Glossary
Amino Acid
A building block of proteins. There are 20 different kinds of amino acids; a protein consists of a specific sequence of amino acids.
Angstrom
A unit of length, one hundred-millionth of a centimeter (approximately 0.000000004 inch); used for describing atomic dimensions.
ATP (adenosine triphosphate)
The compound that serves as a source of energy for the physiological reactions in cells.
Bacterium
A one-celled microorganism that contains no nucleus.
Base
The basic subunit of DNA or RNA. Paired bases--adenine with thymine and guanine with cytosine (uracil replaces thymine in RNA)--make up each "rung" of the "ladder" of the DNA molecule. See nucleotide.
Basic Research
Scientific research that seeks to discover how systems work and develop a base of knowledge that other scientists can use in order to achieve practical goals, such as treatments or cures for diseases.
Biochemistry
The study of the chemical reactions that occur in living organisms.
Cell
The basic subunit of any living organism; the simplest unit that can exist as an independent living system.
Cell Cycle
The sequence of events by which the cell duplicates its contents and divides into two.
Cell Surface Membrane
A complex film of lipids interspersed with proteins. It covers the cell, maintains its integrity, and controls what goes in and what comes out.
Centrifuge
A machine that separates particles according to their size and density by spinning them at varying speeds.
Chloroplast
The chlorophyll-containing organelle
in green plants in which light energy is converted into sugars.
Cholesterol
A waxy lipid produced by animal cells that is a prominent component of cell membranes.
Chromosome
A rod-shaped structure containing genes that is found in the cell nucleus. It is composed of DNA and proteins, and can be seen in a light microscope during some stages of cell division.
Codon
A sequence of three consecutive nucleotides in a DNA or RNA molecule that codes for 1 of the 20 amino acids in proteins or for a signal to start or stop protein production.
Column Chromatography
A technique used to
separate the components of biologically active molecules, which move at different speeds through a hollow column that is filled with a chemically reactive material.
Cristae
The inward folds of a mitochondrion's inner membrane.
Cyanobacteria (formerly called blue-green algae)
Single-celled organisms that perform a type of photosynthesis.
Cytoplasm
All the substance inside a cell, excluding the nucleus but including the other organelles.
Cytoskeleton
A group of non-membrane-bound organelles that supports the cell. Some serve as conduits for the transport of various cell components.
Differentiation
The series of biochemical and structural changes that groups of cells undergo in order to form specialized cells and tissues.
DNA (deoxyribonucleic acid)
The substance of heredity; a large molecule that carries the genetic information necessary for all cellular functions, including the building of proteins. DNA is composed of the sugar deoxyribose, phosphate, and the bases adenine, thymine, guanine, and cytosine.
Electron Microscope
A powerful microscope that uses beams of fast-moving electrons instead of light waves to enable objects to be observed.
Endoplasmic Reticulum
An organelle made up of membranes that form a system of tubes and flattened sacs. Some of the membranes are smooth (the smooth endoplasmic reticulum); others are rough (the rough endoplasmic
reticulum) because they are dotted with ribosomes.
Enzyme
A substance (usually a protein) that speeds up, or catalyzes, a chemical reaction without being permanently altered or consumed.
Eukaryotic Cell
A cell that has a true nucleus surrounded by a membrane. This group includes all animal and plant cells, except cyanobacteria.
Fluid-Mosaic Model
A model of the cell surface membrane in which proteins move about within a bed of semi-fluid lipids.
G Protein
One of a group of proteins involved in signal transduction within the cell.
Gel Electrophoresis
A technique used to separate molecules according to their sizes and charges.
Gene
A unit of heredity; a segment of the DNA molecule containing the code for a specific protein product or function.
Genetic Engineering
See recombinant DNA technology.
Glycolipid
A molecule composed of sugar and fat that forms an important component of cell membranes.
Golgi Apparatus
An organelle composed of membranous sacs that packages proteins into vesicles and sends them to the cell's surface or to lysosomes.
Intermediate Filament
A component of the cytoskeleton that acts to strengthen the cell.
Ion
Any atom or molecule that contains an unequal number of electrons and protons and, therefore, carries a net positive or negative electrical charge.
Ligand
Any molecule that binds to a specific site on a protein or other molecule.
Light Microscope
An instrument that magnifies objects using curved lenses and white light as a source of illumination.
Lipid
A fat or fat-like compound.
Lysosome
A small organelle containing powerful enzymes that can digest a variety of materials.
Microfilament
A threadlike organelle involved in cell motion, particularly muscle contraction.
Micrometer (or micron)
One one-thousandth of a millimeter; 10,000 angstroms; convenient for describing the dimensions of cells and organelles.
Microtubule
A thin, tubular organelle that acts as a structural support for the cell. During cell division, microtubules form the spindle that directs chromosomes to the daughter cells.
Mitochondrion
The cell organelle that converts the energy in sugars into ATP, thereby fueling the cell.
Molecule
The smallest physical unit of an element or compound. A molecule of an element consists of one or more identical atoms. A molecule of a compound consists of two or more different atoms.
Multicellular
Made up of many cells.
Nanometer
One one-thousandth of a micrometer.
Nucleic Acid
Either of two kinds of molecules (DNA and RNA), formed by chains of nucleotides, that carry genetic information.
Nucleotide
A subunit of DNA or RNA that includes one base, one phosphate molecule, and one sugar molecule (deoxyribose in DNA, ribose in RNA). See base.
Nucleus
In eukaryotic cells, the membrane-bound organelle that contains the genetic material.
Organelle
A specialized structure having a definite function in a cell; for example, the nucleus, a mitochondrion, a ribosome.
Peroxisome
A membrane-bound organelle that both generates and breaks down hydrogen peroxide.
Phospholipid
A fatty compound that contains phosphate. Phospholipids make up much of the outer membranes of cells and organelles.
Prokaryotic Cell
A cell that does not have a membrane around its nuclear region; for example, a bacterium.
Protein
A molecule made up of a number of amino acids arranged in a specific order determined by the genetic code. Proteins are essential for all life processes.
Receptor
A specialized molecule of a cell's membrane that receives information from the environment and conveys it to other parts of the cell. The information is transmitted in the form of a specific chemical that must fit the receptor like a key in a lock.
Recombinant DNA Technology
A body of techniques for cutting apart and splicing together different pieces of DNA. When segments of foreign DNA are transferred into another cell or organism, the substance for which they code may be produced along withonal Institutes of Health
Bethesda, Maryland 20892-6200
Last updated: March 5, 1998
| 
