| nucellus | (noun) central part of a plant ovule; contains the embryo sac | - |
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| nucha | (noun) the back side of the neck | Synonyms: nape, scruff |
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| nuclease | (noun) general term for enzymes that catalyze the hydrolysis of nucleic acid by cleaving chains of nucleotides into smaller units | - |
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| nucleole | (noun) a small round body of protein in a cell nucleus; such organelles contain RNA and are involved in protein synthesis | Synonyms: nucleolus |
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| nucleolus | (noun) a small round body of protein in a cell nucleus; such organelles contain RNA and are involved in protein synthesis | Synonyms: nucleole |
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| nucleon | (noun) a constituent (proton or neutron) of an atomic nucleus | - |
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| nucleonics | (noun) the branch of physics that studies the internal structure of atomic nuclei | Synonyms: atomic physics, nuclear physics |
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| nucleoplasm | (noun) the protoplasm that constitutes the nucleus of a cell | Synonyms: karyoplasm |
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| nucleoprotein | (noun) any of several substances found in the nuclei of all living cells; consists of a protein bound to a nucleic acid | - |
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| nucleoside | (noun) a glycoside formed by partial hydrolysis of a nucleic acid | - |
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| nucleosynthesis | (noun) (astronomy) the cosmic synthesis of atoms more complex than the hydrogen atom | - |
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| nucleotide | (noun) a phosphoric ester of a nucleoside; the basic structural unit of nucleic acids (DNA or RNA) | Synonyms: base |
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| nucleus | (noun) a part of the cell containing DNA and RNA and responsible for growth and reproduction | Synonyms: cell nucleus, karyon |
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| (noun) any histologically identifiable mass of neural cell bodies in the brain or spinal cord | - |
| (noun) the central structure of the lens that is surrounded by the cortex | Synonyms: lens nucleus |
| (noun) a small group of indispensable persons or things | Synonyms: core, core group |
| (noun) the positively charged dense center of an atom | - |
| (noun) (astronomy) the center of the head of a comet; consists of small solid particles of ice and frozen gas that vaporizes on approaching the sun to form the coma and tail | - |