in beekeeping, a small colony of bees with a young virgin queen or reserve mated queen.
Nuclei are used widely in beekeeping farms that engage in breeding queens. Nuclei are also used to keep reserve mated queens for replacing those that die in the spring. To form a nucleus, frames with honey, beebread, and brood are placed in hives divided into three or four compartments by dividers or into side compartments of hives with ordinary bee colonies. The compartments are populated by young flightless bees (small numbers introduced from strong colonies), and a virgin queen is introduced. After the queen mates with a drone and begins depositing eggs, she is removed, and another virgin queen is put in her place to be fertilized by the drones, or a new batch of ripe queen cells is placed in the nucleus. In this way, two to four mated queens may be obtained from one nucleus in a summer season.
SH. GASANOV
a body within a cell that, together with the cytoplasm, is an essential cellular component in protozoans and multicellular animals and plants.
The nucleus contains chromosomes and the products of chromosomal activity. All organisms are divided into eukaryotes and prokaryotes, depending on whether their cells are nucleated or nonnucleated, respectively. Although prokaryotes do not have a formed nucleus (the membrane is absent), deoxyribonucleic acid (DNA) is present. The nucleus stores most of the hereditary information of the cell. The genes contained in the chromosomes play a major role in transmitting hereditary characters to some cells and organisms. The nucleus interacts closely and constantly with the cytoplasm. Mediator molecules that transport genetic information to the centers of protein synthesis in the cytoplasm are synthesized in the nucleus. Thus, the nucleus controls the synthesis of all proteins and, through them, all the physiological processes in the cell. Hence, nonnucleated cells and cell fragments obtained experimentally invariably die; the transplantation of nuclei to such cells restores viability. The Czech scientist J. Purkinje Was the first to observe the nucleus in a chicken egg (1825). The British scientist R. Brown was the first to describe the nucleus in plant cells (1831–33), and the German scientist T. Schwann was the first to describe the nucleus in animal cells (1838–39).
A cell usually has one nucleus, situated close to the center and usually having the appearance of a globose or ellipsoid bubble. Sometimes the nucleus has an irregular or complex shape, for example, the nuclei of leukocytes and the macronuclei of infusorians. Binucleate and multinucleate cells are common; they are usually formed by nuclear division unaccompanied by cytoplasmic division or by symplasm, that is, the fusion of several mononuclear cells (for example, striated muscle fibers). Nuclei vary in size from about 1 micrometer (μm) in some protozoans to about 1 mm in some egg cells.
The nucleus is separated from the cytoplasm by a nuclear membrane consisting of two parallel lipoprotein unit membranes that are 7–8 nanometers (nm) thick and have a narrow perinuclear space between them. The nuclear membrane is perforated by pores measuring 60–100 nm in diameter; the outer membrane is continuous with the inner membrane at the edges of the pores. The number of pores varies from cell to cell: from a few to 100–200 per sq μm of nuclear surface. Along the edge of a pore is a ring of solid material, the annulus. The lumen of a pore frequently contains a central granule that measures 15–20 nm in diameter and is joined to the annulus by radial fibers. The fibers and the pore form the pore complex, which apparently regulates the passage of macromolecules across the nuclear membrane, for example, the entry of protein molecules into the nucleus and the exit of ribonucleoprotein particles from the nucleus. The outer membrane is continuous at some points with the membranes of the endoplasmic reticulum; it usually contains protein-synthesizing particles known as ribosomes. The inner membrane sometimes forms evaginations deep into the nucleus.
The nucleus consists of nuclear sap (karyolymph, karyoplasm), which contains formed elements, including nucleoli and chromatin. Chromatin is a more or less irregular clump of chromosomes found in a nondividing nucleus. Composed of DNA and proteins bound together in a complex called deoxyribonucleopro-tein (DNP), chromatin can be detected by Feulgen’s test. When a nucleus divides, all the chromatin condenses in the chromosomes. When mitosis ends, many portions of the chromosomes again loosen; these portions (euchromatin) contain mostly unique (nonrecurrent) genes. The other portions of the chromosomes remain compact (heterochromatin); it is here that the recurrent DNA sequences are arranged. In a nondividing nucleus much of the euchromatin consists of a loose network of DNP fibers 10–30 nm thick; the heterochromatin contains dense masses, or chromocenters, in which the DNP fibers are tightly packed. Some of the euchromatin can also become compact; such euchromatin is considered inert with respect to DNA synthesis. The chromocenters usually border on the nuclear membrane or nucleolus. Some data indicate that DNP fibers are attached to the inner nuclear membrane.
DNA synthesis, or replication, occurs in a nondividing nucleus. The process can be studied by labeling DNA precursors (usually thymidine) contained in the nucleus with radioisotopes. Numerous portions, or replicons, have been shown to exist along the length of the chromatin fibers. Each replicon has its own starting point for DNA synthesis from which replication spreads in both directions. The chromosomes themselves double as a result of DNA replication.
The sensing of genetic information coded in DNA takes place in nuclear chromatin by the synthesis from DNA of molecules of messenger ribonucleic acid (mRNA) and other types of RNA molecules participating in protein synthesis. Specific regions of chromosomes and, accordingly, of chromatin contain recurrent genes that code ribosomal RNA. Nucleoli rich in ribonucleopro-teins (RNP) are formed in these regions. The main function of the RNP is to synthesize the RNA constituting the ribosomes.
The nucleus contains other types of RNA particles besides those in the nucleolus. For example, perichromatin fibers 3–5 nm thick and perichromatin granules 40–50 nm in diameter are found at the boundaries of the loose and compact chromatin. Both probably contain messenger RNA combined with proteins; the perichromatin granules are its inactive form. Perichromatin granules have been observed leaving the nucleus and entering the cytoplasm through pores in the nuclear membrane. There are also interchromatin granules (20–25 nm) and, sometimes, thick (40–60 nm) coiled RNP threads. The nuclei of amoebas contain spiraled RNP threads (30–35 nm × 300 nm); the spirals may enter the cytoplasm and probably contain messenger RNA. In addition to DNA- and RNA-containing structures, some nuclei have purely protein inclusions in the form of spheres (for example, in the nuclei of growing egg cells of many animals and in the nuclei of several protozoans) or bundles of fibers or crystalloids (for example, in the nuclei of many cells in animal and plant tissues and in the macronuclei of some infusorians). The nucleus may also contain phospholipids, lipoproteins, and enzymes (DNA polymerase, RNA polymerase, and the complex of nuclear membrane enzymes, including adenosine triphosphatase).
A variety of specific types of nuclei are found in nature: giant nuclei in growing egg cells (especially of fishes and amphibians), nuclei containing giant polytene chromosomes (for example, those in the salivary gland cells of dipterans), compact spermatozoon nuclei lacking nucleoli, macronuclei of infusorians that are solidly filled with chromatin and that do not synthesize RNA, nuclei whose chromosomes are constantly condensed although nucleoli are formed (as in certain protozoans and in some insect cells), and nuclei whose number of chromosome sets increases twofold or more (polyploidy).
The principal method by which a nucleus divides is mitosis, which is characterized by doubling and condensation of the chromosomes, destruction of the nuclear membrane (except in many protozoans and fungi), and correct disjunction of sister chromosomes in the daughter cells. However, the nuclei of some specialized cells, especially polyploid cells, may divide by simple amitosis. Highly polyploid nuclei may divide not only into two but into many parts, and they can also divide by budding. This may result in the division of whole chromosome sets, or the segregation of genomes.
I. B. RAIKOV