Dr. Paul's Virtually Biology Show.

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D


Decomposer. An organism that obtains its energy from breaking down dead organic matter. A decomposer is often called a saprobe. Many fungi and bacteria are good examples.

Deductive Reasoning. Reasoning from a general concept or theory to a specific prediction. For example, knowing the general laws of gravity, and having mathematical equations for them, allows us to deduce or make specific predictions about the path of a satellite.


Deme. A local freely interbreeding group of organisms; a subpopulation.
Dendrite. The branched part of the neuron involved in receiving chemical signals from other neurons or information from the environment.
Dermal tissue. One of the three main tissue types in plants. This tissue occurs as a single layer of cells on the outer surfaces of leaves and stems where it is called epidermal tissue. Another type of dermal tissue, endodermis, separates the vascular bundle of the root from the surrounding tissues and helps to regulate what materials may enter the vascular system of the plant.
Detritovore. An organism that ingests dead organic matter(detritus) and digests this material internally. Many small animals such as sow bugs, millipedes and worms are detritovores.
Diamond. One of the three natural forms of carbon. The others are graphite and buckminsterfullerine.


Diastole. The period during the cardiac cycle when the ventricles are relaxed. When measuring blood pressure if your blood pressure is 120/70, the "70" refers to your diastolic blood pressure.
Differentiation. In development, refers to the increased and largely irreversible specialization of cells as the organism develops. Recent research into cloning shows that differentiation can be reversed artificially.
Diffusion. A process by which a substance moves from a region of high concentration to a region of low concentration solely due to the thermal energy possessed by the molecules.
Digestion. The breaking down of complex food stuffs into simpler compounds by means of cleavage reactions.
Diploid. Having chromosomes in pairs of similar chromosomes called homologous chromosomes.
Dihybrid cross. A mating involving pure breeding individuals with respect to alternate forms of each of two characteristics. The offspring from this mating are then mated with each other to examine the mode of inheritance of the two characteristics.
DNA. DNA or Deoxyribonucleic acid is a a double stranded nucleic acid. Each strand is mad up of many units each made from phosphate, 5 carbon sugar and one of four nitrogen containing carbon bases. The function of DNA is to serve as genetic material in organisms and in some viruses.

DNA polymerase. A complex enzyme involved in the replication of DNA.
Dominant. In genetics an allele that completely masks the expression of another allele at the same locus on a homologous chromosome is said to be dominant over the second one
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Double bond. A covalent bond in which each of the two atoms in the bond are sharing two electrons. Hence the double bond involves a sharing of four electrons in all. In structural diagrams the double bond is shown as two lines connecting the atoms.

Duplicated Chromosome. A chromosome containing two identical molecules of DNA is called a duplicated chromosome as opposed to an unduplicated chromosome which is a chromosome with one molecule of DNA.