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radioactive isotope:
Form of an atom with an unstable nucleus that emits radiation
as it decays.
radioautography:
see autoradiography
Ras protein:
One of a large family of GTP-binding proteins that help relay
signals from cell-surface receptors to the nucleus. Named for
the ras gene, first identified in viruses that cause rat
sarcomas.
reaction:
In chemistry, any process in which the arrangement of atoms
into molecules is changed.
reading frame:
The phase in which nucleotides are read in sets of three to
encode a protein; an mRNA molecule can be read in any one of
three reading frames.
receptor:
Protein that binds a specific extracellular signaling molecule
(ligand) and initiates a response in the cell. Cell-surface
receptors, such as the acetylcholine receptor and the insulin
receptor, are located in the plasma membrane, with their ligand-binding
site exposed to the external medium. Intracellular receptors,
such as steroid hormone receptors, bind ligands that diffuse
into the cell across the plasma membrane.
recessive:
Refers to the member of a pair of alleles that fails to be expressed in the
phenotype of the organism when the dominant member is present. Also refers to the phenotype of an
individual that has only the recessive allele.
recombinant DNA:
Any DNA molecule formed by joining DNA segments from different sources. Recombinant DNAs are widely used in the cloning of genes, in the genetic modification of organisms, and in molecular biology generally.
recombination:
Process in which chromosomes or DNA molecules are broken and the fragments are rejoined in new combinations. Can occur in the living cell - for example, through crossing-over during meiosis - or in the test tube using purified DNA and enzymes that break and ligate DNA strands.
red blood cell:
see erythrocyte
reduction (verb reduce):
Addition of electron density to an atom, as occurs during the addition of hydrogen to a molecule or the removal of oxygen from it. Opposite of oxidation.
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