It is generally accepted that the differentiated state of a given type of cell is associated with the activity of a particular set of genes, together with the total inactivity of those sets associated with the differentiation of other cell types. It is also clear that the differentiated state of dividing or nondividing cells is often extremely stable. In this article we suggest mechanisms that may account for this stability and that also attempt to explain the ordered switching on or off of genes during development. The phenotype of the organism depends on the genotype, and the genetic contribution from both parents is in almost all cases equal. Since the ultimate control of development resides in the genetic material, the actual program must be written in base sequences in the DNA. It is also clear that cytoplasmic components can have a powerful or overriding influence on genomic activity in particular cells, yet these cytoplasmic components are, of course, usually derived from the activity of genes at some earlier stage in development. A continual interaction between cytoplasmic enzymes and DNA sequences is an essential part of the model to be presented. Modification Enzymes In bacteria, enzymes exist which modify DNA by methylating adenine in the 6-position ( 1 ). These enzymes are extremely specific in their action; they modify bases at particular positions in short defined sequences of DNA, which, at least in some instances, form a palindrome. (A palindrome in DNA is an inverted duplication, with twofold rotational symmetry. The 3′ →...

https://science.sciencemag.org/content/sci/187/4173/226.full.pdf

DNA modification mechanisms and gene activity during development.

Cyclin: A protein specified by maternal mRNA in sea urchin eggs that is destroyed at each cleavage division

Publisher Summary Glutathione (GSH) is the most important nonprotein thiol in living systems and is of widespread occurrence in the intracellular milieu of animals, plants, and microorganisms GSH was isolated and named by the English biochemist Frederick Gowland Hopkins This chapter discusses GSH status, the biologically relevant chemistry of GSH, the forms in which GSH can be present within the cell, along with the GSH content of cells and the methods for analysis of this substance GSH-related biochemical reactions and the biological roles of GSH are discussed in the chapter The use of perturbations in GSH status as a means for investigating GSH-related phenomena and an analysis of the consequences of perturbation are presented A short summary of genetic lesions related to GSH is also included Like chemically induced perturbations in GSH status, genetic lesions provide valuable insights into the role of GSH in normal functions and processes in cells The chapter concludes with some brief comments about the future of the relationship of GSH status to cellular processes

The Glutathione Status of Cells

This review suggests that the intracellular functions of calcium are best understood in terms of calcium's functioning as a second messenger. Further, when functioning as a second messenger, calcium completes its mission not by transferring charge nor by binding to lipid but by binding to specific targets, calcium-modulated proteins. This concept is broadly interpreted to include proteins involved in calcium transport. There is strong evidence that many, if not all, of these calcium-modulated proteins are homologs. Their structures and properties are contrasted to those of extracellular calcium-binding proteins which are not homologous to one another or to the intracellular calcium-modulated proteins. Finally, this line of thought leads to a suggestion of the evolutionary reason for the choice of calcium as the sole inorganic second messenger.

Structure and evolution of calcium-modulated proteins.

The cilium is a complex organelle, the assembly of which requires the coordination of motor-driven intraflagellar transport (IFT), membrane trafficking and selective import of cilium-specific proteins through a barrier at the ciliary transition zone. Recent findings provide insights into how cilia assemble and disassemble in synchrony with the cell cycle and how the balance of ciliary assembly and disassembly determines the steady-state ciliary length, with the inherent length-dependence of IFT rendering the ciliary assembly rate a decreasing function of length. As cilia are important in sensing and processing developmental signals and directing the flow of fluids such as mucus, defects in ciliogenesis and length control are likely to underlie a range of cilium-related human diseases.

Ciliogenesis: building the cell's antenna

Cells of many kinds adhere firmly to glass or plastic surfaces which have been pretreated with polylysine. The attachment takes place as soon as the cells make contact with the surfaces, and the flattening of the cells against the surfaces is quite rapid. Cells which do not normally adhere to solid surfaces, such as sea urchin eggs, attach as well as cells which normally do so, such as amebas or mammalian cells in culture. The adhesion is interpreted simply as the interaction between the polyanionic cell surfaces and the polycationic layer of adsorbed polylysine. The attachment of cells to the polylysine-treated surfaces can be exploited for a variety of experimental manipulations. In the preparation of samples for scanning or transmission electron microscopy, the living material may first be attached to a polylysine-coated plate or grid, subjected to some experimental treatment (fertilization of an egg, for example), then transferred rapidly to fixative and further passed through processing for observation; each step involves only the transfer of the plate or grid from one container to the next. The cells are not detached. The adhesion of the cell may be so firm that the body of the cell may be sheared away, leaving attached a patch of cell surface, face up, for observation of its inner aspect. For example, one may observe secretory vesicles on the inner face of the surface (3) or may study the association of filaments with the inner surface (Fig. 1). Subcellular structures may attach to the polylysine-coated surfaces. So far, we have found this to be the case for nuclei isolated from sea urchin embryos and for the microtubules of flagella, which are well displayed after the membrane has been disrupted by Triton X-100 (Fig. 2).

/pdf/adhesion-of-cells-to-surfaces-coated-with-polylysine-3600ej69p9.pdf

Adhesion of cells to surfaces coated with polylysine. Applications to electron microscopy.

In this communication, the term \"mitotic apparatus\" (abbreviated MA) will refer to the ensemble of structures constituting the \"chromatic\" and \"achromatic\" figures in the classical descriptions of mitosis (e.g., Flemming,I Wilson2). It includes spindles, asters, centrioles, nuclei (before breakdown) and chromosomal structures (after breakdown of the nuclear membrane). The classical pattern of mitosis describes the coordinated behavior of these elements without implying that they are integrated into a physical entity to which such a term as \"mitotic apparatus\" could be applied. Our observations, as will be seen, do indicate the existence of just such a physical association of the various elements, per.mitting their isolation as a single body, and thus the use of the term seems to be justifiable. Heretofore, the physical and chemical study of the MA has been rendered difficult by the necessity of studying it in situ. Most of the earlier information has been adequately summarized by Hughes.3 The most informative avenues of information have been: (a) Staining (e.g., Tahmisian and Brues,4 Mazia, Brewer and Alfert6), (b) polarized light studies (e.g., Inoue and Dan,6 Swann'9), (c) electron microscopy (e.g., Rozsa and Wykcoff7), (d) hydrostatic pressure studies (e.g., Pease8), (e) measurements of anaphase movement of chromosomes (Hughes and Swann9), (f) correlation of dimensional changes of MA with the progress of cytoplasmic division (Dan'0). An enormous literature on the destruction of the MA by antimitotic agents has yielded little insight into its nature. It is evident that the avenues of information have been entirely visual, and that the MA presents exceptional difficulties because it is a structure without a boundary, whose parts are mingled with other cytoplasmic constituents. In the following we describe methods for isolation of the MA in quantity from populations of dividing sea urchin eggs. The somewhat unorthodox methods described were developed after numerous failures with procedures involving disruption of living eggs. Such procedures sometimes yield the MA from a few individuals. Daniellill reports briefly on experiences in liberating the MA from eggs by osmotic rupture.

http://www.pnas.org/content/38/9/826.full.pdf

The Isolation and Biochemical Characterization of the Mitotic Apparatus of Dividing Cells.

An analysis of the partial metabolic derepression of sea urchin eggs by ammonia: The existence of independent pathways

Amoebae of Dictyostelium discoideum were attached to a surface coated with polylysine, and the upper portion of the cells was sheared off with a stream of buffer. Scanning and transmission electron microscopy showed that the cytoplasmic surface of the exposed membrane was covered with fibers consisting of actin-containing filaments. The actin was identified by its solubility properties and its ability to interact with muscle myosin.

https://www.pnas.org/content/pnas/72/5/1758.full.pdf

Daniel Mazia

Papers

Adhesion of cells to surfaces coated with polylysine. Applications to electron microscopy.

The Isolation and Biochemical Characterization of the Mitotic Apparatus of Dividing Cells.

An analysis of the partial metabolic derepression of sea urchin eggs by ammonia: The existence of independent pathways

Visualization of actin fibers associated with the cell membrane in amoebae of Dictyostelium discoideum.

The particulate organization of the chromosome.