R

radially symmetrical RAY-dee-a-lee sim-ET-rik-el An animal body form in which any plane passing from one end to the other divides the body into mirror images. 511

radicle RAD-eh-kil The first root to emerge from a seed. 546

radioles RAY-de-oles Ciliated structures on the anterior end of polychaete worms that filter small organisms and organic particles from seawater. 521

radiometric dating RAD-ee-o-MET-rik DAY-ting Using measurements of natural radioactive decay as a clock to date fossils. 393

radula RAD-yew-la A chitinous, tonguelike structure that mollusks use to eat. 519

reabsorption re-ab-SORP-shun The kidney's return of useful substances to the blood. 778

reactant re-AK-tant A starting material in a chemical reaction. 35

reaction center Clusters of chlorophyll and proteins that receive photon energy in photosynthesis. 153

reaction chain A sequence of releasers that joins several behaviors. 814

reading frame The DNA nucleotide corresponding to the first codon position in mRNA. 332

realized niche re-a-lized neesh The resources in a species' environment that it can actually use, considering competition and other limitations. 838

receptor-mediated endocytosis re-CEP-ter ME-dee-a-ted en-do-ci-TOE-sis Binding of a ligand by a cell surface protein that draws the ligand into the cell in a vesicle. 94

receptor potential A change in membrane potential in a neuron specialized as a sensory receptor, caused by redistribution of ions in response to the strength of the stimulus. 643

recessive re-SESS-ive An allele whose expression is masked by the activity of another allele. 265

reciprocal translocation re-SIP-ro-kal tranz-lo-CAY-shun Two nonhomologous chromosomes exchanging parts. 301

rectum REK-tum A storage region leading from the large intestine to the anus. 764

red blood cell (erythrocyte) A disc-shaped cell, lacking a nucleus, that contains hemoglobin. 594, 714

red marrow Immature blood cells and platelets in cavities in spongy bone. 689

reduction A chemical reaction in which electrons are gained. 117

reduction division Meiosis I, when the diploid chromosome number is halved. 198

reflex arc A neural pathway that links a sensory receptor and an effector. 623

regulatory enzyme REG-u-la-tor-ee EN-zime An enzyme that controls the activity of a biochemical pathway because it can be activated or deactivated by binding a compound other than its substrate under certain conditions. 122

relative dating An estimate of the time an organism lived based on the location of its fossils in rock layers. 392

releaser The specific factor that triggers a fixed action pattern. 814

releasing hormone A hormone produced by the hypothalamus that influences another gland. 666

renal cortex RE-nal COR-tex The outer portion of a kidney. 780

renal medulla RE-nal med-U-la The middle part of a kidney. 780

renal pelvis RE-nal PEL-vis The inner part of a kidney, where urine collects. 780

renal tubule RE-nal TU-bule The tubule part of a nephron, where toxins are added and nutrients recycled to the blood, forming urine. 780

replication fork A locally unwound portion of DNA where replication occurs. 317

respiratory chain A series of electron-accepting enzymes embedded in the inner mitochondrial membrane. 127

resting potential The electrical potential ({neg}65 millivolts) on the inside of a neuron not conducting a nerve impulse. 612

reticular activating system rah-TIK-u-lar AK-tah-vay-ting SIS-tem (RAS) A diffuse network of cell bodies and nerve tracts that extends through the brain stem and into the thalamus; screens sensory input to the cerebrum. 627

retina RET-eh-na A sheet of photoreceptors at the back of the human eye. 648

reverse transcriptase re-VERS tran-SCRIPT-aze An enzyme that constructs a DNA molecule from an RNA molecule. 355, 798

R group R GROOP An amino acid side chain. 47

rhizoids RI-zoydz Hairlike extensions along the lower surface of a bryophyte that anchors the plant and absorbs water and minerals. 493

rhizomes RI-zomes Fleshy, underground stems that run horizontally. 497

rhodopsin ro-DOP-sin A pigment molecule stored in rod cells of the retina. Light splits rhodopsin, which depolarizes the rod cell and provokes an action potential. 644

ribonucleic acid RI-bo-nu-KLAY-ik AS-id (RNA) A single-stranded nucleic acid consisting of nucleotides containing a phosphate, ribose, and nitrogenous bases adenine, guanine, cytosine, and uracil. 51

ribose RI-bose The five-carbon sugar that is a structural component of RNA. 311

ribosomal RNA RI-bo-SOAM-el RNA(rRNA) RNA that, along with proteins, forms a ribosome. 329

ribosome RI-bo-soam A structure built of RNA and protein upon which mRNA anchors during protein synthesis. 66

ribozymes RI-bo-zimes Small RNAs that function as enzymes. 329

RNA polymerase RNA poe-Lim-er-ase An enzyme that takes part in DNA replication and RNA transcription. 317, 327

RNA primer RNA PRI-mer A small piece of RNA, inserted at the start of a piece of DNA to be replicated, which attracts RNA polymerase and is later removed. 317

rods Specialized receptor cells in the retina that provide black-and-white vision. 648

root Part of plant underground. 534

root cap A thimble-shaped protective structure on a root tip. 546

rosettes ro-ZETTZ Nonelongated stems. 541

r-selection A form of natural selection in which individuals are short-lived, reproduce early and have many offspring that get little care. Density-independent factors regulate these populations. 845

rugae RU-guy Folds in the mucosa of the stomach. 760

feedback form | permissions | international | locate your campus rep | request a review copy

digital solutions | publish with us | customer service | mhhe home

---

Copyright ©2001 The McGraw-Hill Companies.
Any use is subject to the Terms of Use and Privacy Policy.
McGraw-Hill Higher Education is one of the many fine businesses of the The McGraw-Hill Companies.