NAD (nicotine adenine dinucleotide) Accepts a hydride ion (H), produced by the Krebs cycle, forming NADH, the main carrier of electrons for oxidative phosphorylation.
NADH dehydrogenase Removes electrons from NADH and passes them down the electron transport chain.
nanometer (nm) Equal to 10-9 meters or 10-3 microns.
National Institutes of Health (NIH) A biomedical research center that is part of the U.S. Department of Health and Human Services. NIH consists of more than 25 research institutes, including the National Institute of Aging (NIA) and the National Cancer Institute (NCI). All of the institutes are funded by the federal government.
natural killer cell (NK cell) A lymphocyte that kills virus-infected cells in the body; also kills foreign cells associated with a tissue or organ transplant.
neuromodulator A chemical released by neurons at a synapse that modifies the behavior of the targeted neuron(s).
neuromuscular junction A special form of synapse between a motor neuron and a skeletal muscle cell.
neuron A cell specially adapted for communication that forms the nervous system of all animals.
neurotransmitter A chemical released by the synapse that activates the targeted neuron.
non-small cell lung cancer A group of lung cancers that includes squamous cell carcinoma, adenocarcinoma, and large cell carcinoma. The small cells are endocrine cells.
northern blotting A technique for the study of gene expression. Messenger RNA (mRNA) is fractionated on an agarose gel and then transferred to a piece of nylon filter paper (or membrane). A specific mRNA is detected by hybridization with a labeled DNA or RNA probe. The original blotting technique invented by E. M. Southern inspired the name. Also known as RNA blotting.
nuclear envelope The double membrane (two lipid bilayers) enclosing the cell nucleus.
nuclear localization signal (NLS) A short amino acid sequence located on proteins that are destined for the cell nucleus, after they are translated in the cytoplasm. nucleic acid DNA or RNA, a macromolecule consisting of a chain of nucleotides.
nucleolar organizer Region of a chromosome containing a cluster of ribosomal RNA genes that gives rise to the nucleolus.
nucleolus A structure in the nucleus where ribosomal RNA is transcribed and ribosomal subunits are assembled.
nucleoside A purine or pyrimidine linked to a ribose or deoxyribose sugar.
nucleosome A beadlike structure, consisting of histone proteins.
nucleotide A nucleoside containing one or more phosphate groups linked to the 5′ carbon of the ribose sugar. DNA and RNA are nucleo-tide polymers.
nucleus Eukaryote cell organelle that contains the DNA genome on one or more chromosomes.
oligodendrocyte A myelinating glia cell of the vertebrate central nervous system.
oligo labeling A method for incorporating labeled nucleotides into a short piece of DNA or RNA. Also known as the random-primer labeling method.
oligomer A short polymer, usually consisting of amino acids (oli-gopeptides), sugars (oligosaccharides), or nucleotides (oligo-nucleotides); taken from the Greek word oligos, meaning few or little.
oncogene A mutant form of a normal cellular gene, known as a proto-oncogene, that can transform a cell to a cancerous phenotype.
oocyte A female gamete or egg cell.
operator A region of a prokaryote chromosome that controls the expression of adjacent genes.
operon Two or more prokaryote genes that are transcribed into a single mRNA.
organelle A membrane-bounded structure, occurring in eukaryote cells, that has a specialized function. Examples are the nucleus, Golgi complex, and endoplasmic reticulum.
osmosis The movement of solvent across a semipermeable membrane that separates a solution with a high concentration of solutes from one with a low concentration of solutes. The membrane must be permeable to the solvent but not to the solutes. In the context of cellular osmosis, the solvent is always water, the solutes are ions and molecules, and the membrane is the cell membrane. osteoblast Cells that form bones.
ovulation Rupture of a mature follicle with subsequent release of a mature oocyte from the ovary.
oxidative phosphorylation Generation of high-energy electrons from food molecules that are used to power the synthesis of ATP from ADP and inorganic phosphate. The electrons are eventually transferred to oxygen, to complete the process; occurs in bacteria and mitochondria.
p53 A tumor suppressor gene that is mutated in about half of all human cancers. The normal function of the p53 protein is to block passage through the cell cycle when DNA damage is detected.
parthenogenesis A natural form of animal cloning whereby an individual is produced without the formation of haploid gametes and the fertilization of an egg.
pathogen An organism that causes disease.
PCR (polymerase chain reaction) A method for amplifying specific regions of DNA by temperature cycling a reaction mixture containing the template, a heat-stable DNA polymerase, and replication primers.
peptide bond The chemical bond that links amino acids together to form a protein.
pH Measures the acidity of a solution as a negative logarithmic function (p) of H+ concentration (H). Thus, a pH of 2.0 (10-2 molar H+) is acidic, whereas a pH of 8.0 (10-8 molar H+) is basic.
phagocyte A cell that engulfs other cells or debris by phagocytosis.
phagocytosis A process whereby cells engulf other cells or organic material by endocytosis. A common practice among protozoans and cells of the vertebrate immune system; from the Greek phagein, "to eat."
phenotype Physical characteristics of a cell or organism.
phosphokinase An enzyme that adds phosphate to proteins.
phospholipid The kind of lipid molecule used to construct cell membranes. Composed of a hydrophilic head-group, phosphate, glycerol, and two hydrophobic fatty acid tails.
phosphorylation A chemical reaction in which a phosphate is covalently bonded to another molecule.
photoreceptor A molecule or cell that responds to light.
photosynthesis A biochemical process in which plants, algae, and certain bacteria use energy obtained from sunlight to synthesize macro-molecules from CO2 and H2O.
phylogeny The evolutionary history of a group of organisms, usually represented diagrammatically as a phylogenetic tree.
pinocytosis A form of endocytosis whereby fluid is brought into the cell from the environment.
pixel One element in a data array that represents an image or photograph.
placebo An inactive substance that looks the same and is administered in the same way as a drug in a clinical trial.
plasmid A minichromosome, often carrying antibiotic-resistant genes, that occurs naturally among prokaryotes; used extensively as a DNA cloning vector.
platelet A cell fragment derived from megakaryocytes and lacking a nucleus that is present in the bloodstream and is involved in blood coagulation.
ploidy The total number of chromosomes (n) that a cell has. Ploidy is also measured as the amount of DNA (C) in a given cell, relative to a haploid nucleus of the same organism. Most organisms are diploid, having two sets of chromosomes, one from each parent, but there is great variation among plants and animals. The silk gland of the moth Bombyx mori, for example, has cells that are extremely polyploid, reaching values of 100,000C, flowers are often highly polyploid, and vertebrate hepatocytes may be 16C.
pluripotency The property by which an undifferentiated animal cell can give rise to most of the body’s cell types.
poikilotherm An animal incapable of regulating its body temperature independent of the external environment. It is for this reason that such animals are restricted to warm tropical climates.
point mutation A change in DNA, particularly in a region containing a gene, that alters a single nucleotide.
polarization A term used to describe the reestablishment of a sodium ion gradient across the membrane of a neuron. Polarization followed by depolarization is the fundamental mechanism by which neurons communicate with one another.
polyacrylamide A tough polymer gel that is used to fractionate DNA and protein samples.
polyploid Possessing more than two sets of homologous chromosomes.
polyploidization DNA replication in the absence of cell division; provides many copies of particular genes and thus occurs in cells that highly active metabolically (see ploidy).
portal system A system of liver vessels that carries liver enzymes directly to the digestive tract.
post-mitotic Refers to a cell that has lost the ability to divide.
probe Usually a fragment of a cloned DNA molecule that is labeled with a radioisotope or fluorescent dye, and used to detect specific DNA or RNA molecules on southern or northern blots.
progenitor cell A cell that has developed from a stem cell but can still give rise to a limited variety of cell types.
proliferation A process whereby cells grow and divide.
promoter A DNA sequence to which RNA polymerase binds to initiate gene transcription.
prophase The first stage of mitosis; the chromosomes are duplicated and are beginning to condense but are attached to the spindle.
protein A major constituent of cells and organisms. Proteins, made by linking amino acids together, are used for structural purposes and regulate many biochemical reactions in their alternative role as enzymes. Proteins range in size from just a few amino acids to more than 200.
protein glycosylation The addition of sugar molecules to a protein.
proto-oncogene A normal gene that can be converted to a cancer-causing gene (oncogene) by a point mutation or through inappropriate expression.
protozoa Free-living, single-cell eukaryotes that feed on bacteria and other microorganisms. Common examples are Paramecium and Amoeba. Parasitic forms inhabit the digestive and urogenital tract of many animals, including humans. P-site The binding site on the ribosome for the growing protein (or peptide) chain.
purine A nitrogen-containing compound that is found in RNA and DNA. Two examples are adenine and guanine.
pyrimidine A nitrogen-containing compound found in RNA and DNA. Examples are cytosine, thymine, and uracil (RNA only).
radioactive isotope An atom with an unstable nucleus that emits radiation as it decays.
randomized clinical trial A study in which the participants are assigned by chance to separate groups that compare different treatments; neither the researchers nor the participants can choose which group. Using chance to assign people to groups means that the groups will be similar and that the treatments they receive can be compared objectively. At the time of the trial, it is not known which treatment is best. random primer labeling A method for incorporating labeled nucleotides into a short piece of DNA or RNA.
reagent A chemical solution designed for a specific biochemical or histochemical procedure.
recombinant DNA A DNA molecule that has been formed by joining two or more fragments from different sources.
refractive index A measure of the ability of a substance to bend a beam of light expressed in reference to air that has, by definition, a refractive index of 1.0.
regulatory sequence A DNA sequence to which proteins bind that regulate the assembly of the transcriptional machinery.
replication bubble Local dissociation of the DNA double helix in preparation for replication. Each bubble contains two replication forks.
replication fork The Y-shaped region of a replicating chromosome; associated with replication bubbles.
replication origin (origin of replication, ORI) The location at which DNA replication begins.
respiratory chain (electron transport chain) A collection of iron-and copper-containing proteins, located in the inner mitochondrion membrane, that use the energy of electrons traveling down the chain to synthesize ATP.
restriction enzyme An enzyme that cuts DNA at specific sites.
restriction map The size and number of DNA fragments obtained after digesting with one or more restriction enzymes.
retrovirus A virus that converts its RNA genome to DNA once it has infected a cell.
reverse transcriptase An RNA-dependent DNA polymerase. This enzyme synthesizes DNA by using RNA as a template, the reverse of the usual flow of genetic information from DNA to RNA.
ribosomal RNA (rRNA) RNA that is part of the ribosome and serves both a structural and functional role, possibly by catalyzing some of the steps involved in protein synthesis.
ribosome A complex of protein and RNA that catalyzes the synthesis of proteins.
rough endoplasmic reticulum (rough ER) Endoplasmic reticulum that has ribosomes bound to its outer surface.
Saccharomyces Genus of budding yeast that are frequently used in the study of eukaryote cell biology.
sarcoma Cancer of connective tissue.
Schwann cell Glia cell that produces myelin in the peripheral nervous system.
screening Checking for disease when there are no symptoms.
senescence Physical and biochemical changes that occur in cells and organisms with age; from the Latin word senex, meaning "old man" or "old age."
signal transduction A process by which a signal is relayed to the interior of a cell where it elicits a response at the cytoplasmic or nuclear level.
smooth muscle cell Muscles lining the intestinal tract and arteries; lack the striations typical of cardiac and skeletal muscle, giving a smooth appearance when viewed under a microscope.
somatic cell Any cell in a plant or animal except those that produce gametes (germ cells or germ cell precursors).
somatic cell nuclear transfer Animal cloning technique whereby a somatic cell nucleus is transferred to an enucleated oocyte. Synonymous with cell nuclear transfer or replacement.
Southern transfer The transfer of DNA fragments from an agarose gel to a piece of nylon filter paper. Specific fragments are identified by hybridizing the filter to a labeled probe; invented by the Scottish scientist E. M. Southern, in 1975; also known as DNA blotting. stem cell Pluripotent progenitor cell found in embryos and various parts of the body that can differentiate into a wide variety of cell types.
steroid A hydrophobic molecule with a characteristic four-ringed structure. Sex hormones, such as estrogen and testosterone, are steroids.
structural gene A gene that codes for a protein or an RNA; distinguished from regions of the DNA that are involved in regulating gene expression but are noncoding.
synapse A neural communication junction between an axon and a den -drite. Signal transmission occurs when neurotransmitters, released into the junction by the axon of one neuron, stimulate receptors on the dendrite of a second neuron.
syncytium A large multinucleated cell. Skeletal muscle cells are syncytiums produced by the fusion of many myoblasts. syngeneic transplants A patient receives tissue or an organ from an identical twin.
tamoxifen A drug that is used to treat breast cancer. Tamoxifen blocks the effects of the hormone estrogen in the body. It belongs to the family of drugs called antiestrogens.
T cell (T lymphocyte) A white blood cell involved in activating and coordinating the immune response.
telomere The end of a chromosome; replaced by the enzyme telom-erase with each round of cell division to prevent shortening of the chromosomes.
telophase The final stage of mitosis in which the chromosomes decon-dense and the nuclear envelope reforms.
template A single strand of DNA or RNA whose sequence serves as a guide for the synthesis of a complementary, or daughter, strand.
therapeutic cloning The cloning of a human embryo for the purpose of harvesting the inner cell mass (embryonic stem cells).
topoisomerase An enzyme that makes reversible cuts in DNA to relieve strain or to undo knots.
totipotency The property by which an undifferentiated animal cell can give rise to all of the body’s cell types. The fertilized egg and blastomeres from an early embryo are the only cells possessing this ability.
transcription The copying of a DNA sequence into RNA, catalyzed by RNA polymerase.
transcription factor A general term referring to a wide assortment of proteins needed to initiate or regulate transcription.
transfection Introduction of a foreign gene into a eukaryote or pro-karyote cell.
transfer RNA (tRNA) A collection of small RNA molecules that transfer an amino acid to a growing polypeptide chain on a ribosome. There is a separate tRNA for amino acid.
transgenic organism A plant or animal that has been transfected with a foreign gene.
trans Golgi network The membrane surfaces where glycoproteins and glycolipids exit the Golgi complex in transport vesicles.
translation A ribosome-catalyzed process whereby the nucleotide sequence of a mRNA is used as a template to direct the synthesis of a protein.
transposable element (transposon) A segment of DNA that can move from one region of a genome to another.
ultrasound (ultrasonography) A procedure in which high-energy sound waves (ultrasound) are bounced off internal tissues or organs producing echoes that are used to form a picture of body tissues (a sonogram).
umbilical cord blood stem cells Stem cells, produced by a human fetus and the placenta, that are found in the blood that passes from the placenta to the fetus.
vector A virus or plasmid used to carry a DNA fragment into a bacterial cell (for cloning) or into a eukaryote to produce a transgenic organism.
vesicle A membrane-bounded bubble found in eukaryote cells. Vesicles carry material from the ER to the Golgi and from the Golgi to the cell membrane.
virus A particle containing an RNA or DNA genome surrounded by a protein coat. Viruses are cellular parasites that cause many diseases.
western blotting The transfer of protein from a polyacrylamide gel to a piece of nylon filter paper. Specific proteins are detected with labeled antibodies. The name was inspired by the original blotting technique invented by the Scottish scientist E. M. Southern in 1975; also known as protein blotting.
xenogeneic transplants (xenograft) A patient receives tissue or an organ from an animal of a different species.
yeast Common term for unicellular eukaryotes that are used to brew beer and make bread. Saccharomyces cerevisiae (baker’s yeast) are also widely used in studies on cell biology.
zygote A diploid cell produced by the fusion of a sperm and egg.