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Natural active immunity Immunity that is established
after the occurrence of a disease.
Natural killer (NK) cell A type of leukocyte that attacks
cancerous or virus-infected cells without previous
exposure to the antigen. NK cell activity is stimulated
by interferon.
Natural passive immunity Immunity conferred by the
mother on the fetus or newborn.
Nitrogen fixation A biological process (usually associated
with plants) whereby certain bacteria convert nitrogen
in the air to ammonia, thus forming a nutrient
essential for plant growth.
Nitrogenous base See Base.
Noncoding DNA DNA that does not encode any product
(RNA or protein). The majority of the DNA in plants
and animals is noncoding.
Nuclease An enzyme that, by cleaving chemical bonds,
breaks down nucleic acids into their constituent
nucleotides.
Nucleic acids Large molecules, generally found in the
cell's nucleus and/or cytoplasm, that are made up of
nucleotides. The two most common nucleic acids are
DNA and RNA.
Nucleotides The building blocks of nucleic acids. Each
nucleotide is composed of sugar, phosphate and one of
four nitrogen bases. The sugar in DNA is deoxyribose
and RNA's sugar is ribose. The sequence of the bases
within the nucleic acid determines the sequence of
amino acids in a protein. See also Base.
Nucleus The structure within eukaryotic cells that
contains chromosomal DNA.
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