A. E. Mirsky

The Interphase Nucleus

Publisher Summary The nucleus of a cell when it is not visibly engaged in division is said to be resting, metabolic, or in interphase. The chromosomes of a cell are contained within its nucleus during interphase. This chapter discusses interphase nucleus with respect to its morphology, chemical composition, and metabolic activity. The nuclei of various cell types differ from each other. The most obvious variations are in size and shape. The nuclei of many cells are round; others are elongated; and occasionally, they have irregular shapes, as in polymorph leucocytes. Of all nuclei, probably those with the most irregular forms are the branched nuclei in the silk-spinning cells of insect larvae. DNA can be detected microscopically after Feulgen staining, and it can also be determined quantitatively in the dried preparations of isolated nuclei. Sperm nuclei have relatively high concentrations of DNA. The membrane surrounding the nucleus is double with a space between the two membranes. The outer of the two membranes is sometimes continuous with the membranes of the general cytoplasmic membrane system. The protamines and histones are basic proteins found combined with DNA in many nuclei.

In vitro incorporation of amino acids into the proteins of isolated nuclear ribosomes.

Abstract Ribonucleoprotein particles of diverse composition and metabolic activity can be extracted from the isolated thymus cell nucleus. If properly supplemented these ribosomes remain capable of active amino acid incorporation into their constituent proteins. This process requires adenosine triphosphate, amino acid activating enzymes, and guanosine triphosphate. Thymus nuclear ribosomes are irreversibly inactivated by brief exposure to hypertonic sucrose solutions, either before or after extraction from the nucleus. While in the nucleus they are resistant to ribonuclease digestion, but after their isolation, ribonuclease attacks the particles and destroys their activity. The addition of deoxyribonucleic acid of diverse sources stimulates amino acid incorporation by isolated nuclear ribosomes, but other polyanions may be inhibitory. This role of deoxyribonucleic acid and the influence of position within the cell nucleus upon the metabolism of nuclear ribosomes is discussed.

The purification and properties of ribosomes from the thymus nucleus.

Abstract Methods are described for the isolation and purification of ribonucleoprotein particles (ribosomes) from isolated calf-thymus nuclei. About half of the total ribonucleic acid of the nucleus can be recovered in the purified ribosome fraction. Nuclear ribosomes can be purified by centrifugation in a sucrose density-gradient. The ribosome peak contains about 36% RNA. Treatment of the peak material with deoxycholate removes much of the protein. The resulting ribosomes have a sedimentation coefficient of 78 S and about 60% of their mass is RNA. The average base composition of this ribosomal RNA is given and compared with that of other nuclear RNA fractions. The changes in state of the nuclear 78-S ribosome with changing pH, ionic strength, and Mg 2+ concentration are described. Electron microscopy of isolated ribosomes after shadow-casting with chromium indicates that the 78-S ribosomes are about 200 A in width.