dark reactions The photosynthetic process in which food (sugar) molecules are formed from carbon dioxide from the atmosphere with the use of ATP; can occur in the dark as long as ATP is present.
death rate The ratio between deaths and individuals in a specified population at a particular time.
deletion The loss of a chromosome segment without altering the number of chromosomes.
dendrites Short, highly branched fibers that carry signals toward the cell body of a neuron. PICTURE
dendrochronology The process of determining the age of a tree or wood used in structures by counting the number of annual growth rings.
deoxyribonucleic acid (DNA) A nucleic acid composed of two polynucleotide strands wound around a central axis to form a double helix; the repository of genetic information. Nucleic acid that functions as the physical carrier of inheritance for 99% of all species. The molecule is double-stranded and composed of two strands in an antiparallel and complementary arrangement. The basic unit, the nucleotide, consists of a molecule of deoxyribose sugar, a phosphate group, and one of four nitrogenous bases. PICTURE 1 PICTURE 2
deoxyribose Five-carbon sugar found in nucleotides of DNA. PICTURE
depth diversity gradient The increase in species richness with increasing water depth until about 2000 meters below the surface, where species richness begins to decline.
dermal system Plant organ system that provides the covering for the plant.
dermis One of the two layers of skin; a connective tissue layer under the epidermis containing elastic and collagen fibers, capillary networks, and nerve endings.
desert biome Characterized by dry conditions and plants and animals that have adapted to those conditions; found in areas where local or global influences block rainfall.
desmosome A circular region of membrane cemented to an adjacent membrane by a molecular glue made of polysaccharides; found in tissues that undergo stretching.
deuterostomes Animals in which the first opening that appears in the embryo becomes the anus while the mouth appears at the other end of the digestive system. Main groups include chordates and echinoderms.
diabetes mellitus, Types I and II A disorder associated with defects in insulin action. Type I diabetes is characterized by inadequate insulin secretion; Type II diabetes is characterized by impaired insulin secretion in response to elevated blood glucose levels or by loss of sensitivity to insulin by target cells.
diaphragm A dome-shaped muscle that separates the thoracic and abdominal cavities.
diastole The filling of the ventricle of the heart with blood. PICTURE
diatomaceous earth Fossilized deposits of diatoms; used for abrasives, polishes and as a filtering agent.
dicots One of the two main types of flowering plants; characterized by having two cotyledons, floral organs arranged in cycles of four or five, and leaves with reticulate veins; include trees (except conifers) and most ornamental and crop plants. PICTURE
dictyosomes Organelles in plant cells composed of a series of flattened membrane sacs that sort, chemically modify, and package proteins produced on the rough endoplasmic reticulum. Also known as the Golgi Apparatus. PICTURE
diencephalon Part of the forebrain; consists of the thalamus and hypothalamus.
diffusion The spontaneous movement of particles from an area of higher concentration to an area of lower concentration. PICTURE
digestion The process of breaking down food into its molecular and chemical components so that these nutrient molecules can cross plasma membranes.
digestive system One of eleven major body organ systems in animals; converts food from the external environment into nutrient molecules that can be used and stored by the body and eliminates solid wastes; involves five functions: movement, secretion, digestion, absorption, and elimination. PICTURE
dihybrid cross In genetics, a cross that involves two sets of characteristics. PICTURE
dinoflagellates Single-celled to colonial protistans characterized by two flagella, one girdling the cell and the other trailing the cell.
dinosaurs Any of the Mesozoic reptiles belonging to the groups designated as ornithischians and saurischians.
diploid Cells that contain homologous chromosomes. The number of chromosomes in the cells is the diploid number and is equal to 2n (n is the number of homologous pairs).
directional selection A process of natural selection that tends to favor phenotypes at one extreme of the phenotypic range. PICTURE
disaccharides 1. Sugars made up of two monosaccharides held together by a covalent bond; e.g., sucrose and lactose. 2. Type of sugar (saccharide) composed of two sugar molecules bonded together with an ester (covalent) bond examples include sucrose, maltose, and lactose. PICTURE
discontinuous variation Occurs when the phenotypes of traits controlled by a single gene can be sorted into two distinct phenotypic classes.
disruptive selection A process of natural selection that favors individuals at both extremes of a phenotypic range. PICTURE
distal tubule The section of the renal tubule where tubular secretion occurs.
divergent evolution The divergence of a single interbreeding population or species into two or more descendant species.
divergent plate boundary The boundary between two tectonic plates that are moving apart.
diversity The different types of organisms that occur in a community.
DNA hybridization The formation of hybrid DNA molecules that contain a strand of DNA from two different species. The number of complementary sequences in common in the two strands is an indication of the degree of relatedness of the species.
DNA ligase In recombinant DNA technology, an enzyme that seals together two DNA fragments from different sources to form a recombinant DNA molecule.
DNA polymerase In DNA replication, the enzyme that links the complementary nucleotides together to form the newly synthesized strand.
dominance The property of one of a pair of alleles that suppresses the expression of the other member of the pair in heterozygotes.
dominance hierarchy A social structure among a group of animals in which one is dominant and the others have subordinate nonbreeding positions.
dominant Refers to an allele of a gene that is always expressed in heterozygotes.
double fertilization A characteristic of angiosperms in which a pollen tube carries two sperm cells to the female gametophyte in the ovule. One sperm cell fuses with the egg cell and gives rise to a diploid embryo The other sperm cell fuses with the two polar cells to form a triploid cell that develops into the endosperm. PICTURE 1 PICTURE 2
duodenum The upper part of the small intestine. PICTURE
duplication An extra copy of a chromosome segment without altering the number of chromosomes.
dystrophin Protein making up only 0.002% of all protein in skeletal muscle but which appears vital for proper functioning of the muscle. Sufferers of muscular dystrophy appear to lack dystrophin.
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