Electron Microscopic Observations on the Submicroscopic Morphology of the Meiotic Nucleus and Chromosomes

Abstract

Since the discovery of the chromosomes and their behavior in mitosis and meiosis, cytologists and cytogeneticists have been largely preoccupied with the nucleus rather than the cytoplasm and have devoted much attention to the problems of reduplication of chromosomes, mutation, and the linear array and function of genes.On the other hand, investigators concerned with submicroscopic morphology worked mainly on the fine structure of cytoplasm and its organelles (1) and as yet relatively little progress has been achieved on the ultrastructural organization of the nucleus and chromosomes. There is a real need for the application of modern techniques of analytical cytology to the problems of chromosome structure in an effort to settle many points of disagreement in the cytological literature (2). Furthermore, the high resolution recently reached by means of the electron microscope on properly fixed and sectioned material offers the possibility of visualizing in situ the nucleoproteins and other macromolecular components which biochemists have been studying in vitro after isolation.From the genetic viewpoint knowledge of the fine structure of chromosomes is of particular importance because of recent evidence which tends to identify the gene with macromolecules of nucleoprotein (3, 4) and to demonstrate, at least for simple genetic systems, that the "one dimensionality and divisibility" of genetic material persist down to its ultimate molecular structure (5).Early attempts to study nuclear components with the electron microscope consisted of observations on isolated lampbrush chromosomes of amphibian ovocytes (6-8) and on nuclear fragments (9). Some workers reported the presence of fibrillar structures about 500 A in diameter (6) that under certain conditions appeared as paired strands (7). However, this has been contradicted by others who found no evidence of single or paired strands in cross-sections of *

Electron Microscope Study of DNA-Containing Plasms

“…It is noteworthy, however, that Grell and Wohlfarth-Bottermann (12) found a fibrillar structure in the chromosomes of a Dinoflagellate, and De Roberfis (9) in the chromosomes of a higher cell; both types contain fibrils with a width of 40 to 80 A, very comparable to those found in bacteria. De Robertis also found the presence of Ca ions to be important during fixation.…”

The Fine Structure of the DNP Component of the Nucleus

“…Stoeckenius (7) and others have speculated that the DNA molecule forms the core of the nucleoprotein filaments and protein, the shell; the basic amino groups of the protein encircling the DNA helix may form bonds with the phosphate radicals of the nucleic acid (26). Small filaments (25 to 200 A in diameter) have been described in chromatin and chromosomes by a number of investigators (10)(11)(12)(13)(14), and also in various DNP-containing organelles, such as the chloroplasts of Chlamydomonas (27), the kinetoplasts of trypanosomes (28), and the nucleoids of bacteria (29) and certain algae (30). It thus seems possible that DNP in the cell has a filamentous structure wherever it occurs.…”

“…Most workers (see 12,13), who have recognized that chromatin is filamentous rather than granular in nature, have described individual "microfibrils," not an interconnected meshwork of filaments. Yet, Yasuzumi (32) has clearly illustrated a net-llke pattern of interlacing filaments in the chromatin of nucleated red blood cells.…”

“…Current concepts of the structure of DNP are based primarily on studies of isolated nucleoproteins (6 9) and chromosomes (10)(11)(12)(13)(14).…”

The ultrastructure of meiotic chromosomes of Lilium longiflorum

“…âl» 1965;Swift, 1 965;Underbrinlc, £t 1967a;Wolfe, 1965aWolfe, , 1965bWolfe, , 1967Wolfe and Grim, 1967;Wolfe and Hewitt, 1966;Wolfe and John, 1965;and Wolfe and Martin, 1967). The extremes in diameter range from 5 nm (DeRobertis, 1956;and Hay and Revel, 1963) through 50 nm (Davies and Tooze, 1964;Gay, 1956;and Ris, 1955). DuPraw (1965cDuPraw ( , 1966 observed an increase in diameter of the fibrils from 23 nm at interphase to greater than 30 nm at mitotic metaphase, and also showed via quantitative electron microscopy (DuPraw and Bahr, 1968Bahr, , 1969 an increase in the packing ratio of DM to fibril length from 56:1 in interphase to 100:1 in metaphase.…”