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nucleus, in physics
The Nature of the Nucleus
Atomic nuclei are composed of two types of particles, protons and neutrons, which are collectively known as nucleons. A proton is simply the nucleus of an ordinary hydrogen atom, the lightest atom, and has a unit positive charge. A neutron is an uncharged particle of about the same mass as the proton. The number of protons in a given nucleus is the atomic number of that nucleus and determines which chemical element the nucleus will constitute when surrounded by electrons.
The total number of protons and neutrons together in a nucleus is the atomic mass number of the nucleus. Two nuclei may have the same atomic number but different mass numbers, thus constituting different forms, or isotopes, of the same element. The mass number of a given isotope is the nearest whole number to the atomic weight of that isotope and is approximately equal to the atomic weight (in the case of carbon-12, exactly equal).
Size and Density
Mass Defect, Binding Energy, and Nuclear Reactions
When nuclear masses are measured, the mass is always found to be less than the sum of the masses of the individual nucleons bound in the nucleus. The difference between the nuclear mass and the sum of the individual masses is known as the mass defect and is due to the fact that some of the mass must be converted to energy in order to make the nucleus stable. This nuclear binding energy is related to the mass defect by the famous formula from relativity, E = mc2, where E is energy, m is mass, and c is the speed of light. The binding energy of a nucleus increases with increasing mass number.
A more interesting property of a nucleus is the binding energy per nucleon, found by dividing the binding energy by the mass number. The average binding energy per nucleon is observed to increase rapidly with increasing mass number up to a mass number of about 60, then to decrease rather slowly with higher mass numbers. Thus, nuclei with mass numbers around 60 are the most stable, and those of very small or very large mass numbers are the least stable.
Two important phenomena result from this property of nuclei. Nuclear fission is the spontaneous splitting of a nucleus of large mass number into two nuclei of smaller mass numbers. Nuclear fusion, on the other hand, is the combining of two light nuclei to form a heavier single nucleus, again with an increase in the average binding energy per nucleon. In both cases, the change to a stable final state is accompanied by the release of a large amount of energy per unit mass of the reacting materials as compared to the energy released in chemical reactions (see nuclear energy).
Models of the Nucleus
Several models of the nucleus have evolved that fit certain aspects of nuclear behavior, but no single model has successfully described all aspects. One model is based on the fact that certain properties of a nucleus are similar to those of a drop of incompressible liquid. The liquid-drop model has been particularly successful in explaining details of the fission process and in evolving a formula for the mass of a particular nucleus as a function of its atomic number and mass number, the so-called semiempirical mass formula.
Another model is the Fermi gas model, which treats the nucleons as if they were particles of a gas restricted by the Pauli exclusion principle, which allows only two particles of opposite spin to occupy a particular energy level described by the quantum theory. These particle pairs will fill the lowest energy levels first, then successively higher ones, so that the “gas” is one of minimum energy. There are actually two independent Fermi gases, one of protons and one of neutrons. The tendency of nucleons to occupy the lowest possible energy level explains why there is a tendency for the numbers of protons and neutrons to be nearly equal in lighter nuclei. In heavier nuclei the effect of electrostatic repulsion among the larger number of charges from the protons raises the energy of the protons, with the result that there are more neutrons than protons (for uranium-235, for example, there are 143 neutrons and only 92 protons). The pairing of nucleons in energy levels also helps to explain the tendency of nuclei to have even numbers of both protons and neutrons.
Neither the liquid-drop model nor the Fermi gas model, however, can explain the exceptional stability of nuclei having certain values for either the number of protons or the number of neutrons, or both. These so-called magic numbers are 2, 8, 20, 28, 50, 82, and 126. Because of the similarity between this phenomenon and the stability of the noble gases, which have certain numbers of electrons that are bound in closed “shells,” a shell model was suggested for the nucleus. There are major differences, however, between the electrons in an atom and the nucleons in a nucleus. First, the nucleus provides a force center for the electrons of an atom, while the nucleus itself has no single force center. Second, there are two different types of nucleons. Third, the assumption of independent particle motion made in the case of electrons is not as easily made for nucleons. The liquid-drop model is in fact based on the assumption of strong forces between the nucleons that considerably constrain their motion. However, these difficulties were solved and a good explanation of the magic numbers achieved on the basis of the shell model, which included the assumption of strong coupling between the spin angular momentum of a nucleon and its orbital angular momentum. Various attempts have been made, with partial success, to construct a model incorporating the best features of both the liquid-drop model and the shell model.
Scientific Notation for the Nucleus and Nuclear Reactions
A nucleus may be represented conveniently by the chemical symbol for the element together with a subscript and superscript for the atomic number and mass number. (The subscript is often omitted, since the element symbol fixes the atomic number.) The nucleus of ordinary hydrogen, i.e., the proton, is represented by 1H1, an alpha particle (a helium nucleus) is 2He4, the most common isotope of chlorine is 17Cl35, and the uranium isotope used in the atomic bomb is 92U235.
Nuclear reactions involving changes in atomic number or mass number can be expressed easily using this notation. For example, when Ernest Rutherford produced the first artificial nuclear reaction (1919), it involved bombarding a nitrogen nucleus with alpha particles and resulted in an isotope of oxygen with the release of a proton: 2He4+7N14→8O17+1H1. Note that the total of the atomic numbers on the left is equal to the total on the right (i.e., 2+7=8+1), and similarly for the mass numbers (4+14=17+1).
Scientific Investigations of the Nucleus
Following the discovery of radioactivity by A. H. Becquerel in 1896, Ernest Rutherford identified two types of radiation given off by natural radioactive substances and named them alpha and beta; a third, gamma, was later identified. In 1911 he bombarded a thin target of gold foil with alpha rays (subsequently identified as helium nuclei) and found that, although most of the alpha particles passed directly through the foil, a few were deflected by large amounts. By a quantitative analysis of his experimental results, he was able to propose the existence of the nucleus and estimate its size and charge.
After the discovery of the neutron in 1932, physicists turned their attention to the understanding of the strong interactions, or strong nuclear force, that bind protons and neutrons together in nuclei. This force must be great enough to overcome the considerable repulsive force existing between several protons because of their electrical charge. It must exist between nucleons without regard to their charge, since it acts equally on protons and neutrons, and it must not extend very far away from the nucleons (i.e., it must be a short-range force), since it has negligible effect on protons or neutrons outside the nucleus.
In 1935 Hideki Yukawa proposed a theory that this nuclear “glue” was produced by the exchange of a particle between nucleons, just as the electromagnetic force is produced by the exchange of a photon between charged particles. The range of a force is dependent on the mass of the particle carrying the force; the greater the mass of the particle, the shorter the range of the force. The range of the electromagnetic force is infinite because the mass of the photon is zero. From the known range of the nuclear force, Yukawa estimated the mass of the hypothetical carrier of the nuclear force to be about 200 times that of the electron. Given the name meson because its mass is between that of the electron and those of the nucleons, this particle was finally observed in 1947 and is now called the pi meson, or pion, to distinguish it from other mesons that have been discovered (see elementary particles).
Both the proton and the neutron are surrounded by a cloud of pions given off and reabsorbed again within an incredibly short interval of time. Certain other mesons are assumed to be created and destroyed in this way as well, all such particles being termed “virtual” because they exist in violation of the law of conservation of energy (see conservation laws) for a very short span of time allowed by the uncertainty principle. It is now known, however, that at a more fundamental level the actual carrier of the strong force is a particle called the gluon.
See G. Gamow, The Atom and Its Nucleus (1961); R. K. Adair, The Great Design: Particles, Fields, and Creation (1987).
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.
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.
REFERENCESRukovodstvo po tsitologii, vol. 1. Moscow-Leningrad, 1965.
Raikov, I. B. Kariologiia prosteishikh. Leningrad, 1967.
DeRobertis, E., W. Nowinsky, and F. Saez. Biologiia kletki. Moscow, 1973. (Translated from English.)
Chentsov, Iu. S., and V. Iu. Poliakov. Ul’trastruktura kletochnogo iadra. Moscow, 1974.
The Nucleus. Edited by A. J. Dalton and F. Haguenau. New York-London, 1968.
The Cell Nucleus, vols. 1–3. Edited by H. Busch. New York-London, 1974.
I. B. RAIKOV