race A subdivision of a species that is capable of interbreeding with other members of the species.
radially symmetrical In animals, refers to organisms with their body parts arranged around a central axis. Such animals tend to be circular or cylindrical in shape.
radiation Energy emitted from the unstable nuclei of radioactive isotopes.
radioactive decay The spontaneous decay of an atom to an atom of a different element by emission of a particle from its nucleus (alpha and beta decay) or by electron capture.
radioisotope Term applied to a radioactive isotope, such as carbon-14 or uranium 238. Radioisotope nuclei are unstable and spontaneously breakdown and emit one of a number of types of radiation.
radiometric time Type of absolute time determined by the relative porportions of radioisotopes to stable daughter isotopes.
ray-finned Taxonomic group of fish, such as trout, tuna, salmon, and bass, that have thin, bony supports holding the Þns away from the body and an internal swim bladder that changes the buoyancy of the body; one of the two main types of bony Þshes.
reabsorption The return to the blood of most of the water, sodium, amino acids, and sugar that were removed during Þltration; occurs mainly in the proximal tubule of the nephron.
receptacle The base that attaches a þower to the stem. PICTURE
receptor Protein on or protruding from the cell surface to which select chemicals can bind. The opiate receptor in brain cells allows both the natural chemical as well as foreign (opiate) chemicals to bind.
recessive Refers to an allele of a gene that is expressed when the dominant allele is not present. An allele expressed only in homozygous form, when the dominant allele is absent.
recombinant DNA molecules New combinations of DNA fragments formed by cutting DNA segments from two sources with restriction enzyme and then joining the fragments together with DNA ligase. Interspecies transfer of genes usually through a vector such as a virus or plasmid.
recombinant DNA technology A series of techniques in which DNA fragments are linked to self-replicating forms of DNA to create recombinant DNA molecules. These molecules in turn are replicated in a host cell to create clones of the inserted segments.
recombination A way in which meiosis produces new combinations of genetic information. During synapsis, chromatids may exchange parts with other chromatids, leading to a physical exchange of chromosome parts; thus, genes from both parents may be combined on the same chromosome, creating a new combination.
red algae Common name for the algae placed in the division Rhodophyta.
red blood cell Component of the blood that transports oxygen with the hemoglobin molecule. See also erythrocyte
red tides Phenomenon associated with population explosions (blooms) of certain types of dinoflagellates; red structures inside the dinoflagellates cause the water to have a reddish color. PICTURE
reduction The gain of an electron or a hydrogen atom. The gain of electrons or hydrogens in a chemical reaction.
reductional division The Þrst division in meiosis; results in each daughter cell receiving one member of each pair of chromosomes. PICTURE
reflex A response to a stimulus that occurs without conscious effort; one of the simplest forms of behavior.
reflex arc Pathway of neurons, effector(s) and sensory receptors that participate in a reflex.
region of division The area of cell division in the tip of a plant root. PICTURE
region of elongation The area in the tip of a plant root where cells grow by elongating, thereby increasing the length of the root. PICTURE
region of maturation (differentiation) The area where primary tissues and root hairs develop in the tip of a plant root. PICTURE
relative time Type of geologic time (absolute time being the other) that places events in a sequence relative to each other.
renal tubule The portion of the nephron where urine is produced. PICTURE
renin An enzyme secreted by the kidneys that converts angiotensinogen into angiotensin II.
replication Process by which DNA is duplicated prior to cell division. PICTURE
reproductive isolating mechanism Biological or behavioral characteristics that reduce or prevent interbreeding with other populations; e.g., the production of sterile hybrids. Establishment of reproductive isolation is considered essential for development of a new species.
reproductive system One of eleven major body organ systems in animals; is responsible for reproduction and thus the survival of the species. PICTURE
reptiles Taxonomic class of vertebrates characterized by scales and amniotic eggs; the first truly terrestrial vertebrate group.
resolution In relation to microscopes, the ability to view adjacent objects as distinct structures.
resource partitioning The division of resources such that a few dominant species exploit most of the available resources while other species divide the remainder; helps explain why a few species are abundant in a community while others are represented by only a few individuals.
respiration 1) breathing as part of gas exchange; or 2) cellular metabolism.
respiratory surface A thin, moist, epithelial surface that oxygen can cross to move into the body and carbon dioxide can cross to move out of the body.
respiratory system One of eleven major body organ systems in animals; moves oxygen from the external environment into the internal environment and removes carbon dioxide from the body. PICTURE
resting potential The difference in electrical charge across the plasma membrane of a neuron.
restriction enzymes A series of enzymes that attach to DNA molecules at speciÞc nucleotide sequences and cut both strands of DNA at those sites. A bacterial enzyme that cuts DNA at a specific recognition sequence. This is a bacterial defense against viral DNA and plasmid DNA and is now used as an important tool in biotechnology.
restriction fragment length polymorphism (RFLP) A heritable difference in DNA fragment length and fragment number; passed from generation to generation in a codominant way.
retina The inner, light-sensitive layer of the eye; includes the rods and cones.
retroviruses Viruses that contain a single strand of RNA as their genetic material and reproduce by copying the RNA into a complementary DNA strand using the enzyme reverse transcriptase. The single-stranded DNA is then copied, and the resulting double-stranded DNA is inserted into a chromosome of the host cell. PICTURE 1 | PICTURE 2 | PICTURE 3 | PICTURE 4
reverse transcriptase An enzyme used in the replication of retroviruses; aids in copying the retrovirus's RNA into a complementary strand of DNA once inside the host cell. PICTURE 1 | PICTURE 2 | PICTURE 3
reverse transcription Process of transcribing a single-stranded DNA from a single-stranded RNA (the reverse of transcription); used by retroviruses as well as in biotechnology.
rheumatoid arthritis A crippling form of arthritis that begins with inþammation and thickening of the synovial membrane, followed by bone degeneration and disÞgurement.
rhizoids Filamentous structures in the plants group known as bryophytes that attach to a substrate and absorb moisture. The term is also applied to similar structures found outside the bryophytes.
rhizome In ferns, a horizontal stem with upright leaves containing vascular tissue.
rhodopsin A visual pigment contained in the rods of the retina in the eye..
ribonucleic acid (RNA) Nucleic acid containing ribose sugar and the base Uracil; RNA functions in protein synthesis. The single starnded molecule transcribed from one strand of the DNA. There are three types of RNA, each is involved in protein synthesis. RNA is made up nucleotides containing the sugar ribose, a phosphate group, and one of four nitrogenous bases (adenine, uracil, cytosine or guanine). PICTURE
ribose Sugar found in nucleotides of RNA and in ATP. PICTURE
ribosomal RNA One of the three types of RNA; rRNA is a structural component in ribosomes.
ribosomal subunits Two units that combine with mRNA to form the ribosomal-mRNA complex at which protein synthesis occurs.
ribosomes Small organelles made of rRNA and protein in the cytoplasm of prokaryotic and eukaryotic cells; aid in the production of proteins on the rough endoplasmic reticulum and ribosome complexes. The site of protein synthesis. The ribosome is composed of two subunits that attach to the mRNA at the beginning of protein synthesis and detach when the polypeptide has been translated. PICTURE
RNA polymerase During transcription, an enzyme that attaches to the promoter region of the DNA template, joins nucleotides to form the synthesized strand of RNA and detaches from the template when it reaches the terminator region. PICTURE
RNA transcript Term applied to RNA transcribed in the nucleus. PICTURE
Rodinia Name applied to the precambrian supercontinent. PICTURE
rods Light receptors in primates' eyes that provide vision in dim light.
root cap Structure that covers and protects the apical meristem in plant roots. Cells forming a protective series of layers over the root meristem. PICTURE
root hairs Extensions of the root epidermis that increase the root's ability to absorb water. PICTURE
root-leaf-vascular system axis Refers to the arrangement in vascular plants in which the roots anchor the plant and absorb water and nutrients, the leaves carry out photosynthesis, and the vascular system connects the roots and leaves, carrying water and nutrients to the leaves and carrying sugars and other products of photosynthesis from the leaves to other regions of the plant. PICTURE
roots Organs, usually occurring underground, that absorb nutrients and water and anchor the plant; one of the three major plant organ systems. PICTURE
root system Plant organ systems that anchors the plant in place, stores excess sugars, and absorbs water and mineral nutrients. That part of the plant below ground level. PICTURE
RuBP Ribulose biphosphate; the 5-carbon chemical that combines with carbon dioxide at the beginning of the Calvin Cycle. PICTURE
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