DICTIONARY OF CELL BIOLOGY
COMPARATIVE GENOMICS CENTRE

Mail Address: Comparative Genomics Centre,
Molecular Sciences Bldg 21, James Cook University,
Townsville, 4811, Queensland, Australia
Telephone: 61-7-4781 6265 Fax:  61-7-4781 6078


TERM DEFINITION
 
Actin Monomers of the protein actin polymerize to form long, thin fibers about 8 nm inA globular protein found in all cells. It is a major component in microfilaments and forms the contractile filaments in muscle cells.
 
Actin filaments Monomers of the protein actin polymerize to form long, thin fibers about 8 nm in diameter. Also termed "microfilaments" and, in muscle, "thin filaments".
 
Active transport The movement of molecules across a membrane which requires ATP or other form of metabolic energy.
 
Anaphase The stage in mitosis in which the chromosomes begin to separate.
 
Antibodies Proteins (immunoglobulins) produced by B lymphocytes and plasma cells in response to stimulation by antigen. A class of globular proteins that bind through hypervariable regions to a range of specific targets and activate through a constant region a variety of immune effector mechanisms.
 
Antigen Substances that can elicit an adaptive immune response and that can react specifically with the corresponding antibodies or T cell receptors.
 
Apoptosis Normal regulated sequence of cell death whereby a cell controls its own destruction and is phagocytosed by macrophages.
 
Basal body A structure at the base of a cilium or flagellum; consists of nine triplet microtubules arranged in a circle with no central microtubule.
 
Brush border The collection of microvilli forming a border on the intestinal side of the epithelial cells of the small intestine.
 
Carrier protein These are integral membrane proteins that bind to a "substrate" and transport it across the membrane. This is accomplished by conformational changes in the protein. Like enzymes, these are invariably substrate specific.
 
Caveolae A small pit, depression, or invagination of the cell membrane - a special form of lipid raft.
 
Cell The basic structural and functional unit of all organisms. Includes the plasma membrane and any external encapsulating structures such as the cell wall and cell envelope.
 
Cell cycle The orderly sequence of events by which a cell duplicates its contents and divides into two. It consists of mitosis (or division) and interphase.
 
Cell signalling The molecular mechanism whereby cells detect and respond to external stimuli and send messages to other cells.
 
Cellular respiration The transfer of energy by the catabolism of molecules to produce ATP, during which oxygen is consumed and carbon dioxide produced. It occurs in the mitochondria of eukaryotes.
 
Central nervous system The part of the nervous system that includes the brain and spinal cord. (Abbrev: CNS)
 
Centriole An array of microtubules which functions in the organisation of the mitotic spindle (centrosome) during cell division in eukaryotes. It is also found at the base of cilia and flagella where it is called the basal body.
 
Centromere A constricted region of a mitotic chromosome that holds sister chromatid together. Also the site on the DNA where the kinetochores form and then capture microtubules from the mitotic spindle.
 
Channel An aqueous channel in a membrane formed by protein and through which selected ions or molecules can pass.
 
Check point The point in the eukaryotic cell division cycle where progress can be halted until conditions are suitable for progression to the next stage.
 
Chemiosmosis The mechanism through which ATP is produced in the inner membrane of the mitochondrion. The electron transport system transfers protons from the inner compartment to the outer and as the protons flow back to the inner compartment, the energy of their movement is used to add phosphate to ADP, forming ATP.
 
Chiasma The site where the exchange of chromosome segments between homologous chromosomes takes place (crossing-over).
 
Cholesterol A lipid molecule with a characteristic four-ringed steroid structure. It is an important component of the plasma membrane and the precursor for the biosynthesis of all steriod hormones.
 
Chromatid A single strand of a chromosome formed by DNA replication and is still joined to the other strand (sister chromatid) at the centromere.
 
Chromatin The ordered and organized complex of DNA and protein that forms the chromosome.
 
Chromosomes A structure composed of a very long molecule of DNA and associated proteins (eg. histones) that carries hereditary information.
 
Cilium A specialized eukaryotic locomotor organelle that consists of a filiform extrusion of the cell surface. Each cilium is bounded by an extrusion of the cytoplasmic membrane, and contains a regular longitudinal array of microtubules, anchored basally in a centriole.
 
Cilia Pleural of cillium.
 
Clathrin coated pit An invagination of the cell membrane of many eukaryotic cells, concerned in receptor-mediated selective transport of many proteins and other macromolecules across the cell membrane.
 
Coated pit An invagination of the cell membrane of many eukaryotic cells, concerned in receptor-mediated selective transport of many proteins and other macromolecules across the cell membrane.
 
Complement An autocatalytic cascade of serum proteins that induces target cell lysis, potentiates inflammation through triggering chemotaxis, and enhances phagocytosis through oposonisation.
 
Cristae Invaginations of the inner membrane of the mitochondrion on which the components of the electron transport chain are located.
 
Crossing over The process whereby two homologous chromosomes break at equivalent positions and then join to produce recombined chromosomes.
 
Cytochrome Haem containing proteins that transfer electrons during cellular respiration and photosynthesis.
 
Cytokinesis The division of the cytoplasm of a plant or animal cell into two. It is distinct from this division of its nucleus.
 
Cytoplasm All of the contents of a cell excluding the plasma membrane and nucleus, but including other subcellular structures.
 
Cytoskeleton The filamentous elements within the cytoplasm of eukaryotic cells that remain after treatment of the cells with mild detergent to remove membrane constituents and soluble components of the cytoplasm. The term embraces intermediate filaments, microfilaments, microtubules, and the microtrabecular lattice.
 
Cytosol That part of the cytoplasm that does not contain membranous or particulate subcellular components.
 
Endocytosis The uptake of material from outside the cell by the invagination of the plasma membrane resulting in the formation of vesicles.
 
Endoplasmic reticulum The irregular network of unit membranes, visible only by electron microscopy, that occurs in the cytoplasm of many eukaryotic cells. The membranes form a complex meshwork of tubular channels, which are often expanded into slitlike cavities called cisternae. The ER takes two forms, rough (or granular), with ribosomes adhering to the outer surface, and smooth (with no ribosomes attached).
 
Endosome A membrane delimited vesicle (0.1 - 0.2 um diameter) formed in the process of endocytosis.
 
Exocytosis The process in which molecules in a membrane-enclosed vesicle fuse with the plasma membrane and are then released outside the cell.
 
flagellum A long, whiplike protrusion from the surface of a eukaryotic cell, whose undulations drive the cell through a liquid medium; similar in structure to a cilium in having an internal arrangement of microtubules in a 9 + 2 array.
 
Flagella Pleural of flagellum.
 
Golgi A compound membranous cytoplasmic organelle of eukaryotic cells, consisting of flattened, ribosome-free vesicles arranged in a more or less regular stack. 
 
Histone A group of basic proteins, rich in arginine and lysine, that are associated with DNA in eukaryotic chromosomes.
 
Immune system A conceptual division of cells and cell products involved in protection from infection and cancers.
 
Immunoglobulin Proteins (antibodies) produced by B lymphocytes and plasma cells in response to stimulation by antigen. A class of globular proteins that bind through hypervariable regions to a range of specific targets and activate through a constant region a variety of immune effector mechanisms.
 
Interphase The quiescent phase of the cell cyle between one mitosis and the synthetic phase of the next.
 
Intermediate Filaments Cytoplasmic fibers averaging 10 nm in diameter (and thus are "intermediate" in size between actin filaments (8 nm) and microtubules (25 nm). Their chief function seems to be to maintain cell integrity.
 
Leukocyte White blood cell.
 
Ligand A molecule such as a hormone or growth factor that binds to a specific site on a receptor protein.
 
Lipid rafts Specialized membrane domains composed mainly of cholesterol and sphingolipids, and are relatively poor in polyunsaturated lipids such as glycerophospholipids.
 
Lysosome Membrane delimited cytoplasmic organelle of eukaryotic cells containing acid hydrolases maintained at a pH of about 5.0.
 
Lymphocyte Small (~10um) white blood cells that mediate the adaptive immune response to an antigen. They are of the T, B or NK cell type.
 
Macrophage A large (>15um diameter) white blood cell that specialises in the uptake of particulate material by phagocytosis.
 
Meiosis The process of cell division in which two nuclear divisions occur with only one chromosome replication. Each of the resulting gametes receives a haploid set of chromosomes. There are four haploid daughter cells resulting from this process.
 
Metaphase The stage of eukaryotic cell division in which the chromosomes are attached to the mitotic spindle at the equator of the cell.
 
Microdomains Specialized membrane domains composed mainly of cholesterol and sphingolipids, and are relatively poor in polyunsaturated lipids such as glycerophospholipids.
 
Microfilaments Monomers of the protein actin polymerize to form long, thin fibers about 8 nm in diameter. Also termed "actin filaments" and, in muscle, "thin filaments".
 
Microtubule Straight, hollow cylinders of a diameter of about 25 nm composed of long chains of the protein tubulin. They serve to give a cell shape, aid in cellular motility and are very important in cell division. Microtubules are dynamic and grow and shrink depending on the needs of the cell.
 
Microvilli Minute projections on the surface of epithelial cells of the villi in the small intestine. They increase the surface area of the intestine to improve absorption of digested nutrients.
 
Mitochondrion A semiautonomous, self replicating organelle that occurs in varying numbers, shapes, and sizes in the cytoplasm of virtually all eukaryotic cells. It is notably the site of tissue respiration.
 
Mitogen A substance that induces mitosis of cells.
 
Mitosis Nuclear division. Consists of four phases: prophase, metaphase, anaphase, telophase.
 
Mitotic spindle A network of microtubules formed during mitosis. These microtubules attach to the centromeres of the chromosomes and help draw the chromosomes.
 
Myosin Protein that uses ATP to drive movements along actin filaments and facilitate muscle contraction.
 
Nuclear membrane The envelope that surrounds and delimits the nucleus of eukaryotic cells. Continuous with membranes of endoplasmic reticulum.
 
Nuclear pore Openings in the membrane of a cell's nuclear envelope that allow the exchange of materials between the nucleus and the cytoplasm.
 
Nucleoid The area of the prokaryotic cytoplasm where the chromatin is localized.
 
Nucleolus A round or oval body in the nucleus of a eukaryotic cell; consists of DNA and RNA and produces ribosomal RNA (pl.: nucleoli).
 
Nucleosome Spherical bodies formed by coils of chromatin. The nucleosomes in turn are coiled to form the Şbers that make up the chromosomes.
 
Nucleus The largest, most prominent organelle in eukaryotic cells; a round or oval body that is surrounded by the nuclear envelope and contains the genetic information necessary for control of cell structure and function.
 
Organelle A subcellular collection of biomolecules that performs a particular function in the cell.
 
Peroxisome Membrane delimited cytoplasmic organelle of eukaryotic cells containing oxidative enzymes (catalase and urate oxidase) that convert fatty acids to acetyl CoA. Also involved in detoxification (e.g. of ethanol).
 
Phagocytes White blood cells that can engulf (by phagocytosis) and destroy microorganisms including viruses and bacteria; cells in this category include neutrophils and monocytes.
 
Phagocytosis The process whereby particular material is taken up by the cell using endocytosis. Macrophages (in the blood supply) and amoeba are common examples of cells that are phagocytic.
 
Pinocytosis A type of endocytosis whereby soluble molecules are taken up from outside the cell through the formation of vesicles.
 
Plasma membrane Double layer of lipid molecules that encloses all cells, and, in eukaryotes, many organelles; may be a single or double lipid bilayer, also includes associated proteins.
 
Proteasome Large multiportein complex responsible for the degradation of cytosolic proteins. These proteins must be marked for destruction by ubiquitination.
 
Receptor Proteins, generally found on the external surface of the plasma membrane, which interact with specific chemicals that circulate in the blood supply. The hormone adrenaline (epinephrine) has a specific receptor on the surface of many cells. Some receptors are the target for drugs designed to interfere with the action of the hormone. Intracellular receptors, such as steroid hormone receptors, bind specific steroids that can diffuse into the cell across the plasma membrane.
 
Replication fork The Y-shaped region of a replicating DNA molecule. It is the point at which the two daughter DNA strands are formed and at the same time separate.
 
Ribosome The site of protein biosynthesis resulting from translation of messenger RNA (mRNA); an intracellular organelle, about 200 A in diameter, consisting of RNA and protein.
 
S phase The period during the cell cycle when DNA is synthesised.
 
Secretory vesicle A membrane bound organelle which contains molecules ready for secretion across the plasma membrane of the cell.
 
Signalling cascade A sequence of reactions/interactions involving proteins which propagates information in the cell. The reactions invariably include one or more phosphorylation/dephosphorylation reactions.
 
Signal sequence An amino acid sequence, normally at the N-terminus of hte protein, that directs a protein to a specific location in the cell eg chloroplast, endoplasmic reticulum.
 
Smooth endoplasmic reticulum Region of the endoplasmic reticulum involved in lipid synthesis. It does not possess ribosomes and is not involved in protein synthesis.
 
Spliceosome An assembly of RNA and protein molecules within the nucleus that performs splicing of mRNA in eukaryotes.
 
Synapse The junction between two nerve cells across which the nerve impulse is transferred.


OTHER GENETICS LINKS OTHER LINKS: Comparative Genomics Centre, James Cook University, Key words: Autoimmune diabetes, Type 1 diabetes mellitus, childhood diabetes, lupus, systemic lupus erythematosus, haemolytic anaemia, hemolytic anemia, Coombs' test, antinuclear antibodies, renal failure, glomerulonephritis, gastritis, type A gastritis, pernicious anemia.