p-aminobenzoic acid /ə-MEEN-ō-ben-ZŌ-ik/ (PABA also 4-aminobenzoic acid or para-aminobenzoic acid) n. A white crystalline organic compound (C₇H₇NO₂) composed of a benzene ring with an amino group and a carboxyl group attached to the carbon atoms 1 and 4, respectively. In humans, gut flora synthesize folic acid from this molecule.
P1-derived artificial chromosome (PAC) n. One type of vector used to clone DNA fragments (100- to 300-kb insert size; average, 150 kb) in Escherichia coli cells (based on the phage P1 genome). See also: cloning vector.
PAGE abbr. Polyacrylamide gel electrophoresis.
pachytene /PACK-ə-teen/ n. The stage of Prophase I during which the two sister chromatids of each chromosome separate from each other. During this stage, the chromosomes look thicker when viewed under a microscope (pachys is Greek for thick). Homologs are still paired at this point. Pachytene is also known as pachynema. Crossing-over: Non-sister chromatids remain in contact throughout pachytene and a kind of localized breakage of the DNA occurs, which is followed by exchanges of DNA between them. This process is called crossing over. Crossing over produces "cross-over chromatids," each composed of distinct blocks of DNA, some blocks derived from the mother, others from the father. MORE INFORMATION
pachyderm /PAK-ə-derm/ n. A mammal belonging to the erstwhile taxonomic group Pachydermata (meaning "thick skinned ones"). This group included elephants, certain artiodactyls, such as hippopotamuses and pigs, as well as some perissodactyls (rhinoceroses, horses).
palate /PAL-ət/ n. The roof of the mouth; the bony portion is the hard palate, the fleshy rear portion is the soft palate.
paleoanthropologist /PALE-ee-ō-an-thrə-PAWL-ə-jist/ A scientist who studies the fossil remains, and the associated artifacts, of the ancestors of modern human beings.
paleocontinent /PALE-ee-ō-KAWN-tə-nent, -nənt/ n. A formerly existing continent, now broken up or fused with other continents due to the effects of rifting and continental drift.
Paleogene (Pg formerly known as Lower Tertiary) /PALE-ee-ə-jeen/ n. The first period of the Cenozoic Era. Extending from 65.5 to 23.03 mya, it includes three epochs: the Paleocene, Eocene, and Oligocene.
Paleolithic /pale-ee-ə-LITH-ik/ n. The Old Stone Age. The Paleolithic was the first period of stone tool use. It extended from the time of the earliest known stone tools, about 2.5 mya (Semaw et al. 1997, Semaw 2000), to the beginning of agriculture around 12,000 B.P.
paleontology /pale-ee-awn-TALL-ə-jee/ n. The scientific study of the life of past geological periods. It deals primarily with the documentation of the chronology of the history of life on earth — paleontologist /pale-ee-awn-TALL-ə-jəst, -jist/
Paleozoic Era /pale-ee-ə-ZŌ-ik/ n. The first, and most lengthy era of the Phanerozoic Eon. It lasted from 542 to 251 million years ago, and is composed of six geologic periods, which are, sequentially, the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. Nearly all of the various plant and animal phyla, with the major exception of the angiosperms, first became abundant during the Paleozoic. GEOLOGIC TIME SCALE
palpation /pawl-PAY-shən/ n. In medical examination: Application of the hands to the external surface of the body for the purpose of determining the state of an underlying organ or tissue.
palindrome /PAL-ən-drōm/ n. With regard to DNA or RNA: A nucleotide sequence having a symmetric structure such that it is the same when read in either direction, for example, CCTAGAGGAGATCC. — palindromic /pal-ən-DRŌM-ick/
paludicolous /pawl-yə-DIK-ə-ləs/ Living in marshes.
palynology /PAL-in-NAWL-ə-jee/ The study of fine organic particulate matter, such as pollen grains and spores, present in air, water or sedimentary deposits.
pancreas /PAN-kree-əs/ n. A large gland, situated behind the stomach in humans. Its thicker end (the "head") is attached to the duodenum. Its slimmer end (the "tail") extends as far as the spleen. The pancreas produces an internal and an external secretion. The internal secretion, or "pancreatic juice," which plays an important role in digestion, arises from the cells of the acini and passes through a network of ducts to the duodenum. The external secretion is produced by the islets of Langerhans, and two of its components, insulin and glucagon, play an essential role in carbohydrate metabolism — pancreatic /pan-kree-AT-ik/ adj.
Pangea (also Pangaea) /pan-JEE-ə/ n. A single great supercontinent that existed from the Permian to the Jurassic. It broke up to produce all of the modern continents. MAP OF PANGEA | SEE ANIMATION OF PANGEA'S BREAKUP
panmictic population /pan-MIK-tik/ A population of organisms in which mating is random.
panmixia /pan-MIX-ee-yə/ (also panmixis) n. See: random mating.
panspermia /pan-SPERM-ee-yə/ n. See: exogenesis.
Panthera /pan-TARE-ə/ n. The genus comprised of the big cats, the lion (Panthera leo), the jaguar (Panthera onca), the leopard (Panthera pardus), and the tiger (Panthera tigris). PICTURES | LION DIET | GEOGRAPHIC DISTRIBUTION OF LIONS
papilla (pl. papillae) /pə-PILL-ə/ n. Any nipple-like process.
papillate /PAH-pill-ate or pə-PILL-ate/ adj. Having papillae.
papillectomy /pah-pill-ECK-təm-ee/ n. Surgical removal of a papilla.
pappose /PAP-ōs/ adj. Having a pappus.
pappus /PAP-əs/ n. (1) in plants: a bristly structure; (2) in animals: a coating of fine, downy hair, especially the early growth of a beard.
papula /PAP-yə-lə/ (1) a pimple; (2) a papule.
papular /PAP-yə-lər/ adj. Pertaining to pimples.
papule /PAP-yool/ n. A raised, red bump on the skin; the term is applied to any bump up to about the size of a pea.
papuliferous /PAP-yə-LIF-ər-əs/ adj. Pimply, having papules.
parapatric /pare-ə-PAT-rik/ adj. Of two populations: Occupying different, non-overlapping, but contiguous geographic regions.
paralysis /pə-RALL-ə-sis/ n. Total loss of function in a muscle or set of muscles — paralyzed /PAIR-ə-līzd/ adj.
parasite /PARE-ə-sīt/ n. An organism that absorbs nutrients on an ongoing basis from some other living organism. Generally, a parasite is smaller than its host — parasitic /PAIR-ə-SIT-ik/ adj.
parasitic /PARE-ə-SIT-ik/ adj. Living as, caused by, or pertaining to a parasite.
parasitism /PARE-ə-sit-iz-əm/ n. A form of symbiosis in which one of the two participants benefits at the expense of the other.
parasitology /pare-ə-sə-TAWL-ə-jee/ n. The study of parasites.
parietal bones /pə-RYE-ə-təl/ n. The two bones forming the roof and sides of the brain cavity.
parthenogen /parth-EN-ə-jen/ n. A parthenogenetic organism.
parthenogenesis /parth-ə-nə-JEN-ə-səs, -nō-JEN/ n. Development of the embryo from an unfertilized egg.
parthenogenetic /parth-ə-nō-jə-NET-ik/ adj. Reproducing by means of parthenogenesis.
parturition /par-ter-RISH-ən/ n. The act of giving birth.
pascal (Pa) /pass-KALL/ n. A unit of pressure, stress, and tensile strength in terms of force per unit area; equal to one newton per square meter.
passive immunity n. Immunity produced by injection of antibodies. Immunity of this sort is ephemeral, lasting only a few weeks or months. See: active immunity.
pathogen /PATH-ə-jen/ n. A disease-causing agent (as a microorganism).
pathogenesis /PATH-ə-JEN-ə-səs/ n. The steps leading to the development of a disease.
pathogenic adj. /PATH-ə-JEN-ik/ Disease-causing.
pathology n. /path-AWL-ə-jee, pə-THAWL-/ (1) the study of diseases; (2) changes caused by a disease.
pathway See: metabolic pathway.
PBS abbr. Primer binding site.
PC abbr. Plastocyanin.
PCC abbr. Premature chromosome condensation.
PCR abbr. Polymerase chain reaction.
PDGF abbr. Platelet-derived growth factor.
PE abbr. Phosphatidylethanolamine.
pediatrics /pee-dee-AT-ricks/ n. The study and medical treatment of the diseases of children.
pedipalp (or pedipalpus) /PED-ə-palp/ n. The second pair of appendages in an arachnid; in a spider, they are like small legs and function in insemination; in a scorpion, whip scorpion, or whip spider, they are the pincers; in a solpugid, they are about the same length as the other legs. MORE ABOUT ARACHNID ANATOMY
peduncle /peh-DƏNK-əl, PEE-dənk-əl/ n. (1) a stalk supporting a flower or an inflorescence; (2) a narrow appendage supporting a sessile animal and connecting it with the substrate; (3) any narrow, connecting band or stalk.
pelagic /pə-LAJ-ik/ adj. Living in the open sea.
Pelomyxa /PEL-ō-mik-sə/ n. A genus of giant amoebae found in the muddy bottoms of fresh water ponds, the largest visible to the naked eye (as large as half a centimeter in diameter, though most are 0.5 to 1.0 mm wide). They are classified as eukaryotes because he have membrane bounded nuclei, but they lack all other organelles characteristic of eukaryotes (Golgi bodies, endoplasmic reticula), mitochondria, chromosomes, and centrioles.
pelvic /PEL-vik/ adj. Of or pertaining to the pelvis.
pelvic opening /PEL-vik/ n. The large opening in the pelvis through which the digestive tract, urethra, and birth canal pass before exiting the body. Large-brained infants, such as those of humans and chimpanzees, require a large pelvic opening so that the head can easily pass during birth. In general, the pelvic opening is larger in females than in males.
pelvis /PEL-vəs/ (pl pelves /PEL-veez/) n. The complex bony structure that in everyday parlance is called the hip bone. It is actually composed of separate bones — the coccyx, the sacrum, and the innominate bones, which are joined together by ligaments.
pelvis renalis See: kidneys.
penetrance /PEN-ə-trəns/ n. The probability of a genotype being expressed. When penetrance is complete, the genotype is expressed in all individuals who have that genotype. When it is incomplete the genotype is expressed in only part of the population.
pennate /PEN-ate/ adj. (1) winged; (2) feathered; (3) wing-shaped.
penniform /PEN-ə-form/ adj. Feather-shaped.
PEP abbr. Phosphoenolpyruvic acid.
PEPC abbr. Phosphoenolpyruvate carboxylase.
PEPCK abbr. Phosphoenolpyruvate carboxykinase.
peptide /PEP-tide/ Two or more amino acids joined in a chain.
peptidoglycan /pep-tid-ō-GLĪ-kən/ n. A polymer found only in prokaryote cell walls. The cell walls of gram-positive bacteria have a thick layer of peptidoglycan, which stains purple in the Gram staining method; in gram-negative bacteria the layer is thinner and stains pink.
perfusion /per-FYOO-zhən/ n. The pumping or flow of a fluid through tissue.
pericardiac /PAIR-ə-KARD-ee-ack/ adj. Of or pertaining to the pericardium.
pericardial /PAIR-ə-KARD-ee-əl/ adj. Of or pertaining to the pericardium.
pericarditic /PAIR-ə-kar-DIT-ick/ adj. Of or pertaining to the pericardium.
pericarditis /PAIR-ə-kar-DITE-əs/ n. Inflammation of the pericardium.
pericardium /PAIR-ə-KARD-ee-əm/ n. A double membrane that surrounds the heart and the roots of the major coronary vessels.
pericarp /PAIR-ə-KARP/ n. The outer wall of a plant ovary (see figure at right).
pericycle /PAIR-ə-sī-kəl/ n. The layer of potentially meristematic cells immediately beneath a root's endodermis.
periderm /PAIR-ə-derm/ n. In plants, the permanent outer layer that replaces the epidermis during secondary growth.
periodontitis /PAIR-ee-ō-dawn-TĪ-tis/ n. A synonym of pyorrhea.
periodontium /PAIR-ee-ō-DAWN-shəm/ n. The tissue surrounding a tooth. — periodontal /PAIR-ee-ō-DAWN-təl/
periophthalmic /PAIR-ee-awf-THAWL-mik/ adj. Around the eyes.
periosteum /PAIR-ə-OST-ee-əm/ n. The fibrous coating investing all surfaces of a bone other than the articular surfaces.
perissodactyl /pə-RISS-ə-DACK-təl/ n. An odd-toed ungulate.
Permian Period (P) /PERM-ee-ən/ The last geologic period of the Paleozoic Era, lasting from 299.0±0.8 to 251.0±0.4 mya; named by Roderick Murchison after the former Russian kingdom of Permia. During the Permian, land vertebrates diversified into what are thought to be the ancestors of such groups as mammals, turtles, lepidosaurs and archosaurs. PICTURE | GEOLOGIC TIME SCALE
peroxisome /pə-RAWKS-ə-sōm/ n. In a eukaryotic cell, small organelles ("microbodies") containing enzymes that transfer hydrogen to oxygen from various substrates. This process produces hydrogen peroxide as a by-product, which is toxic, but peroxisomes also contain an enzyme that prevent any build-up of hydrogen peroxide by breaking it down into water.
petiole /PET-ee-ole/ n. The stalk of a leaf, which joins it to a node on the stem.
pH See: hydrogen ion concentration.
phage n. Short for bacteriophage.
phagocytosis n. Intracellular digestion of macromolecules by a lysosome.
phalanx /FAY-lanks/ (pl phalanges /fə-LAN-jeez/) n. Any of the bones of the fingers or toes — phalangeal /fə-LAN-jee-əl/
pharmacology /FARM-ə-CAWL-ə-jee/ n. The science and study of drugs. pharmacologist /FARM-ə-CAWL-ə-jist/
pharynx /FAIR-inks/ n. The passage connecting the mouth and nasal cavity with the esophagus.
Phanerozoic Eon (Brit. Phanaerozoic) /FAN-er-rō-ZŌ-ik/ n. The time period, running right up to the present, that began with the Cambrian Period, and that is composed of the Paleozoic, Mesozoic, and Cenozoic eras.
Phe abbr. Phenylalanine.
phenetics /fə-NET-icks/ n. A school of taxonomic thought that holds that organisms should be classified on the basis of overall similarity, regardless of the nature of their evolutionary relationship. Compare: cladistics — pheneticist /fə-NET-ə-sist/ n.
phenocopy /FEEN-ō-KAWP-ee/ n. A trait, that seems the same as a genetic trait, but that is the result of environmental influences.
phenology /fə-NAWL-ə-jee/ n. The scientific study of events in the annual cycles of living things, and of the climate's relationship to those events — phenologist /fə-NAWL-ə-jist/ n. — phenological /fə-nə-LAW-jə-kəl/ adj.
phenotype /FEEN-ō-tīp, FEEN-ə-/ n. An observable characteristic of an organism (generally thought of as corresponding to an underlying genotype). — phenotypic /FEEN-ō-tip-ick, FEEN-ə-/ adj.
phenylketonuria (PKU) /feen-əl-kee-tone-ER-ee-yə/ n. A condition characterized by the presence of phenylpyruvic acid in the urine.
phlebotomy /flə-BAWT-ə-mee/ n. The opening of a vein.
phial /FĪL/ n. A vial (a small, usually glass, container for medicines).
phlegm /FLEM/ n. Thick mucus (particularly from the respiratory tract).
phloem /FLŌM/ n. The vascular tissue in plants that carries sugars, especially sucrose, made in the photosynthetic regions of the plant, to other parts of the plant. In everyday parlance, this water-based solution of sugars is called sap. Compare: xylem.
phoronids (or phoronid worms) n. Marine worms comprising the phylum Phoronida. They are commonly called horseshoe worms in reference to the configuration of their gut which is U-shaped, with the anus next to the mouth. They live in chitinous tubes, which they secrete. Another characteristic feature of phoronids is the presence of an umbrella-like feeding apparatus called a lophophore (see figure at right), which is also present in bryozoans and brachiopods, with which phoronids are sometimes grouped as lophophorates. DIAGRAM OF PHORONID ANATOMY | PICTURE SHOWING LOPHOPHORES.
phosphoenolpyruvate carboxykinase (PEPCK) One of the enzymes working in gluconeogenesis.
phosphoenolpyruvate carboxylase (PEPC) An enzyme catalyzing the addition of CO₂ to phosphoenolpyruvate to form the four-carbon compound oxaloacetate.
phosphoenolpyruvic acid (PEP) An important compound involved in glycolysis and gluconeogenesis. Also important as the source of energy for the phosphotransferase system. The anion of phosphoenolpyruvic acid is called phosphoenolpyruvate.
phosphofructokinase (PFK) A kinase acting on fructose 6-phosphate.
phosphoglucose isomerase (PGI) /FOSS-fō-GLUE-cose eye-SOM-er-ase/ An enzyme catalyzing the conversion of glucose-6-phosphate into fructose 6-phosphate (second step of glycolysis).
phosphoglycerate kinase (PGK) /FOSS-fō-GLIS-er-ate KĪ-nase/ A transferase enzyme used in the seventh step of glycolysis.
phosphoglycerate mutase (PGM) /FOSS-fō-GLIS-er-ate MYOOT-ase/ An enzyme catalyzing the eighth step of glycolysis.
phospholipid /FOSS-fō-LIP-əd/ n. A lipid that contains phosphate esters of sphingosine or glycerol. Phospholipids have a head group, which is attracted to water, and a tail group, which is made up of a long hydrocarbon chain repelled by water. Phospholipids are the primary constituent of the lipid bilayers of cells.
phosphorescent /FOSS-fore-RESS-ənt, FOSS-fə-/ adj. Glowing in the dark.
phosphorus (P) /FOSS-fer-əs/ Chemical element; atomic number 15; atomic weight 30.973762. Three isotopes: ³¹P, ³²P, and ³³P. Both ³²P and ³³P are radioactive beta-emitters and are widely used in biology labs. ³²P is used primarily to produce radiolabeled DNA and RNA probes, and ³³P is useful in applications where lower energy beta emissions are desirable, such as DNA sequencing.
phosphorylation /foss-FOR-əl-LAY-shən/ n. The production of a phosphate derivative of an organic molecule, usually by adding a phosphate group from ATP. This process occurs, for example, in respiration and photosynthesis.
photosynthesis /fō-tō-SIN-thə-sis/ n. A process carried out in plants, algae, and bacteria, which uses energy from sunlight to convert carbon dioxide and water into glucose and oxygen. Photosynthesis is the source of atmospheric free oxygen and is the essential starting point for the construction of all organic molecules present in living things. MORE INFORMATION
photosynthetic /fō-tō-sin-THET-ik/ adj. Engaging in photosynthesis.
phototropism /fō-tō-TRAWP-iz-əm, -TRŌP-iz-əm/ n. A tropism in response to light.
phthisis /TĪ-səs/ n. (1) tuberculosis; (2) a progressive wasting away.
phycology /fī-KAW-lə-gee/ The study of algae.
phyllomorphous /fill-ə-MORE-fəs/ (also filiform /FILL-ə-form/ ) adj. Having the appearance or shape of a leaf.
phyllophagous /fə-LAWF-ə-gəs/ adj. Leaf-eating.
phyllophorous /fə-LAWF-(ə)-rəs/ adj. Leaf-bearing.
phylogenetic /FĪ-lə-jə-NET-ik/ With regard to, representing, or pertaining to a phylogeny.
phylogeny /fə-LAWJ-ə-nee/ n. A hypothetical reconstruction, usually in the form of a tree, of the evolutionary relationships of a group of organisms.
physiology /fizz-ee-AWL-ə-jee/ n. The study of the physical function of living organisms.
physometra /fī-sō-MEE-trə/ n. Distention of the uterus by gas.
phytalbumose /fite-AL-boo-mōs/ n. An albumose found in plants.
phytobezoar /FIDE-ō-BEE-zore/ n. A bezoar composed of plant-derived material.
phytohormone /FIDE-ō-HORE-mōn/ n. A plant hormone.
phytolith /FIDE-ō-lith/ n. Microscopic deposits, usually of silicon dioxide, formed within certain plants. Different plants form phytoliths of different, characteristic shapes and sizes. These minute granules become embedded in the teeth of animals eating the plants that produce them. They are therefore often used by paleontologists to determine the diets of extinct animals.
phytoid /FĪT-oid/ adj. (1) plantlike; (2) any disease caused by plant parasites.
phytopathology /FIDE-ō-pə-THAWL-ə-jee/ n. The study of plant diseases — phytopathologist /FIDE-ō-pə-THAWL-ə-jist/
phytophagous /fīt-TAWF-ə-gəs/ adj. Plant-eating.
phytosterol /fī-TAWST-ə-rawl/ n. Any plant sterol.
phytotoxin /FĪ-də-TOX-in/ n. Any toxin produced by a plant.
Pi Inorganic phosphate (also: orthophosphate ion).
PI (1) phosphatidylinositol; (2) principal investigator.
picofarad (pF) /PEEK-ō-mole, PEEK-ə/ n. One-trillionth (10-12) of a farad.
picogram (pg) /PEEK-ō-gram, PEEK-ə/ n. One-trillionth (10-12) of a gram.
picoliter (pl or pL British: picolitre) /PEEK-ō-LEE-ter, PEEK-ə/ n. One-trillionth (10-12) of a liter.
picomole /PEEK-ō-mole, PEEK-ə/ n. One-trillionth (10-12) of a mole.
picosecond (ps) /PEEK-ō-SEK-ənd, PEEK-ə/ n. One-trillionth (10-12) of a second.
picrotoxin (also cocculin) /pik-rō-TAWK-sən/ n. A bitter, crystalline toxin found in Indian berry (Anamirta cocculus). Picrotoxin is a non-competitive antagonist for the GABAA receptor chloride channels. Because GABA is the major inhibitory neurotransmitter in the central nervous system, picrotoxin's interference with it has a stimulant effect.
Pierolapithecus /PEE-ə-rō-lə-PITH-ə-kəs/ An ancient ape recently discovered in Spain. MORE INFORMATION
piscine /PISS-sine, -seen, PĪ-sīn/ adj. Like, pertaining to, or of fish.
piscivorous /pə-SIV-er-əs/ adj. Feeding on fish.
Piroplasma /PEER-ə-PLAZ-mə/ Former name for Babesia.
pistil /PISS-təl/ n. The ovule-bearing organ of a seed plant.
pithecophobia /pith-ə-kə-PHOBE-ee-yə/ Fear or revulsion elicited by the idea of apes being human ancestors.
PK Pyruvate kinase.
pl or pL Picoliter.
placental mammal /plə-SEN-təl/ A eutherian mammal.
plankton /PLANK-tən/ n. Small sea organisms moved primarily, or exclusively, by water currents.
planoccipital /plan-awk-SIP-ə-təl/ adj. Having a relatively flat occipital bone.
plantigrade /PLANT-ə-grade/ adj. Walking with the tarsals and metatarsals flat on the ground.
plasmids /PLAZ-midz/ n. Autonomously replicating extra-chromosomal circular DNA molecules, distinct from the normal bacterial chromosome and nonessential for cell survival under nonselective conditions. Some plasmids are capable of integrating into the host chromosome. Artificially constructed plasmids are often used as cloning vectors.
plasma membrane /PLAZ-mə/ (1) the limiting surface of the cytoplasm of a eukaryotic cell. It consists of a phospholipid bilayer with a variety of embedded molecules that act as channels and pumps, selectively moving particular molecules into and out of the cell. The plasma membrane also plays roles in cell adhesion and in maintaining the shape of the cell via cytoskeleton attachment. Surface molecules on the plasma membrane allow specific recognition of each particular cell type. DIAGRAM (2) the membrane, often called the cell membrane, forming the limiting surface of the cytoplasm of a prokaryotic cell.
plasmodesmata or plasmodesmas (sing plasmodesma or plasmodesm or plasmodesmus) /PLAZ-mə-DEZ-mə pl: -DEZ-məd-ə/ n. Channels, piercing the cell walls of plants, through which cytoplasmic threads connect with adjacent cells.
plastids /PLAST-id/ n. One of the self-replicating organelles present in the cytoplasm of plant cells; different types of plastids (e.g., chloroplasts, leucoplasts, chromoplasts) serve different functions.
plastron /PLAST-rən/ n. The underside of the shell of a turtle. Compare: carapace.
plate tectonics (also continental drift) /tek-TAWN-iks/ The movement over geologic time of large segments of the earth's crust relative to each other. Knowledge of plate tectonics is essential to understanding the present-day distributions of plants and animals. PLATE TECTONICS ANIMATION
platelets /PLATE-lets/ n. See thrombocytes.
platycnemia (also platycnemism) /plat-ik-NEEM-ee-yə/ n. (1) the condition of having an abnormally wide tibia (2) Thick-leggedness.
platycnemic /plat-ik-NEEM-mik/ adj. Affected by platycnemia.
platyrrhine /PLAD-ə-rine/ adj. Of, pertaining to, or being new-world monkeys.
pleiotropy /PLĪ-ō-TRŌP-ee/ n. A condition in which one gene causes a variety of different physical traits (e.g., multiple disease symptoms).
Pleistocene /PLĪST-ə-seen/ n. An epoch of the Quaternary Period of the Cenozoic Era. Often called the Ice Age, extensive continental glaciers existed through much of this epoch, which began at 2.588 mya and ended some 12,000 years ago. GEOLOGIC TIME SCALE
pleural /PLER-əl/ adj. Of, pertaining to, or within the pleurae.
pleural cavity /PLER-əl/ In human anatomy: one of the cavities enclosing each of the lungs.
PLP Pyridoxal phosphate.
ploidy /PLOY-dee/ n. The number of repetitions of the basic chromosome number occurring in a polyploid.
pluridyscrinia /PLUR-ə-dis-KRIN-ee-yə/ n. A disorder simultaneously affecting more than several different = endocrine glands type.
plurigravida /PLUR-ə-GRAV-id-ə/ n. A pregnant woman who has already been pregnant at least two times.
pluripotency /PLUR-ə-PŌT-en-see/ n. The capacity to differentiate into any cell or tissue type.
pneograph /NEE-ō-graf/ n. A machine for recording respiratory movements.
pododynia /pō-dō-DIN-ee-ya/ n. Pain in the feet.
podophthalmic /pō-dawf-THAWL-mik/ adj. Having the eyes on movable footstalks.
poikilothermy /poi-KEE-lə-THERM-ee/ n. Cold-bloodedness.
poison /POY-zən/ (also hazardous substance) n. Any substance that, when taken into the body, is injurious or lethal. Since any substance can be harmful when taken in sufficient quantity, the word poison refers to excessive dosage. However, the substances typically called poisons are injurious even in the case of relatively small quantities.
polar body During oogenesis, one of three small cells produced by each meiosis in addition to the much larger egg cell. A polar body contains little other than a haploid nucleus and serves no function in reproduction.
polar covalent bond A covalent bond between atoms differing in electronegativity.
pollen /PAW-lən/ n. A mass of pollen grains.
pollen grain In flowering plants, a granular microspore that germinates to produce the male gametophyte. Pollen grains are very durable and of great interest as fossils because the shape and size of the grains produced by each type of plant are so distinctive (see picture at right) that they can be used to determine whether any given plant was present during past geologic eras.
pollen mother cell /PAW-lən/ A plant cell (microsporocyte) that produces by meiosis four cells that each develop into a pollen grain. It is derived from the pollen sac.
pollen sacs /PAW-lən/ The pouches in the anther where pollen is formed.
pollen tubes /PAW-lən TOOBZ/ n. The long tubular cells that grow out of pollen grains and that carry the male gametes from the pollen grains to the ovule during fertilization. SEM OF POLLEN TUBES GROWING OUT OF LILY POLLEN GRAINS
pollination /PAWL-ə-NAY-shun/ n. Fertilization by means of pollen.
polygenic trait /PAWL-ee-JEN-ik/ A trait resulting from the combined action multiple genes. Their shared electrons are shifted away from the less electronegative atom, which makes it partially positive and the other atom partially negative.
polymer /PAWL-əm-er/ n. A molecule composed of a chain of repeating subunits.
polymerase /pə-LIM-ə-raze/ n. An enzyme that catalyzes the synthesis of nucleic acid polymer chains.
polymerase chain reaction (PCR) A method for amplifying a DNA base sequence using a heat-stable polymerase and two 20-base primers, one complementary to the (+) strand at one end of the sequence to be amplified and one complementary to the (-) strand at the other end. Because the newly synthesized DNA strands can subsequently serve as additional templates for the same primer sequences, successive rounds of primer annealing, strand elongation, and dissociation produce rapid and highly specific amplification of the desired sequence. PCR can be used to detect the presence of a given sequence in a DNA sample.
polymeric /PAWL-ləm-MARE-ik/ adj. Of, relating to, or being a polymer.
polymerous /pə-LIM-er-əs/ adj. Polymeric.
polymorphic /PAWL-ee-MORE-fik/ adj. Exhibiting polymorphism.
polymorphism /PAWL-ee-MORE-fiz-əm/ n. The existence of two or more distinct genetic variants within a population; usually refers to variation at a particular locus.
polymorphonuclear /PAWL-ee-MORE-fō-N(Y)OOK-lee-er/ adj. Having a nucleus with two or more separate parts.
polyp /PAWL-ip/ n. A wormlike body form common among cnidarians, particularly corals, in which the tube-like body has a mouth at one end, encircled by tentacles, and an anchoring "holdfast" at the opposite end (also an animal having such a body form). MORE INFORMATION
polyphagia /PAWL-ee-PHAGE-ee-ə, -PHAGE-yə/ n. Excessive appetite.
polyploidy /PAWL-ee-PLOID-ee/ n. A condition in which the individual cells of an organism have more than two complete sets of chromosomes. See also: haploid, diploid, triploidy, tetraploidy, hexaploidy, allopolyploid, and autopolyploid. MORE INFORMATION
polyploidization /PAWL-ee-PLOID-eyes-ZAY-shən/ n. Any genetic process producing a polyploid. MORE INFORMATION
polysaccharides (also polysaccharoses) /PAWL-ee-SACK-ə-rides/ n. A group name for those carbohydrates that can be broken down into three or more simple sugars; usually insoluble in water; when soluble they form colloidal solutions. Basic formula: (C₆H₁₂O₆)ₓ. Common examples are starch and cellulose.
polytene chromosome /PAWL-ee-teen/ A type of giant chromosome in which many identical chromatids lie alongside of one another. A pattern of bands is visible because of differential degrees of hypercoiling. Polyteen chromosomes are found in a rather limited variety of organisms, but have been extensively studied in the salivary glands of Drosophila. PICTURE OF POLYTENE CHROMOSOME
polytypic /PAWL-ee-TIP-ik/ adj. Composed of a variety of types, sorts, or forms.
polyuria /PAWL-ee-YER-ee-ə/ n. Excessive urination.
pongid /PAWN-jid/ n. Great ape (i.e., a chimpanzee, bonobo, gorilla, or orangutan); a member of the family Pongidae.
population genetics /jə-NET-iks/ The study of variation in genes in a population of individuals.
porometer /pə-RAWM-ə-der/ An instrument for measuring the diameter of stomata.
positional cloning A technique used to identify genes, usually those that are associated with diseases, based on their location on a chromosome.
positive interference A phenomenon where the occurrence of one crossover event lessens the likelihood of a second crossover event occurring in the same vicinity. Compare: negative interference.
postcranial /pōst-KRANE-ee-yəl/ adj. Pertaining to all parts of the skeleton other than the skull.
postpartum /pōst-PART-əm/ adj. After giving birth.
posterior /paw-STEER-ee-yur/ adj. (1) in human beings: toward the back: dorsal; (2) in animals: toward the rear or tail: caudal; (3) behind.
postoperative /pōst-AWP-er-ə-tiv/ adj. Taking place after surgery.
potassium (K) /pə-TAZ-ee-əm/ Chemical element found in all living tissues. Atomic number 19, atomic weight 39.0983. Potassium-argon dating, which measures radioactive decay of potassium into argon, is one of the most commonly used methods for determining the age of geologic strata.
Precambrian /pree-KAM-bree-ən/ (1) adj. prior to the Cambrian Period; (2) n. informal name for the time period from the formation of Earth to the first abundant appearance of macroscopic hard-shelled animals that defines the beginning of the Cambrian.
precipitate /prə-SIP-ə-tət/ n. A solid, derived from a solute or from a chemical reaction, that settles out in a liquid.
precipitation (also chemical precipitation) /prə-sip-ə-TAY-shən/ n. The production of a precipitate. Precipitation occurs in bodily processes as well as in the deposition of certain geological formations.
precocial /prə-KŌHSH-əl/ adj. Capable of significant independent action soon after hatching or birth. Precocial birds or mammals are those with newly hatched or born young being less dependent on their parents (i.e., ones that can move about, open their eyes, and that have down or hair). Compare: nidifugous, altricial.
predation /prə-DAY-shən/ n. The act of preying on animals.
predator /PRED-ə-ter/ n. An animal that preys on other animals.
prehensile /prə-HEN-səl/ adj. Suited for grasping.
premature chromosome condensation (PCC) A method of studying chromosomes in the interphase stage of the cell cycle.
prey (1) v. to capture an animal in order to eat it (the verb prey is used with the preposition on, as in "to prey on" or "preying on"); (2) n. an animal captured and eaten by some other animal.
pride A group of lions that lives, hunts and feeds together. A pride is generally composed mostly of lionesses and their offspring, as well as a few adult males. Lions are unusually social in comparison with other types of cats. MORE INFORMATION
primate /PRĪ-mate/ A member of the mammalian order Primates ("PRĪ-mə-teez") to which humans, apes, monkeys, and lemurs belong.
primatology /prī-mə-TAWL-ə-jee/ The study of primates — primatologist /prī-mə-TAWL-ə-jəst, -jist/
primordial abiogenesis One of the two major hypotheses concerning the origin of life on Earth. It proposes that life on this planet first arose via spontaneous generation from non-living matter. The opposing hypothesis known as exogenesis.
primordium The progenitor cells of an organ or tissue at its earliest stage of development.
probe In molecular biology, a single-stranded DNA or RNA molecule, or some other biochemical, labeled either radioactively or immunologically, used to detect the presence of a gene, a gene product, or a protein.
proboscidean /prō-bə-SID-ee-ən or -BOSS-əd-/ A member of the mammalian family Proboscidea (/prō-bə-SID-dee-ə or -BOSS-səd-/), which includes the elephants and extinct related forms. ABOUT ELEPHANT EVOLUTION | ELEPHANT DIET
proboscis /prō-BAW-siss, -BAWS-kiss/ (1) n. in invertebrates, a tubular appendage of the mouth employed in feeding; (2) n. in vertebrates, a tubular nasal appendage; (3) frontal proboscis: an anomalous tubular structure attached to the frontal region of the face.
proglottid (also proglottis) /prō-GLAWD-əd, prō-GLAWD-əs/ One of the segments of a tapeworm.
prognosis /prawg-NŌ-səs/ Prediction of how a disease will affect a patient.
prokaryotes /prə- prō-KARE-ee-əts, -yoats/ Simple organisms lacking a membrane-bound nucleus; commonly known as bacteria or "germs". In general, prokaryotes are unicellular, although certain of the more complex forms exhibit a certain degree of cellular differentiation paralleling the specialization seen in the cells of a multicellular organism. Prokaryotes have circular chromosomes and reproduce via binary fission — prokaryotic /prə- prō-KARE-ee-AWT-ick/ ABOUT PROKARYOTIC CELL STRUCTURE
pronucleus /prō-N(Y)OOK-lee-əs/ The nucleus of a sperm, egg, or pollen grain.
proprioception /PRŌ-pree-ə-SEP-shən/ n. Sense of the relative position of parts of the body.
prosector /prō-SEKT-ər/ n. An expert who dissects corpses for anatomical demonstration —prosectorial /PRŌ-sek-TORE-ee-əl/ adj.
prosthesis /PRAWS-thee-səs/ n. An artificial limb or organ.
prosthetic group /PRAWS-thet-ick/ n. A non-protein group, serving some function, and which is attached to a protein.
protein /PRŌ-teen/ A molecule composed of one or more polypeptide chains.
proteolytic /PRŌT-ee-ə-LIT-ik/ adj. Causing, or capable of causing, the cleavage of proteins into separate subunits.
proteome n. The proteins expressed by a cell or organ at a particular time and under specific conditions.
protist /PRŌT-ist/ A eukaryote that is either unicellular, or multicellular without specialized tissues; sometimes treated as a separate kingdom, Protista.
protistology /prō-tist-AWL-ə-jee/ The study of protists — protistologist /prō-tist-AWL-ə-jəst, -jist/
protoctist /prō-TOCK-tist/ (1) a protist; (2) a member of Kingdom Protoctista in the five-kingdom taxonomic system, which is no longer generally recognized.
PrP Prion protein.
PS (1) Photosystem; (2) Phosphatidylserine.
psittacine (also psittaceous) /SID-ə-sine, -kine/ adj. Of or pertaining to parrots.
psittacism /SID-ə-siz-um/ n. Speech in which words are uttered without thought of meaning.
psychology /sī-KAWL-ə-jee/ n. The study of human behavior.
PSTV abbr. Potato spindle tuber virus.
psychrophilous /SĪ-crow-FEEL-əs/ adj. Cold-loving, suited to cold conditions.
pteridophytes /tə-RID-ə-fights/ n. Vascular plants with leaves, stems, and roots, but lacking both seeds and flowers.
ptyalagogue /tī-AL-ə-goag/ n. A stimulant of salivation.
puberty /PYOOB-er-tee/ n. The time when an organism becomes capable of sexual reproduction.
pubescent /pyoo-BESS-ənt/ adj. (1) reaching the age of puberty; (2) having a downy surface covered with soft, fine hairs.
pudenda /pyoo-DEN-də/ n. The external genital organs of a human being, especially of a female.
pulicide /PYOOL-ə-side/ n. Flea killer.
pulmonary /PəL-mən-NARE-ee/ Of or relating to the lungs.
pupa /PYOO-pə/ (pl pupae /PYOO-pee/ ) n. An individual undergoing pupation — pupal /PYOO-pəl/
pupate /PYOO-pate/ v. To undergo pupation.
purgative /PURG-ə-tiv/ n. An agent that causes evacuation of the bowels.
purine /PYOOR-een/ n. One of the two types of nitrogenous bases occurring in nucleic acids. Purines are composed of fused five- and six-member heterocyclic rings of carbon and nitrogen atoms. Two different purines, adenine and guanine, occur in both DNA and RNA. MOLECULAR STRUCTURE OF ADENINE | MOLECULAR STRUCTURE OF GUANINE
purulent /PYOOR-ə-lent/ Producing or containing pus.
putrefaction /pyoo-trə-FAK-shən/ n. A condition of decay or rottenness; usually accompanied by an offensive odor.
putrescence /pyoo-TRESS-səns/ n. Rottenness.
putrid /PYOO-trid/ adj. Rotten.
pygal /PĪ-gəl/ adj. (1) of or pertaining to the buttocks; (2) concerning the lower back.
pyrimidine /pə-RIM-ə-deen/ n. One of the two types of nitrogenous bases occurring in nucleic acids. Pyrimidines are composed of a six-member heterocyclic ring of carbon and nitrogen atoms. The pyrimidines occurring in DNA are cytosine and thymine. In RNA they are cytosine and uracil. MOLECULAR STRUCTURE OF CYTOSINE | STRUCTURE OF THYMINE | MOLECULAR STRUCTURE OF URACIL
pyruvate /pī-ROO-vate/ n. The carboxylate (COOH) ion of pyruvic acid; pyruvate is the end-product of glycolysis.
q abbr. 1) quart; (2) coulomb; (3) symbol used to indicate the long arm of a chromosome.
Q10 abbr. Coenzyme Q₁₀.
QH2 abbr. Reduced coenzyme Q₁₀.
quadriplegia /KWAW-drə-PLEE-zhə/ n. Condition of being paralyzed in all four limbs.
quadriplegic /KWAW-drə-PLEE-jik/ adj. Being paralyzed in all four limbs.
quadrupedal /kwaw-DROOP-ə-dəl or KWAW-droo-ped-əl/ adj. Walking on four legs. Compare: bipedal.
quadrupedalism /KWAW-droo-PEED-ə-liz-əm/ n. A form of locomotion in which four legs are used. Compare: bipedalism.
quadruplet /kwaw-DROOP-lət, -let/ n. Any of four children produced by a single pregnancy.
quahog /KWAW-hawg/ n. The edible hard clam Mercenaria mercenaria. Native to the eastern shores of N. America (from the Yucatán Peninsula north to Prince Edward Island). PICTURE
quail /KWALE/ n. Mid-sized pheasants belonging to the genera Coturnix, Anurophasis, Perdicula, Ophrysia, Callipepla, Colinus, Oreortyx, and Cyrtonyx. PICTURE
Quaternary Period /KWAT-er-NAIR-ee/ n. The most recent of the three periods of the Cenozoic Era, lasting from 2.588 mya to the present. It includes two geologic epochs: the Pleistocene and the Holocene. GEOLOGIC TIME SCALE
quintipara /kwin-TIP-er-ə/ n. A woman who has birthed five babies.
quintuplet /kwin-TOO-plət/ n. One of five babies resulting from a single pregnancy.
quokka /KWAW-kə/ n. A short-tailed wallaby (Setonix brachyurus). PICTURE
quotidian /kwō-TID-ee-ən/ adj. Daily.
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m | n | o | p-q | r | s | t | u-z
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