Science / Biology Glossary

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; in . . . View Full Definition

Dihybrid Cross: In genetics, a cross that involves two sets of characteristics.

Dinoflagellates: Single-celled to colonial protistans characterized by two flagella, one girdling the cell and the other trailing the cell. Some dinoflagellates exist in coral, in a symbiotic relationship. T . . . View Full Definition

Dinosaurs: Any of the Mesozoic diapsids (once considered to be reptiles) belonging to the groups designated as ornithischians and saurischians.

Dioecious: Term applied to plants having separate male and female plants.

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.

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 es . . . View Full Definition

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.

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 degr . . . View Full Definition

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.

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