor tissues. Of primary importance are the mechanisms involved in aberration induction and in the action of modifying agents that alter the overall frequency or relative frequencies of the different aberration types. The types of aberrations that are induced following irradiation or treatment with certain radiomimetic chemicals are classified according to the unit of breakage or exchange that is involved. The types of structural changes that can occur within or between chromosomes and chromatids are of two kinds: the simple deletion and the exchange. The simple deletion appears to be the result of a single break in the chromosome or chromatid and the exchange is a new rearrangement following the joining of breakage ends that result from separate breaks. Comparisons among different types of ionizing radiations and among similar radiation types having different energies for their capacity for producing chromosome aberrations are of considerable fundamental importance in the study of aberration production.

Chromosome Aberrations Induced by Ionizing Radiations

Gametophytes of homosporous ferns

order into a subject that has not been much dis cussed in the past.1 The amount of relevant work is considerable, and it is spread over most of cell biology. In consequence, it is quite certain that I shall have failed to notice many important papers, particularly in those subjects with which I am unfamiliar. I should be glad to hear of these omis sions. I must also apologize to the numerous au thors of whose work I have taken note, but whose names are concealed beneath that of some re viewer. In certain subjects, especially radiation biology and nucleic acids, the volume of work makes it impossible, in a review of this sort, to do justice to individuals.

https://cancerres.aacrjournals.org/content/canres/17/8/727.full.pdf

The control of cell division; a review. I. General mechanisms.

Reproductive Biology of the Pteridophyta. III. A study of the Blechnaceae

The effects of solutions of benzimidazole and 5-aminouracil onAllium cepa root meristems immersed in them have been studied cytologically. Benzimidazole and 5-aminouracil, analogues of purines and pyrimidines respectively, reduce the number of nuclear divisions being initiated. A marked decline in frequency of cells in mitosis occurs at the beginning of treatment with benzimidazole (300 p.p.m.) but not until 6 or more hours later with 5-aminouracil (125 p.p.m.).

Some cytological aspects of antagonism in synthesis of nucleic acid

THE CHROMOSOME number has been found to differ from cell to cell within the root tips of Paphiopedilum Wardii Summerhayes, one of the mottledleaved Burmese lady slipper orchids. Lack of constancy as regards somatic chromosome number within an individual is not unknown in plants. An occasional cell, for example, or a number of scattered cells, or a group of associated cells within a root may be tetraploid or octaploid. Langlet's (1927) term "polysomaty" is commonly applied in these cases. There are, however, species or varieties in which the somatic complement of chromosomes fluctuates in the vicinity of the diploid number with no evidence of duplication of the entire set. Apparently these variations have diverse histories and are to be explained differently. The chromosome or chromosomes contributing to variability may be supernumerary to the normal complement and much smaller than its members. The B chromosomes of maize are an example (Kuwada, 1919; Fisk, 1927; Longley, 1927) as are the-centric fragments surviving from the last previous meioses referred to by Darlington (1937). The variability, on the other hand, may involve replication of members of the complement. There has been little observation on mitoses leading to varying numbers other than through polysomaty. Since the mechanism by which the change in number is brought about can be followed with considerable ease in P. Wardii, a -brief investigation was made. The process involves only certain members of the chromosome set and leads to cells polysomic, in Blakeslee's terminology, for those members of the complement. The causal mechanism bears close resemblance to that of polysomaty. These facts lead to the adoption of the word "aneusomaty"2 as a name for the phenomenon. Root tips were fixed in Randolph's modification of Navashin's fluid, imbedded in paraffin, sectioned, and stained with crystal violet-iodine. Counts of chromosomes were made in serial sections from the root tip through the embryonic region. Although the plants supplying the root tips came from two different sources, they may be members of a single clone since the species is rare in cultivation and is propagated vegetatively. The idiogram (fig. 6) shows the complement to contain seven pairs of atelomitic chromosomes of graduated lengths, two shorter pairs having about the same length. Each member of one pair of chromosomes has a secondary constriction which delimits a region similar in size to the smallest chromosomal type of the complement. The remainder of the chromosomes are rod-shaped; they vary 1 Received for publication March 19, 1945. 2 The author wishes to express his appreciation for Prof. C. E. Allen's advice in selecting an appropriate name. from 27 to 31 in number according to the present observations. Their lengths are graduated so that the longer and shorter rod-shaped chromosomes can be distinguished easily. Some of the types of intermediate length have morphological peculiarities which are sufficient to distinguish them but this group is generally difficult to assort. The rod-shaped chromosomes are not all strictly telomitic; the locations of their centric regions vary from apparently terminal to sufficiently subterminal so that an extremely short second arm is visible. The somatic number as determined in equatorial plates ranges from 41 to 45 (fig. 3 and 2, respectively). The shortest type of chromosome (no. 20) is almost spherical; it possesses two horn-like proj ections, one to a chromatid, at the end anterior in polar congression. Because of its shape and the fact that it is from one-half to two-thirds the length of the next shortest chromosome, it is recognized easily. Two to six (fig. I and 5) chromosomes of this type are present in various plates; they may occur in several kinds of associations lying on the periphery of the spindle or they may be scattered throughout the plate. The latter condition is more frequent. The total number of chromosomes, omitting those of type 20, varies from 38 to 40 (fig. 4 and 1). Two additional types of chromosomes have different frequencies of occurrence in equatorial plates. One of these (no. 19) likewise belongs to the class of smaller chromosomes. It has a swollen midregion and tapers toward each end, the end anterior in polar migration sometimes being stretched. It is almost as easily distinguishable as the shortest chromosomal type. Two, three, or four (fig. 4, 2, and 1) may be present in somatic plates. The other variant (no. 10) is one of the medium-sized chromosomal types, being roughly three times as long as type 20. Like chromosomes of type 20 and 19, it has a single projection or two small projections, depending upon the view and state of division, at its forward end. Two or three of these (fig. 5 and 3) may be present in an equatorial plate. Table 1 presents an analysis of the equatorial plates shown in figures 1-5. The variable number of occurrences of three chromosomal types allows similar total number of chromosomes to be reached by different means. The data in table 1 dealing with figures 2 and 5 demonstrate this point. Chromosomes of type 20 are the most variable in numbers. So frequent are misdivisions of chromosomes of this type that some stages in their progress have been observed. The apparent explanation is that the two daughter chromatids of a type 20 chromosome may dissociate at the end opposite the centric region before or during the early stages

Production of variable aneuploid numbers of chromosomes within the root tips of paphiopedilum wardii

Apogamy in doodia caudata

THE DIVISION of chromosome material is associated with the activity of the spindle apparatus which does not stain like chromatin and is often called the achromatic figure. Its structure in many plant and animal cells both in vivo and after a variety of technical procedures has been studied by many workers. Their observations have disagreed in many ways. A number of interpretations of the achromatic figure's mode of function have arisen. Many earlier accounts of mitosis have been reviewed by Hughes (1952), Schrader (1953), and Ris (1955). Schrader lists a number of theories dealing with the mode of action of the spindle apparatus; none of these, his evaluation makes clear, satisfies all established facts. Clearly the nature of the forces which bring about mitosis, promote the formation of the spindle and carry the mitosis to its conclusion is puzzling.

The achromatic figure during mitosis in maize endosper

When roots of Vicia faba are immersed in a solution of a thymine analogue, 5-aminourcil, 50 ppm. chromosome aberrations are induced at anaphase. Among these are chromosomes which are bent so that a polar group has a bell-shaped outline, chromosomes which disjoin faultily and chromosomes which contain with increased frequency regions devoid of material stainable with the Feulgen procedure.Sites of differential response (Feulgen-negative regions in the terminology of Duncan and Woods) have recognizable locations. They lie in or near regions which other authors have identified as heterochromatin. The frequency with which these sites become Feulgen-negative is highest in roots treated with 5-aminouracil for 12 hours, lower in roots treated for 24 hours and lowest in control roots and roots which are recovering in distilled water 24-48 hours after removal from a 24 hour treatment with 5-aminouracil. The change in frequency from 12 to 24 hour treatments constitutes a partial recovery.In roots treated with a mixture of 5-aminouracil 50 ppm. and cytidine sulfate 126 ppm. the frequency is intermediate between that of control roots and of roots treated with 5-aminouracil alone. The Feulgen-negative regions, therefore, are considered segments of chromosomes where interference in pyrimidine metabolism is critical, an exogenous source antagonizing the antimetabolite 5-aminouracil. Other aberrancies induced by treatment with 5-aminouracil are less intense.Since comparable sites of sister chromosomes at anaphase are often similary affected and since Feulgen-negative regions can be demonstrated in metaphase chromosomes, a preanaphase cause is indicated. Their incidence in roots treated with 5-aminouracil is highest when the mitotic index is lowest, a fact associating their origin with inhibition of DNA synthesis. DNA synthesis, of course, occurs during interphase. Perhaps initiation of Feulgennegative regions is an aspect of its interference in DNA metabolism.

Robert E. Duncan

Papers

Some cytological aspects of antagonism in synthesis of nucleic acid

Production of variable aneuploid numbers of chromosomes within the root tips of paphiopedilum wardii

Apogamy in doodia caudata

The achromatic figure during mitosis in maize endosper

The Structural Chromosome Aberrations Produced By 5-Aminouracil and Their Prevention by an Additional Thymine Source