Double circulation A transportation system for the blood that has separate pulmonary and systemic systems. The heart pumps blood to the lungs and back, then to the body and back via a network of blood vessels. In humans (but not all animals) the blood travels through the heart twice on each complete journey around the body. There is no mixing of the two kinds of blood (oxygen-rich blood is completely separated from oxygen-poor blood). A double circulation system maintains the high blood pressure needed for efficient transport of materials around the body.
Double fertilization Restricted to angiosperms, the flowering plants, it is a process where one male sperm cell pollinates an egg to form a zygote, a diploid embryo, while another male sperm joins with two other polar nuclei to form a triploid cell, becoming the endosperm in the ovule. Corn is an example.
Double helix Two strands of DNA coiled about a central axis, usually a right-handed helix. The two sugar-phosphate backbones wind around the outside of the bases (A = adenine, G = guanine, T = thymine, C = cytosine). The strands are antiparallel, thus the phos-phodiester bonds run in opposite directions. As a result, the structure has major and minor grooves at the surface. Each adenine in one strand of DNA is hydrogen bonded to a thymine in the second strand; each guanine is hydrogen bonded to a cytosine.
Double prodrug (pro-prodrug) A biologically inactive molecule that is transformed in vivo in two steps (enzymatically and/or chemically) to the active species.
Down’s syndrome The most common and readily identifiable chromosomal abnormality associated with mental retardation. There are 47 instead of the usual 46 chromosomes, and the extra chromosome, chromosome 21, changes the orderly development of the body and brain, showing several symptoms including a characteristic body type, mental retardation, increased susceptibility to infections, and various heart and other organ abnormalities. It is caused by one of the parent’s gametes not dividing properly or where one of the parents has chromosome 14 and 21 merge.
This photograph depicts a newborn with the genetic disorder Down’s syndrome due to the presence of an extra 21st chromosome. The estimated incidence of Down’s syndrome is between 1:1,000 to 1:1,1000 live births. Each year approximately 3,000 to 5,000 children are born with this chromosomal disorder.
Approximately 4,000 children with a Down’s syndrome are born in the United States each year, or about one in every 800 to 1,000 live births. The incidence is higher for women over age 35, but the condition can occur at any age for women.
Drug Any substance presented for treating, curing, or preventing disease in human beings or in animals. A drug can also be used for making a medical diagnosis or for restoring, correcting, or modifying physiological functions (e.g., the contraceptive pill).
Drug disposition Refers to all processes involved in the absorption, distribution, metabolism, and excretion of drugs in a living organism.
Drug latentiation The chemical modification of a biologically active compound to form a new compound, which in vivo will liberate the parent compound. Drug latentiation is synonymous with prodrug design.
Drug targeting A strategy aiming at the delivery of a compound to a particular tissue of the body.
Drumlin An oval or elongated hill of glacial drift that looks like an overturned canoe from the air.
Drupe A fleshy or pulpy fruit with a single seed enclosed in a pit.
Dual-action drug A compound that combines two desired different pharmacological actions at a similarly efficacious dose.
Duodenum The first part of the small intestine; short, wide, U-shaped, about 12 inches long, and closest to the stomach. The bile duct (gallbladder) and pancreatic duct (pancreas) both open into the duodenum.
After food mixes with stomach acid, it moves into the duodenum and mixes with digestive juices from the pancreas, liver, and gallbladder, which starts the process of breaking down food into its constituent parts.
Duplication A specific kind of mutation: production of one or more copies of any piece of DNA, including a gene or even an entire chromosome. A chromosome structural aberration from an error in meiosis. Duplication of a portion of a chromosome resulting from fusion with a fragment from a homologous chromosome.
Dwarfism Animals that evolve on islands are affected by gigantism or dwarfism, the evolution of body form as either large (e.g., Komodo dragon weighs up to 365 pounds) or small (e.g., Island fox in the Channel Islands). Island animal populations tend to acquire different sizes than their mainland counterparts. Dwarfism may be due to limited food supply, but the reasons for both gigantism and dwarfism are not known fully.
A duplication is a particular kind of mutation resulting in the production of one or more copies of any piece of DNA, including a gene or even an entire chromosome.
Dynein A molecular motor, a complex believed to be made of 12 distinct protein parts, that performs basic transportation tasks critical to the cell. Converts chemical energy stored in an ATP molecule into mechanical energy that moves material though the cell along slender filaments called microtubules. one of the most important functions occurs during cell division, when it helps move chromosomes into proper position. It also plays a part in the movement of eukaryotic flagel-la and cilia.
Molecular motors play a critical role in a host of cell functions, such as membrane trafficking and cell movement during interphase, and for cell asymmetry development. During cell division, they are responsible for establishing the mitotic or meiotic spindle, as well as segregating chromosomes and dividing the cell at cytokinesis. It is the last part of the mitotic cycle during which the two daughter cells separate. Motors either move along actin tracks (members of the myosin superfamily) or microtubules (the dynein and kinesin superfamilies). Based on the Greek dunamis, meaning "power."
The dynein motor, a cellular complex believed to be composed of 12 distinct protein parts, performs fundamental transportation tasks critical to the cell. Defects in its structure can prove fatal. This machine converts chemical energy stored in an ATP molecule into mechanical energy that moves material though the cell along slender filaments called microtubules. One of the dynein motor’s most important functions occurs during cell division, when it helps move chromosomes into proper position.
Dysentery Sickness that usually involves the abdomen causing cramps, vomiting, and swelling. Caused by a bacterium, Shigella bacillus, or a proto-zoon, Entamoeba histolytica.
Ecdysone A juvenile steroid hormone that affects arthropods and that belongs to the larger class of ecdy-steroids, sterol derivatives that as a whole affect a variety of conditions relating to molting and metamorphosis, including eliciting molting, regulating growth of motor neurons, controlling choriogenesis, stimulating growth and development of imaginal discs, initiating breakdown of larval structures during metamorphosis, and eliciting the deposition of cuticle by the epidermis. In insects, ecdysone primarily elicits and stimulates molting. It acts on specific genes, stimulating the synthesis of proteins involved in these bodily changes, and is produced by prothoracic glands in insects and Y-organs (a gland near the external adductor muscles) in crustaceans. Ecdysone—formerly called alpha ecdysone, and beta ecdysone, or ecdysterone (now called 20-hydroxy-ecdysone [20-HE])—is believed to be the active form. Ecdysone is not the active molting hormone. Various tissues, including the fat body, convert ecdysone to 20-hydroxyecdysone, the active form of molting hormone.
Echinoderm Diversified marine animals (phylum Echinodermata) that include the classes Crinoidea (sea lilies), Asteroidea (starfish), Ophiuroidea (brittle stars or snake stars), Echinoidea (sea urchins and sand dollars), and Holothuroidea (sea cucumbers).
Echolocation A form of sensory perception used by animals like bats to orient themselves in flight, detect objects, seek food, and communicate. Bats, for example, send out a series of short, high-pitched sounds, called echoes, that travel, hit an object, and bounce back, giving the bat the ability to judge distance, size, shape, and motion.
Eclosion The emergence of an adult insect from the pupa case or, less commonly, the hatching of an egg.
EC nomenclature for enzymes A classification of enzymes according to the Enzyme Commission of the International Union of Biochemistry and Molecular Biology. Enzymes are allocated four numbers, the first of which defines the type of reaction catalyzed; the next two define the substrates; and the fourth is a catalog number. Categories of enzymes are EC 1, oxidoreduc-tases; EC 2, transferases; EC 3, hydrolases; EC 4, lyases; EC 5, isomerases; EC 6, ligases (synthetases).
Ecological efficiency Each transfer of energy from one trophic level to another has an ecological efficiency associated with it. Ecological efficiency refers to the transfer of energy up trophic levels; it is the ratio of secondary productivity to primary productivity consumed.
Ecological efficiency goes down as you move up the trophic levels. Ecological efficiencies generally range from 5 to 20 percent, meaning that this percentage of primary-producer biomass consumed is converted into new consumer biomass.
Ecological efficiency depends on assimilation efficiency (that portion of the consumed energy assimilated) and on net production efficacy (that portion of the consumed energy converted into biomass). Assimilation efficiencies are greater for carnivores (50-90 percent) than for herbivores (20-60 percent).
Ecological niche The totality of biotic and abiotic resources an organism interacts with while living in its environment.
Ecological succession A transitional change in the biological community, where a group of plant and/or animal species gives way to another set of species over time, in response to a sequence of events such as fire, storms, human activities, or other natural or human-made occurrences. The term also refers to the normal evolution of a community from pioneer stage to climax community when equilibrium between species and its environment occurs.
Ecology The study of all life forms and their interactions with their environment.
Ecosystem Any natural system—including biotic and abiotic parts—that interacts as a unit to produce a stable functioning system through cyclical exchange of materials.
Ectoderm The outer layer of an embryo’s three primary germ layers (endoderm, mesoderm, ectoderm) that gives rise to the nervous system and epidermis in vertebrates.
Ectoparasite A parasite that feeds from the exterior of its host.
Ectotherm A cold-blooded organism that relies on obtaining its heat from certain behavior techniques and from the external environment (sun); e.g., snakes, alligators, lizards, fish, or amphibians.
Effector cell A cell that performs a specific function in response to a stimulus. A gland or muscle cell that responds to stimuli from the body. Cells with full immune functions capable of participating in the immune response by destroying foreign cells or tissues; effector lymphocytes can mediate the removal of pathogens from the body.
Efficacy Describes the relative intensity with which agonists vary in the response they produce, even when they occupy the same number of receptors and with the same affinity. Efficacy is not synonymous with intrinsic activity.
Efficacy is the property that enables drugs to produce responses. It is convenient to differentiate the properties of drugs into two groups: those that cause them to associate with the receptors (affinity) and those that produce stimulus (efficacy). This term is often used to characterize the level of maximal responses induced by agonists. In fact, not all agonists of a receptor are capable of inducing identical levels of maximal response. Maximal response depends on the efficiency of receptor coupling, i.e., from the cascade of events that, from the binding of the drug to the receptor, leads to the observed biological effect.
EF-hand A common structure to bind Ca2+ in calmodulin and other Ca2+-binding proteins consisting of a helix (E), a loop, and another helix (F).
Egg The mature female reproductive cell.
Ehrlich, Paul (1854-1915) German/Polish Immunol-ogist Paul Ehrlich was born on March 14, 1854, near Breslau, Germany (now Wroclaw, Poland), to Ismar Ehrlich and his wife Rosa Weigert, whose nephew was the great bacteriologist Karl Weigert.
Ehrlich was educated at the Breslau Gymnasium and then at the Universities of Breslau, Strassburg, Freiburg-im-Breisgau, and Leipzig. He received his doctorate in medicine in 1878 for his dissertation on the theory and practice of staining animal tissues based on the work of aniline dyes discovered by W. H. Perkin in 1853.
In 1878 Ehrlich was appointed assistant professor at the Berlin Medical Clinic, where he continued his work with dyes used for staining tissues, classifying them as being basic, acid, or neutral. His work on staining granules in blood cells laid the foundation for future work on hematology (the study of blood and blood-forming tissues) and in the field of staining of tissues.
In 1882 Ehrlich published his method of staining the tubercle bacillus that Robert Koch had discovered, and it was this technique that later became the precursor for the currently used Gram method of staining bacteria. Ehrlich himself had a bout of tuberculosis. Ehrlich also discovered the blood-brain barrier when he noticed that the dyes injected into an animal brain would not stain.
In 1899 he became director of the newly created Royal Institute of Experimental Therapy in Frankfurt and of the Georg-Speyer Haus, founded by Frau Franziska Speyer for chemotherapy studies, which was built next door to Ehrlich’s institute. It is here that he began work on serum antitoxins and chemotherapy and came up with the concept of the "magic bullet," a compound that could be made to selectively target a disease-causing organism, killing only that organism. His research programs were guided by his theory that the germicidal capability of a molecule depended on its structure, especially its side chains, which could bind to the disease-causing organism. After many trials searching hundreds of agents with the help of the nearby Cas-sella chemical works, which donated samples of new compounds produced in their laboratory, in 1909 he found a cure for syphilis. The agent he identified was arsphenamine, trade name Salvarsan (the 606th substance tested) and later Neosalvaran (the 914th substance tested). Ehrlich became one of the founders of chemotherapy.
Ehrlich received the Tiedemann Prize of the Senck-enberg Naturforschende Gesellschaft at Frankfurt/Main in 1887, the Prize of Honor at the XVth International Congress of Medicine at Lisbon in 1906, the Liebig Medal of the German Chemical Society in 1911, and in 1914 the Cameron Prize of Edinburgh. In 1908 he shared, with Ilya Ilyich Mechnikov, the Nobel Prize in recognition for his work on immunity.
The Prussian government elected him privy medical counsel in 1897, to a higher rank of the counsel in 1907, and in 1911 he reached the highest rank possible, real privy counsel with the title of excellency. He died on August 20, 1915, from a stroke.
