cAMP

Cyclic adenosine monophosphate; a nucleotide derivative that acts as a signaling molecule in cells.

Cancer: The pathological condition of having certain cells that divide without control, compromising the health of an individual.

Carcinogens: Molecules or energy (e.g., radiation) that can convert cells from normal to transformed (cancerous). They can act either directly or indirectly on cells. In the latter case they can be converted from inactive carcinogens to active carcinogens in the liver.

Carcinoma: The most common form of human cancer. It is characterized by unregulated division of epithelial cells.

Carrier Proteins: Membrane proteins that transport molecules across membranes.

Catabolite Repressor: Mediates the repression of transcription of many genes involved in sugar metabolism (including the ones in the lactose operon) in bacteria.

Catalysis: The process of facilitating a chemical reaction by reducing the activation energy.

Catalyze: To facilitate a chemical reaction by reducing its activation energy. This process does not affect the equilibrium of a reversible reaction.

Cdc2 (Cell division Cycle 2 or Cdk1): A cyclin-dependent protein that works with cyclin B as the mitosis-promoting factor in the eukaryotic cell division cycle.

Cdk (Cyclin Dependent Kinase): Any of a group of proteins involved in the cell cycle. They are active only when bound to a cyclin. The Cdk's stimulate movement through various parts of the cell cycle through phosphorylation.

Cdk1 (Cell Division Cycle 2 or Cdc2): A cyclin-dependent protein that works with cyclin B as the mitosis-promoting factor in the eukaryotic cell division cycle.

Cell Line: A clone of cells usually derived from either an embryo or a tumor. A cell line is regarded as immortal because it will always continue to propagate so long as it is replenished with fresh nutrients.

Cell Strain: A clone of cells usually derived from typical animal or plant tissue. It is a mortal group of cells because the cells will eventually reach cell crisis (i.e., die) once they have divided a certain number of times. The life expectancy of animal cell strains is dictated, in part, by the age of the subject from which the cells were isolated.

Centrifuge: A machine that can create gravitational forces used to separate cells, cellular structures, and molecules according to their differential behaviors in a centrifugal field. The separation is achieved owing to differences in the mass, shape, and density of the structures being separated.

Chain Elongation: The process of adding amino acids to a growing chain during protein synthesis.

Channel Formers: Molecules that embed themselves in membranes and pass ions down their gradients. They are a type of ionophore.

Chemiosmotic Phosphorylation: A theory originally proposed by Peter Mitchell which suggests that the buildup of both a pH and an electrical gradient across a membrane can be coupled to the phosphorylation of ATP from ADP.

Chlorophyll: The main pigment molecule family in most vascular plants. Chlorophylls have unstable porphyrin pigments containing magnesium surrounded by five rings. One ring has a long hydrocarbon tail that helps anchor the chlorophyll to the membrane.

Chloroplasts: Double-membrane organelles in plant cells that contain chlorophyll. They are the sites of chemiosmotic photophosphorylation, which occurs in the thylakoid membranes, as well as carbon fixation, which occurs in the stroma.

Cholesterol: An amphipathic lipid that contains the four-ring steroid arrangement. Cholesterol is often found in cell membranes, where it regulates fluidity. It is also the precursor for most other steroids and is shipped into the cell through receptor-mediated endocytosis.

Chromatid: One of a pair of homologous chromosomes formed during the S phase and joined at the centromere. They separate during mitosis, with one chromatid going to each of the two new daughter cells.

Chromatin: Found in the nucleus of eukaryotic cells and consisting of DNA, histones, and nonhistone proteins. Euchromatin is usually less dense than heterochromatin. Experiments suggest that euchromatin represents the DNA that is being actively transcribed.

Clathrate Structure: An ordered cage of water molecules formed around hydrophobic molecules in an aqueous medium.

Codon: Sequence of three nucleotides in DNA or mRNA which specifies that a particular amino acid be added during protein synthesis.

Cofactor: Small molecules or metal ions required in certain enzyme-catalyzed reactions.

Confocal Microscope: A microscope developed in the 1980s that uses a laser for imaging single spots that it can store in a computer and display as a complete image. The confocal microscope increases the practical limit of resolution of the microscope, since much of the stray reflected image generated in a standard microscope is deflected because of the confocal pinhole in the confocal microscope.

Conformation: The precise shape of a molecule defined by the three-dimensional spatial relationship of the atoms that make up the molecule.

Connexin: The functional unit of the gap or electrical junction. It consists of a pore surrounded by a ring of six proteins. Ions or small molecules travel through the pore and cause signaling and electrical changes in cells joined by gap junctions.

Constitutive Expression: The constant expression seen in biological processes that are always turned on and thus can't be regulated by an external stimulus such as a hormone. It is the opposite of "regulated expression."

Covalent Bond: An attractive chemical force generated by equal sharing of one or more electrons between two atoms.

Critical Concentration (Cc): That concentration of either tubulin or actin monomers above which polymerization of the microtubule or actin filament occurs. When the concentration of the local monomers falls below the critical concentration, the microtubule or actin filament disassembles.

Cyanide: A toxin that stops the electron transport system in mitochondria and thus kills cells and organisms by decreasing the amount of ATP available to run metabolic processes.

Cyclin: A molecule that can combine with cyclin-dependent protein kinases to activate different stages in the cell cycle. Cyclins increase or decrease in amount according to the cell cycle stage.

Cytokinesis: The division of the cytoplasm during cell division.