nucleus

/nooh"klee euhs, nyooh"-/, n., pl. nuclei /-klee uy'/, nucleuses.

1. a central part about which other parts are grouped or gathered; core: A few faithful friends formed the nucleus of the club.

2. Biol. a specialized, usually spherical mass of protoplasm encased in a double membrane, and found in most living eukaryotic cells, directing their growth, metabolism, and reproduction, and functioning in the transmission of genic characters. See illus. under ameba.

3. Physics. the positively charged mass within an atom, composed of neutrons and protons, and possessing most of the mass but occupying only a small fraction of the volume of the atom.

4. Anat. a mass of nerve cells in the brain or spinal cord in which nerve fibers form connections.

5. Also called condensation nucleus. Meteorol. a particle upon which condensation of water vapor occurs to form water drops or ice crystals.

6. Chem. a fundamental arrangement of atoms, as the benzene ring, that may occur in many compounds by substitution of atoms without a change in structure.

7. Astron. the condensed portion of the head of a comet.

8. Phonet.

a. the central, most prominent segment in a syllable, consisting of a vowel, diphthong, or vowellike consonant, as the a-sound in cat or the l-sound in bottled; peak.

b. the most prominent syllable in an utterance or stress group; tonic syllable.

[1695-1705; < L: kernel, syncopated var. of nuculeus, equiv. to nucu(la) little nut (nuc-, s. of nux nut + -ula -ULE) + -leus n. suffix]

Syn. 1. center, kernel, heart.

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I

Central, positively charged core of an atom.

It consists of positively charged protons and neutral neutrons, known collectively as nucleons, held together by the strong force. The number of nucleons can range from 1 to about 270, depending on the element. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Some nuclei, especially heavier ones, are unstable, or radioactive (see radioactivity), emitting energy in the form of alpha rays (see alpha decay), beta rays (see beta decay), or gamma rays. The nucleus makes up nearly all the mass but only a minute fraction of the volume of the atom.

II

Specialized structure occurring in most cells (except bacteria) and separated from the rest of the cell by the nuclear membrane.

This membrane seems to be continuous with the cell's endoplasmic reticulum and has pores that permits the passage of large molecules. The nucleus controls and regulates the cell's activities (e.g., growth and metabolism) and carries the genes. Nucleoli are small bodies often seen within the nucleus that play an important part in the synthesis of RNA and protein. A cell normally contains only one nucleus.

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      in physics, the core of an atom. See atom.

▪ biology

      in biology, a specialized structure occurring in most cells (except bacteria and blue-green algae) and separated from the rest of the cell by a double layer, the nuclear membrane. This membrane seems to be continuous with the endoplasmic reticulum (a membranous network) of the cell and has pores, which probably permit the entrance of large molecules. The nucleus controls and regulates the activities of the cell (e.g., growth and metabolism) and carries the genes, structures that contain the hereditary information. Nucleoli are small bodies often seen within the nucleus; they play an important part in the synthesis of ribonucleic acid ( RNA) and protein. The gellike matrix in which the nuclear components are suspended is the nucleoplasm.

      A cell normally contains only one nucleus; under some conditions, however, the nucleus divides but the cytoplasm does not. This produces a multinucleate cell (syncytium) such as occurs in skeletal muscle fibres. Some cells—e.g., the human red blood cell—lose their nuclei upon maturation. See also cell.

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