Ca2+/calmodulin-dependent protein kinase (CaM kinase) Protein kinase whose activity is regulated by the binding of Ca2+-activated calmodulin (Ca2+/calmodulin), and which indirectly mediates the effects of Ca2+ by phosphorylation of other proteins. cadherin A member of a family of proteins that mediates Ca2+-dependent cell cell adhesion in animal tissues. caged molecule Organic molecule designed to change into an active form
when irradiated with light of a specific wavelength. An example is caged ATP. calcium pump (Ca2+ ATPase) Transport protein in the membrane of the
sarcoplasmic reticulum of muscle cells (and elsewhere) that pumps Ca2+ out of
the cytoplasm into the sarcoplasmic reticulum using the energy of ATP
hydrolysis. calmodulin Ubiquitous calcium-binding protein whose binding to other
proteins is governed by changes in intracellular Ca2+ concentration. Its binding
modifies the activity of many target enzymes and membrane transport proteins. calorie Unit of heat. One calorie (small "c") is the amount of heat needed to
raise the temperature of 1 gram of water by 1°C. A kilocalorie (1000 calories)
is the unit used to describe the energy content of foods. Calvin cycle (Calvin-Benson cycle) Major metabolic pathway by which CO2
is incorporated into carbohydrate during the second stage of photosynthesis
(carbon fixation) in plants. Also called the carbon-fixation cycle. CAM see cell adhesion molecule CaM kinase see Ca2+/calmodulin-dependent protein kinase CaM-kinase II Multifunctional Ca2+/calmodulin-dependent protein kinase
found in all animal cells that undergoes autophosphorylation when activated. It
is especially abundant in brain and is thought to have a role in learning and
memory in vertebrates. cAMP see cyclic AMP capacitation Poorly understood process that sperm must go through in the
female reproductive tract before they are competent for fertilization. capsid Protein coat of a virus, formed by the self-assembly of one or more
protein subunits into a geometrically regular structure. carbohydrate General term for sugars and related compounds containing
carbon, hydrogen, and oxygen, usually with the empirical formula (CH2O)n carbon fixation Process by which green plants incorporate carbon atoms from
atmospheric carbon dioxide into sugars. The second stage of photosynthesis. carbon-fixation cycle see Calvin cycle carbonyl group Pair of atoms consisting of a carbon atom linked to an oxygen
atom by a double bond (C=O). carboxyl group Carbon atom linked both to an oxygen atom by a double bond
and to a hydroxyl group. Molecules containing a carboxyl group are weak
acids carboxylic acids ( ). carboxyl terminus (C terminus) The end of a polypeptide chain that carries a
free a-carbonyl group. carcinogen Any agent, such as a chemical or a form of radiation, that causes
cancer. carcinogenesis Generation of a cancer. carcinoma Cancer of epithelial cells. The most common form of human
cancer. cardiac muscle Specialized form of striated muscle found in the heart,
consisting of individual heart muscle cells linked together by cell junctions. carrier protein Membrane transport protein that binds to a solute and
transports it across the membrane by undergoing a series of conformational
changes. cartilage Form of connective tissue composed of cells (chondrocytes)
embedded in a matrix rich in type II collagen and chondroitin sulfate. caspase Any of a family of intracellular proteases that are involved in
initiating the cellular events of apoptosis. catabolism General term for the enzyme-catalyzed reactions in a cell by which
complex molecules are degraded to simpler ones with release of energy.
Intermediates in these reactions are sometimes called catabolites. catalyst Substance that can lower the activation energy of a reaction, thus
increasing its rate. caveola (caveolae) Invaginations at the cell surface that bud off internally to
form pinocytic vesicles. Thought to form from lipid rafts, regions of membrane
rich in certain lipids. CD28 Cell-surface protein on T cells that binds the co-stimulatory B7 protein
on "professional" antigen-presenting cells, providing an additional signal
required for the activation of a naïve T cell by antigen. CD4 Co-receptor protein found on helper T cells that binds to class II MHC
molecules outside the antigen-binding site. CD8 Co-receptor protein found on cytotoxic T cells that binds to class I MHC
molecules outside the antigen-binding site. cdc gene see cell-division-cycle gene Cdk inhibitor protein (CKI) Protein that binds to and inhibits cyclin-Cdk
complexes, primarily involved in the control of G1 and S phases. Cdk-activating kinase (CAK) Protein kinase that phosphorylates Cdks in
cyclin-Cdk complexes, activating the Cdk. Cdk see cyclin-dependent kinase cDNA DNA molecule made as a copy of messenger RNA and therefore
lacking the introns that are present in genomic DNA. cDNA clones represent
DNA cloned from cDNA and a collection of such clones, usually representing
the genes expressed in a particular cell type or tissue, is a cDNA library. cell adhesion molecule (CAM) Protein on the surface of an animal cell that
mediates cell cell binding or cell matrix binding. cell body Main part of a nerve cell that contains the nucleus. The other parts
are the axons and dendrites. cell coat see glycocalyx cell cortex Specialized layer of cytoplasm on the inner face of the plasma
membrane. In animal cells it is an actin-rich layer responsible for movements
of the cell surface. cell cycle (cell-division cycle) Reproductive cycle of a cell: the orderly
sequence of events by which a cell duplicates its contents and divides into two. cell division Separation of a cell into two daughter cells. In eucaryotic cells it
entails division of the nucleus (mitosis) closely followed by division of the
cytoplasm (cytokinesis). cell fate In developmental biology, describes what a particular cell at a given
stage of development will normally give rise to. cell fusion Process in which the plasma membranes of two cells fuse down at
the point of contact between them, allowing the two cytoplasms to mingle. cell junction Specialized region of connection between two cells or between a
cell and the extracellular matrix. cell line Population of cells of plant or animal origin capable of dividing
indefinitely in culture. cell plate Flattened membrane-bounded structure that forms by fusing vesicles
in the cytoplasm of a dividing plant cell and is the precursor of the new cell
wall. cell wall Mechanically strong extracellular matrix deposited by a cell outside
its plasma membrane. It is prominent in most plants, bacteria, algae, and fungi.
Not present in most animal cells. cell-cycle control system Network of regulatory proteins that governs
progression of a eucaryotic cell through the cell cycle. cell-division-cycle gene (cdc gene) Gene that controls a specific step or set of
steps in the eucaryotic cell cycle. Originally identified in yeasts. cell-free system Fractionated cell homogenate that retains a particular
biological function of the intact cell, and in which biochemical reactions and
cell processes can be more easily studied. cell-mediated immune response That part of an adaptive immune response in
which antigen-specific T cells are activated to perform various functions such
as killing infected cells and activating macrophages. cellularization The formation of cells around each nucleus in a multinucleate
cytoplasm, transforming it into a multicellular structure. cellulose Structural polysaccharide consisting of long chains of covalently
linked glucose units. It provides tensile strength in plant cell walls. centimorgan see genetic map distance central lymphoid organ (primary lymphoid organ) Lymphoid organ in
which lymphocytes develop. In adult mammals these are the thymus and bone
marrow. central nervous system (CNS) Main information-processing organ of the
nervous system. In vertebrates it consists of the brain and spinal cord. centriole Short cylindrical array of microtubules, closely similar in structure to
a basal body. A pair of centrioles is usually found at the center of a centrosome
in animal cells. centromere Constricted region of a mitotic chromosome that holds sister
chromatids together. It is also the site on the DNA where the kinetochore
forms that captures microtubules from the mitotic spindle. centrosome cycle Duplication of the centrosome (during interphase) and
separation of the two new centrosomes (at the beginning of mitosis), which
provides two centrosomes to form the poles of the mitotic spindle. centrosome Centrally located organelle of animal cells that is the primary
microtubule-organizing center and acts as the spindle pole during mitosis. In
most animal cells it contains a pair of centrioles. CG island Region of DNA with a greater than average density of CG
sequences; these regions generally remain unmethylated. CGN see cis Golgi network channel protein Membrane transport protein that forms an aqueous pore in the
membrane through which a specific solute, usually an ion, can pass. chaperone (molecular chaperone) Protein that helps other proteins avoid
misfolding pathways that produce inactive or aggregated polypeptides. checkpoint Point in the eucaryotic cell-division cycle where progress through
the cycle can be halted until conditions are suitable for the cell to proceed to
the next stage. chelate Combine reversibly, usually with high affinity, with a metal ion such
as iron, calcium, or magnesium. chemical group Set of covalently linked atoms, such as a hydroxyl group
( OH) or an amino group ( NH2), the chemical behavior of which is well
characterized. chemiosmotic coupling Mechanism in which a gradient of hydrogen ions (a
pH gradient) across a membrane is used to drive an energy-requiring process,
such as ATP production or the rotation of bacterial flagella. chemokine Small secreted protein that attracts cells, such as white blood cells,
to move towards its source. Important in the functioning of the immune system. chemotaxis Directed movement of a cell or organism towards or away from a
diffusible chemical. chiasma (chiasmata) X-shaped connection visible between paired
homologous chromosomes in division I of meiosis, and which represents a site
of crossing-over. chlorophyll Light-absorbing green pigment that plays a central part in
photosynthesis in bacteria, plants, and algae. chloroplast Organelle in green algae and plants that contains chlorophyll and
carries out photosynthesis. It is a specialized form of plastid. cholesterol Lipid molecule with a characteristic four-ring steroid structure that
is an important component of the plasma membranes of animal cells. chondrocyte (cartilage cell) Connective-tissue cell that secretes the matrix of
cartilage. chromaffin cell Cell that stores adrenaline in secretory vesicles and secretes it
in times of stress when stimulated by the nervous system. chromatid One copy of a chromosome formed by DNA replication that is still
joined at the centromere to the other copy. The two identical chromatids are
called sister chromatids. chromatin Complex of DNA, histones, and nonhistone proteins found in the
nucleus of a eucaryotic cell. The material of which chromosomes are made. chromatography Biochemical technique in which a mixture of substances is
separated by charge, size, or some other property by allowing it to partition
between a moving phase and a stationary phase. (See affinity chromatography, DNA affinity chromatography, high-performance liquid chromatography.) chromosomal crossing-over The exchange of DNA between paired
homologous chromosomes in division I of meiosis. It is a sign of genetic
recombination and the crossovers (chiasmata) are visible in the light
microscope. (20 10.) chromosome Structure composed of a very long DNA molecule and
associated proteins that carries part (or all) of the hereditary information of an
organism. Especially evident in plant and animal cells undergoing mitosis or
meiosis, where each chromosome becomes condensed into a compact rodlike
structure visible under the light microscope. chromosome condensation Process by which a chromosome becomes packed
up into a more compact structure prior to M phase of the cell cycle. cilium (cilia) Hairlike extension of a eucaryotic cell containing a core bundle
of microtubules and capable of performing repeated beating movements. Cilia
are found in large numbers on the surface of many cells, and are responsible
for the swimming of many single-celled organisms. circadian clock Internal cyclical process that produces a particular change in a
cell or organism with a period of around 24 hours, for example the sleepwakefulness
cycle in humans. cis face Face of a Golgi stack at which material enters the organelle. It is
adjacent to the cis Golgi network. cis Golgi network (CGN) Network of interconnected cisternae and tubules
which receives vesicles from the endoplasmic reticulum and transfers material
to the cis face of the Golgi apparatus. cisterna (cisternae) Flattened membrane-bounded compartment, as found in
the endoplasmic reticulum or Golgi apparatus. citric acid cycle (tricarboxylic acid (TCA) cycle, Krebs cycle) Central
metabolic pathway found in aerobic organisms. Oxidizes acetyl groups derived
from food molecules to CO2 and H2O. In eucaryotic cells it occurs in the
mitochondria. CKI see Cdk inhibitor protein class I MHC molecule One of the two classes of MHC molecule. It is present
on almost all cell types and presents viral peptides on the surface of virusinfected
cells, where they are recognized by cytotoxic T cells.
class II MHC molecule One of the two classes of MHC molecule. It is present
on "professional" antigen-presenting cells and presents foreign peptides to
helper T cells. class switching The change from making one class of immunoglobulin (for
example IgM) to making another class (for example IgG) that many B cells
undergo during the course of an immune response. classical pathway A pathway for activating the complement system that is
initiated by IgG or IgM antibodies bound to the surface of a microbe. clathrin Protein that assembles into a polyhedral cage on the cytosolic side of
a membrane so as to form a clathrin-coated pit, which buds off by endocytosis
to form an intracellular clathrin-coated vesicle. clathrin-coated pit Region of plasma membrane of animal cells that is coated
with the protein clathrin on its cytosolic face. Such regions are continually
forming and budding off by endocytosis to form intracellular clathrin-coated
vesicles containing extracellular fluid and the materials dissolved in it. cleavage (1) Physical splitting of a cell into two. (2) Specialized type of cell
division seen in many early embryos whereby a large cell becomes subdivided
into many smaller cells without growth. clonal selection theory Theory that explains how the adaptive immune system
can respond to millions of different antigens in a highly specific way. From a
population of lymphocytes with a vast repertoire of randomly generated
antigen specificities, a given foreign antigen activates (selects) only those cells
with the corresponding antigen specificity. clone Population of cells or organisms formed by repeated (asexual) division
from a common cell or organism. Also used as a verb: "to clone a gene" means
to produce many copies of a gene by repeated cycles of replication. cloning vector A small DNA molecule, usually derived from a bacteriophage
or plasmid, which is used to carry the fragment of DNA to be cloned into the
recipient cell, and which enables the DNA fragment to be replicated. coated vesicle Small membrane-bounded organelle with a cage of proteins
(the coat) on its cytosolic surface. It is formed by the pinching off of a coated
region of membrane (coated pit). Some coats are made of clathrin, whereas
others are made from other proteins. codon Sequence of three nucleotides in a DNA or messenger RNA molecule
that represents the instruction for incorporation of a specific amino acid into a
growing polypeptide chain. coenzyme Small molecule tightly associated with an enzyme that participates
in the reaction that the enzyme catalyzes, often by forming a covalent bond to
the substrate. Examples include biotin, NAD+, and coenzyme A. coenzyme A Small molecule used in the enzymatic transfer of acyl groups in
the cell. (See also acetyl CoA.) cofactor Inorganic ion or coenzyme that is required for an enzyme's activity. cohesin, cohesin complex Complex of proteins that holds sister chomatids
together along their length before their separation. coiled-coil Especially stable rodlike structure in proteins which is formed by
two of these a helices coiled around each other. collagen fibril Extracellular structure formed by self-assembly of secreted
fibrillar collagen subunits. An abundant constituent of the extracellular matrix
in many animal tissues. collagen Fibrous protein rich in glycine and proline that is a major component
of the extracellular matrix and connective tissues. Exists in many forms: type I,
the most common, is found in skin, tendon, and bone; type II is found in
cartilage; type IV is present in basal laminae. colony-stimulating factor (CSF) General name for the numerous signal
molecules that control the differentiation of blood cells. colorectal tumor Common carcinoma of the epithelium lining the colon and
rectum. combinatorial control Describes the control of a step in a cellular process,
such as the initiation of DNA transcription, by a combination of proteins rather
than by any individual one. communicating junction Type of cell junction that allows the passage of
chemical or electrical signals from one cell to another. compartment Regions in the embryo that are formed exclusively from the
descendants of a few founder cells; there is no cell movement beween
compartments once delimited. complement system System of serum proteins activated by antibody antigen
complexes or by microorganisms. Helps eliminate pathogenic microorganisms
by directly causing their lysis or by promoting their phagocytosis. complementary DNA see cDNA complementary Two nucleic acid sequences are said to be complementary if
they can form a perfect base-paired double helix with each other. complex oligosaccharide Chain of sugars attached to a glycoprotein that is
generated by trimming of the original oligosaccharide attached in the
endoplasmic reticulum and subsequent addition of further sugars. complex Assembly of molecules that are held together by noncovalent bonds.
Protein complexes perform most cell functions. condensation reaction Chemical reaction in which two molecules are
covalently linked through OH groups with the removal of a molecule of
water. condensin, condensin complex Complex of proteins involved in chromosome
condensation prior to mitosis. Target for the M-Cdk. conditional mutation A mutation that changes a protein or RNA molecule so
that its function is altered only under some conditions, such as at an unusually
high or an unusually low temperature. confocal microscope Type of light microscope that produces a clear image of
a given plane within a solid object. It uses a laser beam as a pinpoint source of
illumination and scans across the plane to produce a two-dimensional 'optical
section.' conformation The spatial arrangement of atoms in three dimensions in a
macromolecule such as a protein or nucleic acid. connective tissue Any supporting tissue that lies between other tissues and
consists of cells embedded in a relatively large amount of extracellular matrix.
Includes bone, cartilage, and loose connective tissue. connective-tissue cell Any of the various cell types found in connective tissue,
e.g. fibroblasts, cartilage cells (chondrocytes), bone cells (osteoblasts and
osteocytes), fat cells (adipocytes) and smooth muscle cells. connexon Water-filled pore in the plasma membrane formed by a ring of six
protein subunits. Part of a gap junction: connexons from two adjoining cells
join to form a continuous channel between the two cells. consensus sequence Average or most typical form of a sequence that is
reproduced with minor variations in a group of related DNA, RNA, or protein
sequences. The consensus sequence shows the nucleotide or amino acid most
often found at each position. The preservation of a consensus implies that the
sequence is functionally important. constitutive secretory pathway Pathway present in all cells by which
molecules such as plasma membrane proteins are continually delivered to the
plasma membrane from the Golgi apparatus in vesicles that fuse with the
plasma membrane. constitutive Produced in constant amount; opposite of regulated. contact-dependent signaling Cell cell communication in which the signal
molecule remains bound to the signaling cell and only influences cells that
physically contact it. contractile ring Ring containing actin and myosin that forms under the
surface of animal cells undergoing cell division and contracts to pinch the two
daughter cells apart. convergent extension Cellular rearrangement within a tissue that causes it to
extend in one dimension (e.g. length) and shrink in another (e.g. width). cooperativity Phenomenon in which the binding of one ligand molecule to a
target molecule promotes the binding of successive ligand molecules. Seen in
the assembly of large complexes, as well as in enzymes and receptors
composed of multiple allosteric subunits, where it sharpens the response to a
ligand. cortical granule Specialized secretory vesicle present under the plasma
membrane of unfertilized eggs, including those of mammals; after fertilization
it is involved in preventing the entry of further sperm. co-translational Describes import of a protein into the endoplasmic reticulum
before the polypeptide chain is completely synthesized. co-transport (coupled transport) Membrane transport process in which the
transfer of one molecule depends on the simultaneous or sequential transfer of
a second molecule. coupled reaction Linked pair of chemical reactions in which the free energy
released by one of the reactions serves to drive the other. covalent bond Stable chemical link between two atoms produced by sharing
one or more pairs of electrons. crista (cristae) (1) (2) One of the folds of the inner mitochondrial membrane.
(3) A sensory structure in the inner ear. critical concentration Concentration of a protein monomer, such as actin or
tubulin, that is in equilibrium with the assembled form of the protein (i.e.
assembled into actin filaments or microtubules respectively). crossing-over see chromosomal crossing-over cryoelectron microscopy Electron microscopy technique in which the objects
to be viewed, such as macromolecules and viruses, are rapidly frozen. cryptochrome Flavoprotein responsive to blue light, found in both plants and
animals. In animals it is involved in circadian rhythms. cut-and-paste transposition Type of movement of a transposable element in
which it is cut out of the DNA and inserted into a new site by a special
transposase enzyme. cyclic AMP (cAMP) Nucleotide that is generated from ATP by adenylyl
cyclase in response to stimulation of many types of cell-surface receptors.
cAMP acts as an intracellular signaling molecule by activating cyclic-AMPdependent
kinase (protein kinase A, PKA). It is hydrolyzed to AMP by a
phosphodiesterase. cyclic AMP-dependent protein kinase (protein kinase A, PKA) Enzyme
that phosphorylates target proteins in response to a rise in intracellular cyclic
AMP. cyclic GMP Small soluble intracellular signaling molecule formed from GTP
by the enzyme guanylyl cyclase in response to photoreceptor stimulation in the
retina. cyclin Protein that periodically rises and falls in concentration in step with the
eucaryotic cell cycle. Cyclins activate crucial protein kinases (called a cyclindependent
protein kinase, or Cdk) and thereby help control progression from
one stage of the cell cycle to the next. cyclin-Cdk complex Protein complexes that are formed periodically during
the eucaryotic cell cycle as the level of cyclin increases, and in which the
cyclin-dependent kinase (Cdk) becomes partially activated. cyclin-dependent kinase (Cdk) Protein kinase that has to be complexed with
a cyclin protein in order to act. Different Cdk-cyclin complexes trigger
different steps in the cell-division cycle by phosphorylating specific target
proteins. cytochrome b-c1 complex Second of the three electron-driven proton pumps
in the respiratory chain. It accepts electrons from ubiquinone. cytochrome oxidase complex Third of the three electron-driven proton pumps
in the respiratory chain. It accepts electrons from cytochrome c and generates
water using molecular oxygen as an electron acceptor. cytochrome Colored, heme-containing protein that transfers electrons during
cellular respiration and photosynthesis. cytokine Extracellular signal protein or peptide that acts as a local mediator in
cell cell communication. cytokine receptor Type of cell-surface receptor whose ligands are cytokines
such as interferons, growth hormone and prolactin, and which acts through the
Jak-STAT pathway. cytokinesis Division of the cytoplasm of a plant or animal cell into two, as
distinct from the division of its nucleus (which is mitosis) cytoplasm Contents of a cell that are contained within its plasma membrane
but, in the case of eucaryotic cells, outside the nucleus. cytoskeleton System of protein filaments in the cytoplasm of a eucaryotic cell
that gives the cell shape and the capacity for directed movement. Its most
abundant components are actin filaments, microtubules, and intermediate
filaments. cytosol Contents of the main compartment of the cytoplasm, excluding
membrane-bounded organelles such as endoplasmic reticulum and
mitochondria. Originally defined operationally as the cell fraction remaining
after membranes, cytoskeletal components, and other organelles have been
removed by low-speed centrifugation. cytotoxic T cell Type of T cell responsible for killing infected cells.
