Science / Biology Glossary

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 c . . . View Full Definition

Deoxyribose: Five-carbon sugar found in nucleotides of DNA.

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.

Devonian: Period of geologic time from 410 - 360 million years before the present. Life on land diversified, with the amphibians appearing late in this period. Plants underwent major changes, includin . . . View Full Definition

Diabetes Mellitus: 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 res . . . View Full Definition

Diaphragm: A dome-shaped muscle that separates the thoracic and abdominal cavities.

Diastole: The filling of the ventricle of the heart with blood.

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 (exce . . . View Full Definition

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 Go . . . View Full Definition

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.

Digestion: The process of breaking down food into its molecular and chemical components so that these nutrient molecules can cross plasma membranes.

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