Audio Glossary

You need Adobe Flash Player to listen to the pronunciations.

  1. dark reactions: A series of chemical reactions that directly use carbon dioxide to synthesize sugars. The dark reactions do not require light and take place in the stroma of chloroplasts. Compare light reactions.
  2. decomposer: An organism that breaks down dead tissues into simple chemical components, thereby returning nutrients to the physical environment.
  3. deletion: A mutation in which one or more nitrogen bases are removed from the DNA sequence of a gene or chromosome. Compare insertion and substitution.
  4. dendrite: An extension of a neuron that receives signals from adjacent cells.
  5. density-dependent: Of or referring to a factor, such as food shortage, that limits the growth of a population more strongly as the density of the population increases. Compare density-independent.
  6. density-independent: Of or referring to a factor, such as weather, that can limit the size of a population but does not act more strongly as the density of the population increases. Compare density-dependent.
  7. deoxyribonucleic acid: See DNA.
  8. depolarization: A decrease in the charge difference between the inside and the outside of the plasma membrane of a neuron; initiates an action potential. Compare polarization.
  9. depth perception: The ability to accurately determine the relative distances of objects in space.
  10. dermal tissue: The tissue type in plants that forms the outer covering of the body.
  11. desert: A terrestrial biome, the communities of which are dominated by plants that grow in regions with low precipitation, usually 25 centimeters per year or less.
  12. deuterostome: Any of a group of animals, including sea stars and vertebrates, in which the second opening to develop in the early embryo becomes the mouth. Compare protostome.
  13. development: The process by which an organism grows from a single cell to its adult form.
  14. diastole: The relaxation phase of the cardiac cycle. The diastolic pressure corresponds to the second and lower of the two values reported in a blood pressure reading. Compare systole.
  15. differentiation: The process by which a cell develops physical characteristics that make it suited for a specialized function.
  16. diffusion: The passive movement of a molecule from areas of high concentration of that molecule to areas of low concentration of that molecule.
  17. digestion: The chemical breakdown of food.
  18. digestive system: A tubular passageway through which food generally moves in a one-way flow and in which animals convert the food they eat into a form their bodies can absorb and use.
  19. diploid: Of or referring to a cell or organism that has two complete sets of homologous chromosomes (2n). Compare haploid.
  20. directional selection: A type of natural selection in which individuals with one extreme of an inherited characteristic have an advantage over other individuals in the population, as when large individuals produce more offspring than small and medium-sized individuals. Compare disruptive selection and stabilizing selection.
  21. disruptive selection: A type of natural selection in which individuals with either extreme of an inherited characteristic have an advantage over individuals with an intermediate phenotype, as when both small and large individuals produce more offspring than medium-sized individuals. Compare directional selection and stabilizing selection.
  22. distribution: The geographic area over which a species is found.
  23. disturbance: An event, such as a fire or windstorm, that kills or damages some organisms in a community, thereby creating an opportunity for other organisms to become established.
  24. diversity: The composition of an ecological community, which has two components: the number of different species that live in the community and the relative abundances of those species.
  25. DNA: Deoxyribonucleic acid, a polymer of nucleotides that stores the information needed to synthesize proteins in living organisms.
  26. DNA chip: A small surface, or "chip," roughly the size of a dime on which thousands of samples of DNA are placed in a regimented order.
  27. DNA fingerprinting: The use of DNA analysis to identify individuals and determine the relatedness of individuals.
  28. DNA hybridization: Base pairing of DNA from two different sources.
  29. DNA library: A collection of an organism's DNA fragments that is stored in a host organism, such as a bacterium.
  30. DNA polymerase: The key enzyme that cells use to copy their DNA; in DNA technology, used in the polymerase chain reaction to make many copies of a gene or other DNA sequence.
  31. DNA primer: A short segment of DNA used in PCR amplification that is designed to pair with one of the two ends of the gene being cloned by PCR.
  32. DNA probe: A short sequence of DNA (usually tens to hundreds of bases long) that can pair with a particular gene or other specific region of DNA.
  33. DNA repair: A three-step process in which damage to DNA is repaired. Damaged DNA is first recognized, then removed, and then replaced with newly synthesized DNA.
  34. DNA replication: The duplication, or copying, of a DNA molecule. DNA replication begins when the hydrogen bonds connecting the two strands of DNA are broken, causing the strands to unwind and separate. Each strand is then used as a template for the construction of a new strand of DNA.
  35. DNA segregation: The process by which the DNA of a dividing cell is divided equally between two daughter cells.
  36. DNA sequence: The sequence or order in which the nitrogen bases adenine (A), cytosine (C), guanine (G), and thymine (T) are arranged throughout all or part of an organism's DNA.
  37. DNA sequencing: A procedure, usually automated, used to determine the sequence of bases in a DNA fragment.
  38. DNA technology: The set of techniques that scientists use to manipulate DNA.
  39. domain: A level of biological classification above the kingdom. The three domains are the Bacteria, the Archaea, and the Eukarya.
  40. dominant: Of or referring to an allele that determines the phenotype of an organism when paired with a different (recessive) allele. Compare recessive.
  41. dopamine: A neurotransmitter released in the brain that is associated with feelings of happiness.
  42. dormancy: A state of arrested growth in a plant. Seed dormancy occurs when a mature plant embryo (inside of a seed) halts its growth, and ends when the embryo is stimulated by favorable conditions to begin growing again. In some plant species, bud dormancy occurs during winter and ends when the plant resumes growth in the spring.
  43. double fertilization: The two fertilization events that occur when a flowering plant reproduces sexually. In one of these fertilization events, a sperm cell fertilizes the egg, producing a diploid cell (zygote) that will grow into the next generation's flowering plant; in the other, a second sperm cell fertilizes a large central cell within the embryo sac, producing the endosperm that will nourish the developing embryo.
  44. double helix: The structure of DNA, in which two long strands of covalently bonded nucleotides are held together by hydrogen bonds and twisted into a spiral coil.
  45. doubling time: The time it takes a population to double in size. Doubling time can be used as a measure of how fast a population is growing.
  46. drag: A force that resists the motion of any moving object.
  47. duplication: A mutation in which a fragment from one chromosome fuses to the homologous chromosome, increasing the length of the chromosome that receives the fragment.