nucleus

nucleus:

see cellcell,
in biology, the unit of structure and function of which all plants and animals are composed. The cell is the smallest unit in the living organism that is capable of integrating the essential life processes. There are many unicellular organisms, e.g.
..... Click the link for more information. , in biology.

nucleus,

in physics, the extremely dense central core of an atomatom
[Gr.,=uncuttable (indivisible)], basic unit of matter; more properly, the smallest unit of a chemical element having the properties of that element. Structure of the Atom
..... Click the link for more information. .

The Nature of the Nucleus

Composition

Atomic nuclei are composed of two types of particles, protons and neutrons, which are collectively known as nucleons. A protonproton,
elementary particle having a single positive electrical charge and constituting the nucleus of the ordinary hydrogen atom. The positive charge of the nucleus of any atom is due to its protons.
..... Click the link for more information. is simply the nucleus of an ordinary hydrogen atom, the lightest atom, and has a unit positive charge. A neutronneutron,
uncharged elementary particle of slightly greater mass than the proton. It was discovered by James Chadwick in 1932. The stable isotopes of all elements except hydrogen and helium contain a number of neutrons equal to or greater than the number of protons.
..... Click the link for more information. 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 elementelement,
in chemistry, a substance that cannot be decomposed into simpler substances by chemical means. A substance such as a compound can be decomposed into its constituent elements by means of a chemical reaction, but no further simplification can be achieved.
..... Click the link for more information. 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 isotopesisotope
, in chemistry and physics, one of two or more atoms having the same atomic number but differing in atomic weight and mass number. The concept of isotope was introduced by F.
..... Click the link for more information. , of the same element. The mass number of a given isotope is the nearest whole number to the atomic weightatomic weight,
mean (weighted average) of the masses of all the naturally occurring isotopes of a chemical element, as contrasted with atomic mass, which is the mass of any individual isotope. Although the first atomic weights were calculated at the beginning of the 19th cent.
..... Click the link for more information. of that isotope and is approximately equal to the atomic weight (in the case of carbon-12, exactly equal).

Size and Density

The nucleus occupies only a tiny fraction of the volume of an atom (the radius of the nucleus being some 10,000 to 100,000 times smaller than the radius of the atom as a whole), but it contains almost all the mass. An idea of the extreme densitydensity,
ratio of the mass of a substance to its volume, expressed, for example, in units of grams per cubic centimeter or pounds per cubic foot. The density of a pure substance varies little from sample to sample and is often considered a characteristic property of the
..... Click the link for more information. of the nucleus is revealed by a simple calculation. The radius of the nucleus of hydrogen is on the order of 10⁻¹³ cm so that its volume is on the order of 10⁻³⁹ cm³ (cubic centimeter); its mass is about 10⁻²⁴ g (gram). Combining these to estimate the density, we have 10⁻²⁴ g/10⁻³⁹ cm³ ≈ 10¹⁵ g/cm³, or about a thousand trillion times the density of matter at ordinary scales (the density of water is 1 g/cm³).

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 relativityrelativity,
physical theory, introduced by Albert Einstein, that discards the concept of absolute motion and instead treats only relative motion between two systems or frames of reference.
..... Click the link for more information. , 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 energynuclear energy,
the energy stored in the nucleus of an atom and released through fission, fusion, or radioactivity. In these processes a small amount of mass is converted to energy according to the relationship E = mc², where E is energy, m
..... Click the link for more information. ).

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 principleexclusion principle,
physical principle enunciated by Wolfgang Pauli in 1925 stating that no two electrons in an atom can occupy the same energy state simultaneously. The energy states, or levels, in an atom are described in the quantum theory by various values of four different
..... Click the link for more information. , which allows only two particles of opposite spin to occupy a particular energy level described by the quantum theoryquantum theory,
modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles; the quantum theory and the theory of relativity together form the theoretical basis of modern physics.
..... Click the link for more information. . 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 RutherfordRutherford, Ernest Rutherford, 1st Baron,
1871–1937, British physicist, b. New Zealand. Rutherford left New Zealand in 1895, having earned three degrees from the Univ.
..... Click the link for more information. 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 radioactivityradioactivity,
spontaneous disintegration or decay of the nucleus of an atom by emission of particles, usually accompanied by electromagnetic radiation. The energy produced by radioactivity has important military and industrial applications.
..... Click the link for more information. 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 interactionsstrong interactions,
actions between elementary particles mediated, or carried, by gluons. They are responsible for the binding of protons and neutrons in the nucleus and interactions between quarks.
..... Click the link for more information. , 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 photonphoton
, the particle composing light and other forms of electromagnetic radiation, sometimes called light quantum. The photon has no charge and no mass. About the beginning of the 20th cent.
..... Click the link for more information. 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 mesonmeson
[Gr.,=middle (i.e., middleweight)], class of elementary particles whose masses are generally between those of the lepton class of lighter particles and those of the baryon class of heavier particles. From a technical point of view mesons are strongly interacting bosons; i.
..... Click the link for more information. 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 pionpion
or pi meson,
lightest of the meson family of elementary particles. The existence of the pion was predicted in 1935 by Hideki Yukawa, who theorized that it was responsible for the force of the strong interactions holding the atomic nucleus together.
..... Click the link for more information. , to distinguish it from other mesons that have been discovered (see elementary particleselementary particles,
the most basic physical constituents of the universe. Basic Constituents of Matter

Molecules are built up from the atom, which is the basic unit of any chemical element. The atom in turn is made from the proton, neutron, and electron.
..... Click the link for more information. ).

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 lawsconservation laws,
in physics, basic laws that together determine which processes can or cannot occur in nature; each law maintains that the total value of the quantity governed by that law, e.g., mass or energy, remains unchanged during physical processes.
..... Click the link for more information. ) for a very short span of time allowed by the uncertainty principleuncertainty principle,
physical principle, enunciated by Werner Heisenberg in 1927, that places an absolute, theoretical limit on the combined accuracy of certain pairs of simultaneous, related measurements.
..... Click the link for more information. . It is now known, however, that at a more fundamental level the actual carrier of the strong force is a particle called the gluon.

Bibliography

See G. Gamow, The Atom and Its Nucleus (1961); R. K. Adair, The Great Design: Particles, Fields, and Creation (1987).

nucleus

(new -klee-ŭs) 1. The dark kilometer-sized body that is the permanent portion of a comet and is thought by most researchers to be the fount of all cometary activity. The density is about 0.2 g cm^–3. It contains about 75% by mass ice (mainly water ice, 85%, but with liberal amounts of CO₂, CO, H₂ CO, CH₃ OH, and NH₃) and 25% by mass dust thought to have a composition similar to carbonaceous chondritic meteorites (see carbonaceous chondrite). It is often described as a dirty snowball. A comet of mass 10¹⁸ grams would have a nucleus of diameter about 12 km. A positive identification of the nucleus of comets has recently been achieved using radar. The nucleus of comet Halley was imaged by the Giotto spacecraft in March 1986, and was found to be a huge potato-shaped object, 16 km long and 8 km wide. It was active only over 10% of its surface and was spinning every 54 hours, precessing every 7.4 days. A comet's nucleus of dirty ice is surrounded by a thin layer (a few cm) of insulating dust from which the ice has sublimated. This enables it to survive over 2000 perihelion passages. At each passage the nucleus loses on average a layer of material one meter thick. In Halley's comet this amounted to 3 × 10¹⁴ g. This material forms the coma and comet tails and also any associated meteoroid stream. 2. The small core of an atom, consisting of protons and neutrons bound together by strong nuclear forces. The nucleus has a positive charge equal to Ze , where e is the magnitude of the electron charge and Z the number of protons present – the atomic number. The total number of protons plus neutrons is the mass number, A . A given element is characterized by its atomic number but may, within limits, have different numbers of neutrons in its nuclei, giving rise to different isotopes of the element. The total mass of the protons and neutrons bound together in a nucleus is less than when the particles are unbound. This mass difference is equivalent to the energy required to bind the particles together. Nuclei are represented by their chemical symbols to which numbers are attached; usually the mass number is added as a left superscript to indicate a particular isotope, as in ⁴He. The radii of nuclei are commonly measured in femtometers (1 fm = 10^–15m). The femtometer is sometimes referred to as a ‘fermi’. 3. The central region of a galaxy.

Nucleus

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

Nucleus

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.

REFERENCES

Rukovodstvo 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

nucleus

[′nü·klē·əs] (astronomy) The small permanent body of a comet, believed to have a diameter between one and a few tens of kilometers, and to be composed of water and volatile hydrocarbons. (cell and molecular biology) A small mass of differentiated protoplasm rich in nucleoproteins and surrounded by a membrane; found in most animal and plant cells, contains chromosomes, and functions in metabolism, growth, and reproduction. (computer science) That portion of the control program that must always be present in main storage. The main storage area used in the nucleus (first definition) and other transient control program routines. (hydrology) A particle of any nature upon which, or a locus at which, molecules of water or ice accumulate as a result of a phase change to a more condensed state. (neuroscience) A mass of nerve cells in the central nervous system. (nuclear physics) The central, positively charged, dense portion of an atom. Also known as atomic nucleus. (science and technology) A central mass about which accretion takes place.

nucleus

In ancient construction, the internal part of the flooring, consisting of a strong cement, over which the pavement was laid, bound with mortar.

nucleus

1. Biology (in the cells of eukaryotes) a large compartment, bounded by a double membrane, that contains the chromosomes and associated molecules and controls the characteristics and growth of the cell 2. Anatomy any of various groups of nerve cells in the central nervous system 3. Astronomy the central portion in the head of a comet, consisting of small solid particles of ice and frozen gases, which vaporize on approaching the sun to form the coma and tail 4. Physics the positively charged dense region at the centre of an atom, composed of protons and neutrons, about which electrons orbit 5. Chem a fundamental group of atoms in a molecule serving as the base structure for related compounds and remaining unchanged during most chemical reactions 6. Botany the central point of a starch granule 7. Logic the largest individual that is a mereological part of every member of a given class

nucleus

[noo´kle-us] (pl. nu´clei) (L.) 1. cell nucleus; a spheroid body within a cell, contained in a double membrane, the nuclear envelope, and containing the chromosomes and one or more nucleoli. The contents are collectively referred to as nucleoplasm. The chromosomes contain deoxyribonucleic acid (DNA), which is the genetic material that codes for the structure of all the proteins of the cell.2. a mass of matter" >gray matter in the central nervous system, especially such a mass marking the central termination of a cranial nerve.3. in organic chemistry, the combination of atoms forming the central element or basic framework of the molecule of a specific compound or class of compounds.4. the dense core of an atom, made of protons and neutrons held together by the strong nuclear force. Traveling in orbit around it is a cloud of negatively charged particles called electrons. The number of protons in the atomic nucleus gives a substance its identity as a particular element. Called also atomic nucleus. adj., adj nu´clear.

The nucleus and nucleolus of a cell. From Mahon and Manuselis, 2000.nucleus ambi´guus the nucleus of origin of motor fibers of the glossopharyngeal, vagus, and accessory nerves in the medulla oblongata.nucleus an´sae lenticula´ris (nucleus of ansa lenticularis) a collection of neurons in the ansa lenticularis as it curves around the medial edge of the globus pallidus.arcuate nuclei of medulla oblongata, nu´clei arcua´ti medul´lae oblonga´tae small irregular areas of gray substance on the ventromedial aspect of the pyramid of the medulla oblongata.atomic nucleus nucleus (def. 3).basal nuclei (nu´clei basa´les) specific interconnected subcortical masses of matter" >gray matter embedded in each hemisphere" >cerebral hemisphere and in the upper brainstem, comprising the corpus striatum (caudate and lentiform nuclei), body" >amygdaloid body, claustrum, and external, extreme, and capsules" >internal capsules. Called also basal ganglia.caudal olivary nucleus a folded band of gray substance enclosing a white core, which produces the elevation on the medulla oblongata known as the olive.caudate nucleus (nucleus cauda´tus) an elongated, arched gray mass closely related to the lateral ventricle throughout its entire extent, which, together with the putamen, forms the neostriatum.nucleus ceru´leus a compact aggregation of pigmented neurons lying below the locus ceruleus.cochlear nuclei, anterior and posterior the nuclei of termination of sensory fibers of the cochlear nerve (see anatomic Table of Nerves in the Appendices); they partly encircle the inferior cerebellar peduncle at the junction of the medulla oblongata and pons.dentate nucleus (nucleus denta´tus) the largest of the deep cerebellar nuclei, lying in the matter" >white matter of the cerebellum just lateral to the nucleus" >emboliform nucleus.droplet nuclei small particles of pathogen-containing respiratory secretions expelled into the air by coughing, which are reduced by evaporation to small, dry particles that can remain airborne for long periods; one possible mechanism for transmission of infection from one individual to another.emboliform nucleus (nucleus embolifor´mis) a small cerebellar nucleus lying between the nucleus" >dentate nucleus and the nucleus" >globose nucleus and contributing to the superior peduncles" >cerebellar peduncles.fastigial nucleus (nucleus fasti´gii) the most medial of the deep cerebellar nuclei, near the midline in the roof of the fourth ventricle.globose nucleus (nucleus globo´sus) a cerebellar nucleus lying between the nucleus" >emboliform nucleus and the nucleus fastigii and projecting its fibers via the superior peduncle" >cerebellar peduncle.intracerebellar nuclei four accumulations of matter" >gray matter embedded in the matter" >white matter of the cerebellum, comprising the nucleus" >dentate nucleus, nucleus" >emboliform nucleus, nucleus fastigii, and nucleus" >globose nucleus.lenticular nucleus (lentiform nucleus) the part of the corpus striatum somewhat resembling a biconvex lens, divided into a larger external or lateral part called the putamen and a smaller light colored internal or medial part called the globus pallidus.motor nucleus any collection of cells in the central nervous system giving origin to a motor nerve.olivary nucleus a folded band of gray matter that encloses a white core and produces the elevation called the olive on the medulla oblongata; it receives heavy projections from the spinal cord, mesencephalon, and cerebral cortex and projects fibers via the contralateral inferior cerebellar peduncle.nuclei of origin, nu´clei ori´ginis groups of nerve cells in the central nervous system from which arise the motor, or efferent, fibers of the cranial nerves.paraventricular nucleus (nucleus paraventricula´ris) a band of cells in the wall of the third ventricle in the supraoptic part of the hypothalamus; many of its cells are neurosecretory in function and project to the neurohypophysis, where they secrete oxytocin (and, to a lesser extent, antidiuretic hormone).pontine nuclei, nu´clei pon´tis masses of nerve cells scattered throughout the ventral part of the pons, in which the longitudinal fibers of the pons terminate, and whose axons in turn cross to the opposite side and form the middle cerebellar peduncle, which projects fibers to the neocerebellum.nucleus pro´prius a column of large neurons that extends throughout the posterior horn of the spinal cord.nucleus pulpo´sus (pulpy nucleus) a semifluid mass of fine white elastic fibers forming the center of an intervertebral disk.red nucleus (nucleus ru´ber) an oval mass of gray matter (pink in fresh specimens) in the anterior part of the tegmentum and extending into the posterior part of the hypothalamus; it receives fibers from the cerebellum.sensory nucleus the nucleus of termination of the afferent (sensory) fibers of a peripheral nerve.supraoptic nucleus (nucleus supraop´ticus) one just above the lateral part of the optic chiasm; many of its cells are neurosecretory in function and project to the neurohypophysis, where they secrete antidiuretic hormone (ADH) and, to a lesser extent, oxytocin; other cells are osmoreceptors that stimulate ADH release in response to increased osmotic pressure.tegmental nucleus, laterodorsal several nuclear masses of the reticular formations of the pons and midbrain, especially of the latter, where they are in close approximation to the superior cerebellar peduncles.thoracic nucleus (nucleus thora´cicus) thoracic column.vestibular nuclei, nu´clei vestibula´res the four cellular masses in the floor of the fourth ventricle: superior (rostral or cranial), lateral, medial, and inferior (caudal) vestibular nuclei; in them are the terminations of the branches of the vestibular nerve (see anatomic Table of Nerves in the Appendices). The nuclei give rise to a widely dispersed special sensory system through projections to motor nuclei in the brain stem and cervical cord, to the cerebellum, and to motor cells throughout the spinal cord; they also have connections that provide for conscious perception of, and autonomic reactions to, labyrinthine stimulation.

nu·cle·us

, pl.

nu·cle·i

(nū'klē-ŭs, nū'klē-ī), 1. In cytology, typically a rounded or oval mass of protoplasm within the cytoplasm of a plant or animal cell; it is surrounded by a nuclear envelope that encloses euchromatin, heterochromatin, and one or more nucleoli, and undergoes mitosis during cell division.
See also: virion. Synonym(s): karyon2. By extension, because of similar function, the genome of microorganisms (microbes), which is relatively simple in structure, lacks a nuclear envelope or membrane and does not undergo mitosis during replication.
See also: virion. Synonym(s): nucleoid (3) 3. In neuroanatomy, a group of nerve cell bodies in the brain or spinal cord that can be demarcated from neighboring groups on the basis of either differences in cell type or the presence of a surrounding zone of nerve fibers or cell-poor neuropil. 4. Any substance (for example, foreign body, mucus, crystal) around which a urinary or other calculus has formed. 5. The central portion of an atom (composed of protons and neutrons) where most mass and all the positive charge are concentrated. 6. A particle on which a crystal, droplet, or bubble forms. 7. A characteristic arrangement of atoms in a series of molecules; for example, the benzene nucleus is a series of aromatic compounds. [L. a little nut, the kernel, stone of fruits, the inside of a thing, dim. of nux, nut]

nucleus

(no͞o′klē-əs, nyo͞o′-)n. pl. nu·clei (-klē-ī′) or nu·cleuses 1. Biology A membrane-bound organelle within a eukaryotic cell that contains most of the cell's genetic material. DNA transcription takes place in the nucleus.2. Anatomy A group of specialized nerve cells or a localized mass of gray matter in the brain or spinal cord.3. Physics The positively charged central region of an atom, composed of protons and neutrons, about which negatively charged electrons orbit. Extremely small and dense, the nucleus contains almost all of the mass of an atom.4. Chemistry A group of atoms bound in a structure, such as a benzene ring, that is resistant to alteration in chemical reactions.

nucleus

Histology
The organelle in a eukaryotic cell which contains the genomic information (DNA), replicative and transcriptional machinery (RNA), and binding proteins needed to copy the genomic information and encode the structural and functional proteins required for cell activity.
Imaging
An MRI term for the core or centre of most atoms, which contains protons (positive charge) and neutrons (no electrical charge). For the common isotope of hydrogen, the nucleus has a single proton.
Neuroanatomy
An aggregate of neuronal cell bodies sharing a common function (e.g., accessory nucleus, caudate nucleus, nucleus ambiguus).
Organic chemistry
The part of a molecule that is the major determinant of its chemical behaviour (e.g., benzene ring, β-lactam ring, cyclopentano-perhydrophenanthrene in steroids).

nu·cle·us

, pl. nucleuses (nū'klē-ŭs, -ĕz) 1. cytology Typically a rounded or oval mass of protoplasm within the cytoplasm of a plant or animal cell; it is surrounded by a nuclear envelope, which encloses euchromatin, heterochromatin, and one or more nucleoli and undergoes mitosis during cell division.
Synonym(s): karyon. 2. By extension, because of similar function, the genome of microorganisms (microbes), which is relatively simple in structure, lacks a nuclear membrane and does not undergo mitosis during replication.
Synonym(s): nucleoid (3) . 3. neuroanatomy a group of nerve cell bodies in the brain or spinal cord that can be demarcated from neighboring groups on the basis of either differences in cell type or the presence of a surrounding zone of nerve fibers or cell-poor neuropil. 4. Any substance (e.g., foreign body, mucus, crystal) around which a urinary or other calculus is formed. 5. The central portion of an atom (composed of protons and neutrons) where most of the mass and all of the positive charge are concentrated. 6. A particle on which a crystal, droplet, or bubble forms. 7. A characteristic arrangement of atoms in a series of molecules; e.g., the benzene nucleus in a series of aromatic compounds.
See also: virion

nucleus

1. Of a body cell, the central structure consisting of the tightly bundled genetic material DNA surrounded by a nuclear membrane. 2. Of an atom, the central core of protons and, except in the case of hydrogen, neutrons which is surrounded by a rapidly moving cloud of electrons, widely separated from it. The forces which bind together the protons and neutrons are immensely powerful and it is these forces which are released in an atomic explosion. From the Latin nucleus , a nut or kernel.

nucleus

an organelle of eukaryotic cells that is bounded by a NUCLEAR MEMBRANE and contains the chromosomes whose genes control the structure of proteins within the cell.
(anatomy) the mass of nerve cell bodies, connected by tracts of nerve fibres, which occur in the vertebrate brain.

Nucleus

The central part of a cell that contains most of its genetic material, including chromosomes and DNA.Mentioned in: Gene Therapy

nucleus

1. A mass of grey matter composed of nerve cell bodies in any part of the brain or spinal cord and dealing with a common function. 2. Core or central portion of the cell body of a neuron, containing cellular DNA in particular. Plural: nuclei.
abducens nucleus Nucleus of the abducens nerve (sixth cranial nerve) located in the lower part of the pons and whose axons supply the lateral rectus muscle.
accessory oculomotor nucleus See Edinger-Westphal nucleus.
nucleus of the crystalline lens See crystalline lens.
Edinger-Westphal nucleus Part of the oculomotor nucleus, it is situated posterior to the main nucleus and contains the parasympathetic component of the complex. Axons from the Edinger-Westphal pass out along the third (or oculomotor) nerve to synapse in the ciliary ganglion. Postganglionic fibres pass through the short ciliary nerves to the sphincter pupillae and ciliary muscles. The nucleus also receives fibres concerned with accommodation and fibres from the pretectal nucleus dealing with pupil light reflexes. Syn. accessory oculomotor nucleus; accessory parasympathetic nucleus. See pretectal nucleus; pupil light reflex.
lateral nucleus Part of the oculomotor nucleus which supplies, via the oculomotor nerve, all the extraocular muscles except the superior oblique and the lateral rectus muscles.
lateral geniculate nucleus See lateral geniculate body.
oculomotor nucleus This is the nucleus of the oculomotor nerve (third cranial nerve). It is a complex mass of cells located in the midbrain at the level of the superior colliculus and beneath the cerebral aqueduct (of Sylvius) which connects the third and fourth ventricles. It is divided into several subnuclei. See oculomotor nerve; Edinger-Westphal nucleus; Perlia's nucleus; trochlear nucleus.
olivary nucleus See pretectum; pupil light reflex.
Perlia's nucleus Midline part of the oculomotor nucleus. It is rudimentary in man and primates and may provide part of the innervation of the superior rectus muscle.
pretectal nucleus A complex group of nerve cells in the midbrain anterior to the superior colliculi. One of these, the pretectal olivary nucleus, receives retinal inputs via the optic tract and superior brachium and sends axons to both Edinger-Westphal nuclei. It constitutes a centre of the pupil light reflex. Another, the nucleus of the optic tract, may be involved in the control of reflex eye movements. Other fibres from the pretectal nucleus innervate the cornea, the iris, the ciliary muscle and the extraocular muscles (except the lateral rectus and superior oblique muscles), as well as the levator palpebrae muscle.
trochlear nucleus A nucleus of the trochlear nerve (fourth cranial nerve) located at the level of the inferior colliculus and below the posterior end of the oculomotor nerve nucleus, it sends fibres to the contralateral superior oblique muscle.

nu·cle·us

, pl. nucleuses (nū'klē-ŭs, -ĕz) 1. [TA] In cytology, typically a rounded or oval mass of protoplasm within the cytoplasm of a plant or animal cell. 2. By extension, because of similar function, genome of microorganisms (microbes), which is relatively simple in structure, lacks a nuclear envelope or membrane and does not undergo mitosis during replication.
Synonym(s): nucleoid (3) . 3. [TA] In neuroanatomy, group of nerve cell bodies in brain or spinal cord that can be demarcated from neighboring groups on the basis of either differences in cell type or the presence of a surrounding zone of nerve fibers or cell-poor neuropil. 4. Any substance (e.g., foreign body, mucus, crystal) around which a urinary or other calculus has formed.

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The Nature of the Nucleus

Composition

Size and Density

Mass Defect, Binding Energy, and Nuclear Reactions

Models of the Nucleus

Scientific Notation for the Nucleus and Nuclear Reactions

Scientific Investigations of the Nucleus

Bibliography

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Synonyms for nucleus

noun centre

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noun a source of further growth and development

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noun a part of the cell containing DNA and RNA and responsible for growth and reproduction

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noun the positively charged dense center of an atom

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noun a small group of indispensable persons or things

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noun (astronomy) the center of the head of a comet

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noun any histologically identifiable mass of neural cell bodies in the brain or spinal cord

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noun the central structure of the lens that is surrounded by the cortex

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