Dr. Paul's Virtually Biology Show.

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R

Radial symmetry. A body plan in which the body parts originate from the central of the organism like spokes in a wheel. Star fish and jellyfish and among the creatures which share this type of symmetry.
Recessive allele. An alternate form of a gene that has no obvious effect on the phenotype of a heterozygous individual.
Recombination. A name given for one of any number of processes in prokaryotes and eukaryotes that serve to produce new combinations of genetic material already existing in individuals. Thus crossing over and independent assortment would be examples of recombination in meiosis. Sexual reproduction as a whole is a type of recombination.
Reduction. The addition of electrons and often hydrogen ions to a substance. Contrast oxidation.
Reflex. An automatic response to a stimulus consisting usually of a small number of steps, perhaps just a muscle twitch. A motor program as an elaborate sequence of automatic responses is become a common term. However, the distinction is not always clear cut.

Releaser. A general term for a stimulus that triggers a fixed action pattern.

Reproductive success. Just as it sounds this term refers to the number of offspring an organism has. This is not the same as fitness which is reproductive success relative to the other members of the population.

Respiration. This term has two uses: Cellular respiration refers to the oxidation of food stuffs to produce ATP. Respiration also refers to gas exchange (releasing Carbon dioxide and taking in oxygen)between the organism and the environment.
Restriction enzymes. A class of enzymes found in bacteria that cut strands of DNA at sites determined by certain base sequences. In nature, these enzymes help the bacteria cope with and manipulate foreign DNA. These enzymes are important tools in genetic engineering because they help scientists isolate DNA fragments containing genes.
Retrovirus. An RNA virus that uses an enzyme called reverse transcriptase to make a DNA molecule, sometimes called copy DNA, which then gets inserted into the host cell's genetic material.
Reverse transcriptase. An enzyme found in retroviruses that enables the production of a DNA copy from the viral RNA. Since the usual flow of information in living things is from DNA to an RNA transcript these enzymes are called "reverse" transcriptase.
Ribonucleic acid. A single stranded nucleic acid containing ribose and using uracil in place of thymine. Various RNA's are involved in protein synthesis and comprise the genetic material of certain viruses including the retroviruses.
Ribose. A five carbon monosaccaride best known for its role as a component of nucleotides and the nucleic acid RNA. A related monosaccharide, differing by lacking an oxygen, is called deoxyribose. This sugar is found in DNA.
Ribosome. A very small two part organelle containing RNA and proteins that is involved in the translation step of protein synthesis. In eukaryotic cells, ribosomes may be free in the cytoplasm or bound to the surface of the rough endoplasmic reticulum(Rough ER).

Ribosomal RNA. The RNA found as part of the ribosome and that serve in part as a ribozyme involved in forming peptide bonds.

Ribozyme. An RNA molecule that serves as an enzyme. The Ribosomal RNA in the ribosome acts as an enzyme to catalyze the formation of peptide bonds during protein synthesis

RuBP. Ribulose bi phosphate. A five carbon sugar with two phosphate groups that is the starting material for the Calvin cycle. Carbon dioxide is attached to this compound to form a six carbon unstable intermediate which breaks into two three carbon molecules of PGA.

Rough endoplasmic reticulum(Rough ER). A network of membranous sacs connected to the nuclear envelope that is covered with ribosomes. Rough ER is a site for active synthesis and collection of proteins.