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

Reports of changes in RNA and protein metabolism associated with cell expansion are numerous. Results often depend on whether studies on cell expansion were done with intact plants or with excised parts. A net increase in protein content is usually associated with intact cell extension (6, 15, 17, 30), while excised tissues usually show a net decrease (9, 18, 30, 33). A possible exception to the latter phenomenon was reported by Thimann and Loos (32) for excised potato and artichoke tissue. In these tissues, auxin stimulated the synthesis of protein over a 4-day period. Burroughs and Bonner (8) demonstrated protein synthesis associated with only slight variation in protein content during growth of excised coleoptile tissue. However, auxin did not affect the amount of C14-amino acid incorporated into protein of the coleoptile tissue during 6 hours. Changes in specific activities and amounts of certain enzymes occur in association with cell elongation (3, 7, 14, 27, 28). Studies on changes in RNA metabolism associated with cell expansion are equally complex. There is usually a progressive increase in RNA content per cell from the apical meristem through the zone of radial enlargement (15,17), but in corn mesocotyl tissue (19), there was a net decrease in RNA per cell associated with cell expansion and maturation. The association of RNA with various cell organelles, as measured by differential centrifugation, changes in certain tissues during cell expansion and maturation (10, 19, 23, 29). In excised tissues, a net decrease in RNA is usually associated with cell expansion (10, 20, 33, 35). With corn mesocotyl, growth-promoting concentrations of auxin enhanced the rate of RNA breakdown (20, 33), but this was not observed in soybean hypocotyl tissue (18, 21). In the expanding zone of excised soybean hypocotyl, auxin enhanced C14-nucleotide incorporation into RNA (21), but this did not occur with corn mesocotyl (20). The differences in RNA metabolism in elongating zones of soybean hypocotyl and corn mesocotyl may be attributable to differences in the degradative metabolism of RNA in these tissues. Corn mesocotyl has been shown to have more ribonuclease activity than soybean hypocotyl (34), and low concentrations of auxin enhance ribonuclease activity in corn mesocotyl during excised growth (30). There is little or no change in ribonuclease activity of soybean hypocotyl during excised growth with or without auxin. The growth rate of corn root tips correlates positively with the RNA content of the tissue (35). Woodstock and Skoog (36) hypothesized that the amount of RNA synthesis by the cells of the root apex determines their subsequent rate of elongation. They also showed that IAA inhibited growth and RNA synthesis in the corn root. Many lines of evidence support the view that RNA and protein synthesis may function during cell expansion, although no definitive experiments lhave been done. Developments in the field of protein and nucleic acid biochemistry provide a background of information on specific inhibitors which facilitate the study of RNA and protein syntheses. First, the antibiotic actinomycin D inhibits DNA-dependent synthesis of RNA by RNA polymerase at low concentrations (16) and leads to an inhibition of protein synthesis (13, 22). DNA synthesis is inhibited only at higher concentrations (16). Secondly, puromycin is an effective inhibitor of protein synthesis (1, 26). Presumably by formation of abnormal messenger RNA, 8-azaguanine inhibits protein synthesis in bacteria while only slightly affecting RNA synthesis (9). Also, the demonstration that part or all messenger or informational RNA is unstable (rapid turnover) in many systems (22, 25) would implicate continued RNA synthesis for the normal complete development of a cell. Using the elongating zone of the soybean hypocotyl and actinomycin D, puromycin, and 8-azaguanine as specific inhibitors, the essentiality of RNA and protein synthesis for cell elongation was investigated. The data show that RNA and protein synthesis are essential for cell elongation in soybean hypocotyl tissue. There also is suggestive evidence that the role of auxin in regulating cell elongation may be associated with the control of RNA and/or protein synthesis.

Ribonucleic Acid and Protein Synthesis as Essential Processes for Cell Elongation

Ever since meiosis was recognized as a process there has been a continuing interest in its temporal aspects. Two main types of meiotic timing experiments have been conducted: first, experiments to estimate the duration of meiosis (and sometimes its stages); second, experiments to locate the sensitive stage(s) when exposure of meiocytes to various treatments can affect meiotic chromosome behaviour (e.g. pairing or recombination). Such experiments have played an important role in increasing our under-standing of the meiotic process. The duration of meiosis has been estimated in about 70 organisms, including two prokaryotes (yeast and Chlamydomonas) and the following eukaryotes: 1 Basidiomycete (Coprinus lagopus), 2 Gymnosperms (Larix decidua and Thuja plicata gracilis), at least 39 angiosperms, and at least 26 animal species. The duration of female meiosis has been estimated in far fewer species than male meiosis. However, estimates of the duration of female meiosis are available for 6 angiosperms, Drosophila melanogaster, Xenopus laevis, and several mammals. Comparison of these data shows that the duration of meiosis is one of the most variable aspects of the meiotic process, ranging from less than 6 h in yeast to more than 40 years in the human female. Developmental holds at different stages of meiosis are common in plants and animals, and inevitably prolong the meiotic division. However, even among species without developmental holds, the duration of meiosis is very variable. For instance, in animals it ranges from about 1-2 days in male Drosophila melanogaster to more than 24 days in male Homo sapiens and several Orthopterans. Despite the large variation in the duration of meiosis three generalizations can be made: (i) first prophase is always very long compared with the remaining meiotic stages, (ii) the rate of meiotic development is very slow compared with the rate of development in dividing somatic meristem cells of the same organisms under the same conditions, (iii) the duration of meiosis is characteristic of the genotype and species. Four main factors have been recognized which effect or determine the duration of meiosis, namely (1) environmental factors (e.g. temperature); (2) nuclear DNA content; (3) ploidy level of the organism; and, (4) the genotype. Because nuclear DNA content plays a major role in determining the duration of meiosis, it has been suggested that DNA influences the rate of meiotic development in two ways: first through its informational content (the genotype), and second indirectly by the physical and mechanical effects of its mass independently of its informational content (i.e. the nucleotype). Thus, the observed duration of meiosis is the result of a complex genotype-nucleotype-environment interaction. With the obvious exception of variation caused by developmental holds, changes in the duration of meiosis usually involve proportional changes in the durations of all its stages. This is true irrespective of whether the variation in meiotic time is associated with changes in temperature, nuclear DNA amount, ploidy level, or sex difference. While results for animal species show some evidence of a similar phenomenon, the relative proportions of meiosis taken by individual meiotic stages is clearly much more variable between animal species than between plants. The duration of meiosis often has much wider implications for the organism than those affecting the meiotic process per se. Examples are given from which it is concluded, first, that the duration of meiosis can limit the type of life cycle which a species can display; and second, that in many species the duration of the meiosis is an essential adaptive feature suited to its life cycle type in its normal environment.

The time and duration of meiosis

Publisher Summary This chapter focuses on the nature of the stimulus to cell division, the course of wound repair, and the biochemistry associated with these events. Most studies on wound healing in higher plants have concentrated on mitotic and postmitotic events. The temperature rise in onion epidermal cells occurs immediately after wounding and reaches a maximum of about 0.08°. An increase in the level of several enzymes has been observed in cells responding to a wound stimulus. This increase often occurs prior to cell division. Fine structure studies of wound-healing, particularly in its early stages, seem to be a fertile field for further investigations and necessary for a better understanding of the events involved. Such studies can yield information as to the transmittability of the wound stimulus. It is impossible to draw any convincing overall schemes explaining wound healing. The data presented clearly point to the participation of hormonal factors, some of which are not yet characterized, as well as other factors in promoting cell division and differentiation. Studies, particularly of a quantitative nature, must be undertaken if this biological problem is to be addressed.

Wound-Healing in Higher Plants

The Development of Cells in the Growing Zones of the Root

A new technique for determining the durations of the different phases of division is described. It is shown that at 15o C. the mean durations of the several phases in the root tip of the pea are as follows : interphase 23 hours ; prophase 2 hours ; metaphase 25 minutes; anaphase 5 minutes; telophase 22 minutes. The effects of different temperatures have been determined, and it is shown that all stages are accelerated by an increase in temperature from 15o to 25° C., the ratios of the durations at the two temperatures being about 2-0 or slightly less. It is suggested that since interphase is the longest stage of the division cycle, in terms of growth effects on this stage are the most significant.

R. Brown

Papers

The Effects of Temperature on the Durations of the Different Stages of Cell Division in the Root-tip

The Effects of Sugar and Potassium on Extension Growth in the Root

The Influence of Mature Tissue 2

Enzyme Changes in Relation to Cell Growth in Excised Root Tissues1