Online Biology Dictionary P-Q

p (1) phosphate; (2) symbol used to indicate the short arm of a chromosome; (3) pico- (10^-12)

p- Para-.

P (1) phosphorus; (2) proline; (3) Permian.

p-aminobenzoic acid (PABA also 4-aminobenzoic acid or para-aminobenzoic acid). /uh-MEEN-oh-ben-ZOH-ik/ A white crystalline organic compound (C₇H₇NO₂). It composed of a benzene ring with an amino group and carboxylic group attached to the carbon atoms 1 and 4, respectively. In humans, gut flora synthesize folic acid from this molecule, a process that humans would not be able to carry out unaided.

P1-derived artificial chromosome (PAC) 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 bacteriophage P1 genome. See also: cloning vector.

PAGE Polyacrylamide gel electrophoresis.

palate /PAL-uht/ The roof of the mouth; the bony portion is the hard palate, the fleshy rear portion is the soft palate.

paleoanthropologist /pale-ee-oh-an-thruh-PAWL-uh-jist/ A scientist who studies the fossil remains, and the associate artifacts, of the ancestors of modern human beings.

Paleocene /PALE-ee-oh-seen/ An epoch of the Tertiary Period of the Cenozoic Era. It lasted from 65.5 ± 0.3 to 55.8 ± 0.2 mya. GEOLOGIC TIME SCALE

paleocontinent /PALE-ee-oh-CAWN-tuh-nent, -nuhnt/ A formerly existing continent, now broken up or fused with other continents due to the effects of rifting and continental drift.

Paleolithic /pale-ee-oh-LITH-ick/ The old stone age. The paleolithic is the first period of stone tool use. It extends 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-uh-jee/ The scientific study of the life of past geological periods. It deals primarily with the documentation of the the chronology of the history of life on earth.

paludicolous /pawl-yuh-DICK-uh-luhs/ Living in marshes.

Pan The genus to which the chimpanzee (Pan troglodytes) and the bonobo (Pan paniscus) belong. ABOUT CHIMPANZEE HABITAT | PICTURE OF A BONOBO

Pangea in the early Mesozoic Credit: Hakim Djendi

Pangea (also Pangaea) /pan-JEE-uh/ A single great supercontinent that existed from the Permian to Jurassic. It broke up to produce all of the modern continents. SEE ANIMATION OF PANGEA'S BREAKUP

panmictic population /pan-MICK-tick/ A population of organisms in which mating is random.

panmixia /pan-MIX-ee-yuh/ (also panmixis) See: random mating.

panspermia /pan-SPERM-ee-yuh/ See: exogenesis.

Panthera /pan-TARE-uh/ 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 | ABOUT THE DIET OF LIONS | GEOGRAPHIC DISTRIBUTION OF LIONS

papilla (pl. papillae) /puh-PILL-uh/ Any nipplelike process.

papillate /PAH-pill-ate or puh-PILL-ate/ Having papillae.

papillectomy /pah-pill-ECK-tuhm-ee/ Surgical removal of a papilla.

pappose /PAP-ohs/ Having a pappus.

pappus /PAP-uhs/ (1) in plants: a bristly structure; (2) in animals: a coating of fine, downy hair, especially the early growth of a beard.

papula /PAP-yuh-luh/ (1) a pimple; (2) a papule.

papular /PAP-yuh-luhr/ Pertaining to pimples.

papule /PAP-yool/ A raised, red bump on the skin; the term is applied to any bump up to about the size of a pea.

papuliferous /PAP-yuh-LIF-uhr-uhs/ Pimply, having papules.

para- /PAIR-uh/ Chemistry: in para-substitution, the substituents occupy positions 1 and 4. SEE EXPLANATORY DIAGRAM.

paracentric inversion /pare-uh-SENT-rick/ An inversion that does not include the centromere. Compare pericentric inversion.

parapatric /pare-uh-PAT-rick/ Of two populations: Occupying different, non-overlapping, but contiguous geographic regions.

parasite /PARE-uh-sight/ An organism that absorbs nutrients from a living organism. Generally, a parasite is smaller than its host.

parasitism /PARE-uh-sit-iz-uhm/ A form of symbiosis in which one of the two participants benefits at the expense of the other.

parietal bones /puh-RYE-uh-tuhl/ The two bones forming the roof and sides of the brain cavity.

partially fertile (also semi-fertile) With regard to a hybrid, capable of producing offspring, but producing fewer viable gametes than do the parents that crossed to produce the hybrid.

parthenogen /parth-EN-uh-jen/ A parthenogenetic organism.

parthenogenesis /parth-uh-nuh-JEN-uh-suhs, -noh-JEN/ Development of the parthenogenesisembryo from an unfertilized egg.

parthenogenetic /parth-uh-noh-juh-NET-ik/ Reproducing by means of parthenogenesis.

pascal (Pa) /PASS-call/ A unit of pressure, stress, and tensile strength in terms of force per unit area; equal to one newton per square meter.

passive immunity Immunity produced by injection of antibodies. Immunity of this sort is ephemeral, lasting only a few weeks or months. See: active immunity.

passive transport Diffusion of molecules across a biological membrane without the assistance of carrier proteins (this term is used in opposition to facilitated diffusion).

pathogen /PATH-uh-jen/ A disease-causing agent (as a microorganism).

pathogenic Disease-causing.

pathology /path-AWL-uh-jee, puh-THAWL-/ (1) the study of diseases; (2) changes caused by a disease.

PBS Primer binding site.

PC Plastocyanin.

PCC Premature chromosome condensation.

PCR Polymerase chain reaction.

PDGF Platelet-derived growth factor.

PE Phosphatidylethanolamine.

pediatrics /pee-dee-AT-ricks/ The study and medical treatment of the diseases of children.

pedipalp or pedipalpus. The second pair of appendages in an arachnid; in a scorpion, the claws; in a spider, they are like small legs.

peduncle. A stalk supporting an inflorescence.

pelagic. /puh-LAJ-ik/ Living in the open sea.

Pelomyxa 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.

Human Pelvis (male) - Frontal view

pelvic opening 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 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-uh-truhns/ 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.

PEP Phosphoenolpyruvic acid.

PEPC Phosphoenolpyruvate carboxylase.

PEPCK Phosphoenolpyruvate carboxykinase.

pepsin A gastric enzyme, which is proteolytic in acidic solutions.

peptide Two or more amino acids joined in a chain.

peptide bond A covalent bond between two amino acids; forms when the amino group of one amino acid bonds to the carboxyl group of another and water is eliminated. DIAGRAM

pericardial /PEAR-uh-KARD-ee-uhl/ Of or pertaining to the pericardium.

pericardium /PEAR-uh-KARD-ee-uhm/ The sac enclosing the heart and the adjacent portions of the great blood vessels.

1. Endocarp, 2. Seed 3. Mesocarp 4. Epicarp. 1, 3, and 4 together make up the pericarp.

pericarp /PEAR-uh-KARP/ The outer wall of a plant ovary (see figure at right).

pericentric inversion /PEAR-uh-SENT-rick/ An inversion that includes the centromere. Compare paracentric inversion.

pericycle /PEAR-uh-sigh-kuhl/ The layer of potentially meristematic cells immediately beneath a root's endodermis.

periderm /PEAR-uh-derm/ In plants, the permanent outer layer that replaces the epidermis during secondary growth.

periosteum /PAIR-uh-OST-ee-uhm/ The fibrous coating investing all surfaces of a bone other than the articular surfaces.

peritoneal cavity /PAIR-uh-TONE-ee-uhl/ The body cavity, defined by the peritoneum, and containing all of the abdominal organs except the kidneys.

peritoneum /PAIR-uh-TONE-ee-uhm/ In human anatomy: the serous membrane lining the peritoneal cavity.

Permian Period (P) /PERM-ee-uhn/ 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 such groups as mammals, turtles, lepidosaurs and archosaurs.

peroxisome /puh-ROCKS-uh-soam/ 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.

PFGE Pulsed-field gel electrophoresis.

PFK Phosphofructokinase.

PG Prostaglandin.

PGI Phosphoglucose isomerase

PGK Phosphoglycerate kinase.

PGM Phosphoglycerate mutase.

pH See: hydrogen ion concentration.

phage Short for bacteriophage.

phagocytosis Intracellular digestion of macromolecules by a lysosome.

pharynx The passage connecting the mouth and nasal cavity with the esophagus.

Phanerozoic (Brit. Phanaerozoic) Eon /FAN-er-ro-ZOH-ik/ 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 Phenylalanine.

phenetics 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.

phenocopy /FEEN-oh-CAWP-ee/ A trait, that seems the same as a genetic trait, but that is caused by environmental influences.

phenotype /FEEN-oh-type/ The observable characteristics of an organism. See also: genotype.

phenylalanine (phe or F) /FEEN-uhl-AL-uh-neen/ One of the essential amino acids.CODONS | MOLECULAR STRUCTURE | PHENYLALANINE METABOLISM

phenylketonuria (PKU) /feen-uhl-kee-tone-ER-ee-yuh/ A condition characterized by the presence of phenylpyruvic acid in the urine.

phoronids (or phoronid worms) 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.

Phosphate Ion

phosphate /FOSS-fate/ An ester of phosphoric acid. The phosphate ion has one central phosphorus atom surrounded by four oxygen atoms in a tetrahedral arrangement.

phosphoenolpyruvate carboxylase (PEPC) An enzyme catalyzing the addition of CO₂ to phosphoenolpyruvate to form the four-carbon compound oxaloacetate.

phosphoenolpyruvate carboxykinase (PEPCK) One of the enzymes working in gluconeogenesis.

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-foe-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-foe-GLIS-er-ate KIGH-nase/ A transferase enzyme used in the seventh step of glycolysis.

phosphoglycerate mutase (PGM) /FOSS-foe-GLIS-er-ate MYOOT-ase/ An enzyme catalyzing the eighth step of glycolysis.

phospholipid /FOSS-foe-LIP-uhd/ A lipid that contains phospate 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.

phosphorus (P) /FOSS-fer-uhs/ 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-uhl- LAY-shun/ 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 /foe-toe-SIN-thuh-sis/ 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. DIAGRAM

phototaxis (pl phototaxes or phototaxies) also phototaxy /foe-toe-TAX-uhs, pl: -TAX-eez/ A tendency on the part of a motile organism to move toward or away from light.

phototropism /foe-toe-TRAWP-iz-uhm, -TROPE-iz-uhm/ A tropism in response to light.

phylogeny /fuh-LAWJ-uh-nee/ A hypothetical reconstruction, usually in the form of a tree, of the evolutionary relationships of a group of organisms.

phylum (pl phyla) /FIGH-luhm, FIGH-luh/ In taxonomy, a division of a kingdom that itself contains one or more classes. CHART OF RELATIVE TAXONOMIC RANKS

physometra /figh-soh-MEE-truh/ Distention of the the uterus by gas.

phytohormone /FIGHT-toe-HORE-moan/ A plant hormone.

phytopathology /FIGHT-toe-puh-THAWL-uh-jee/ The study of plant diseases.

phytophagous /fight-TAWF-uh-guhs/ Plant-eating.

phytoplankton /FIE-toe-PLANK-tuhn/ Plankton composed of plant life.

Pi Inorganic phosphate (also: orthophosphate ion).

PI (1) phosphatidylinositol; (2) principal investigator.

picogram /PIGH-ko-gram, PEE-/ One-trillionth (10^-12) of a gram.

picrotoxin (also cocculin) /pick-roh-TAWK-suhn/ A bitter, crystalline toxin found in Indian berry (Anamirta cocculus). Picrotoxin is a non-competitive antagonist for the GABA_A receptor chloride channels. Because GABA is the major inhibitory neurotransmitter in the central nervous system, picrotoxin's interference with it has a stimulant effect.

PIP2 Phosphatidylinositol-4,5-bisphosphate.

Piroplasma /PEER-uh-PLAZ-muh/ Former name for Babesia.

pistil /PISS-tuhl/ The ovule-bearing organ of a seed plant.

Pithecanthropus /pith-thuh-KAN-thuhp-uhs, pith-thuh-kan-THROPE-uhs/ The genus to which the hominid Homo erectus was formerly assigned.

PK Pyruvate kinase.

PKU Phenylketonuria.

placenta /pluh-SEN-tuh/ An oval, vascular structure in the uterus that nourishes the fetus.

placental mammal /pluh-SEN-tuhl/ A eutherian mammal.

plankton /PLANK-tuhn/ Small sea organisms moved primarily, or exclusively, by water currents.

plasmids /PLAZ-midz/ 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 cells /PLAZ-muh/ Antibody-secreting cells derived by differentiation from B cells.

Plasma membrane (click to enlarge)

plasma membrane
/PLAZ-muh/ 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. A form of plasma membrane is also found in prokaryotes, but there it is often instead called the cell membrane.

plasmodesmata or plasmodesmas (sing plasmodesma or plasmodesm or plasmodesmus) /PLAZ-muh-DEZ-muh pl: DEZ-muhd-uh/ Channels through the cell walls of plants through which cytoplasmic threads connect with adjacent cells.

Plasmodium /plaz-MODE-ee-uhm/ A genus of protozoan blood parasites transmitted by mosquitoes; causative agents of the various forms of malaria. LIFE CYCLE

plastids /PLAST-ids/ Plant organelles serving various functions; e.g., chloroplasts, leucoplasts, chromoplasts.

plastocyanin /PLAST-oh-SIGH-uh-nin -nuhn/ An important copper-containing protein involved in electron-transfer during photosynthesis.

plasmolysis (plaz-MAWL-uh-suhs) Separation of the plasma membrane from the cell wall resulting from shrinkage of the cytoplasm in a hypertonic environment.

platycnemia (also platycnemism) /plat-ick-NEEM-ee-yuh/ (1) the condition of having an abnormally wide tibia (2) Thick-leggedness.

platycnemic /plat-ick-NEEM-mick/ Affected by platycnemia.

Platyhelminthes (also Platyelminthes) /plad-ee-hel-min(t)-theez/ The phylum that includes the tapeworms and flatworms.

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

pleiotropy /PLY-oh-TROPE-ee/ A condition in which one gene causes a variety of different physical traits (e.g., multiple disease symptoms).

Pleistocene /PLEIST-uh-seen/ An epoch of the Quaternary Period of the Cenozoic Era. It began at 2.588 mya and ended 12,000 years ago. GEOLOGIC TIME SCALE

pleura (pl pleurae) /PLER-uh; pl: PLER-ee or PLER-eye/ One of the serous membranes that enclose the lungs and line the thorax.

pleural /PLER-uhl/ Of, pertaining to, or within the pleurae.

pleural cavity /PLER-uhl/ In human anatomy: one of the cavities enclosing each of the lungs.

Pliocene /PLY-oh-seen/ An epoch of the Tertiary Period of the Cenozoic Era. It lasted from 5.332 to 2.588 mya. GEOLOGIC TIME SCALE

PLP Pyridoxal phosphate.

ploidy /PLOY-dee/ The number of repetitions of the basic chromosome number occurring in a polyploid.

pluripotency /PLER-uh-POTE-en-see/ The capacity to differentiate into any cell or tissue type.

pm Micrometer.

pneograph /NEE-oh-graf/ A machine for recording respiratory movements.

p/o ratio (also p:o ratio) The ratio of moles ATP formed from ADP to gram atoms of oxygen consumed (a measure of mitochondrial oxidative phosphorylation).

pododynia /poh-doh-DIN-ee-ya/ Pain in the feet.

poikilothermy /poi-KEE-luh-THERM-ee/ Cold-bloodedness.

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-luhn/ A mass of pollen grains produced by a seed plant. PICTURE OF BEE COVERED WITH POLLEN

Different types of plants produce pollen grains of different shapes and sizes (click to enlarge)

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 above) that they can be used to determine whether any given plant was present during past geologic eras.

pollen mother cell /PAW-luhn/ 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-luhn/ The pouches in the anther where pollen is formed.

pollinium (pl pollinia) /paw-LIN-ee-uhm/ A clump of pollen grains produced by a single anther and transferred as a unit during pollination.

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-uhm-er/ A molecule composed of a chain of repeating subunits.

polymerase /puh-LIM-uh-raze/ 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.

polymorphic /PAWL-ee-MORE-fik/ Exhibiting polymorphism.

polymorphism /PAWL-ee-MORE-fiz-uhm/ The existence of two or more distinct genetic variants within a population; usually refers to variation at a particular locus.

polymorphonuclear /PAWL-ee-MORE-foe-N(Y)OOK-lee-er/ Having a nucleus with two or more separate parts.

polypeptide /PAWL-ee-PEP-tide/ A chain of amino acids covalently linked by peptide bonds.

polyphagia /PAWL-ee-PHAGE-ee-uh, -PHAGE-yuh/ Excessive appetite.

polyploidy /PAWL-ee-PLOID-ee/ 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-shun/ Any genetic process producing a polyploid. MORE INFORMATION

polysaccharides (also polysaccharoses) /PAWL-ee-SACK-uh-rides/ 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.

A polytene chromosome from a fly salivary gland (click to enlarge)

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.

polytypic /PAWL-ee-TIP-ik/ Composed of a variety of types, sorts, or forms.

polyuria /PAWL-ee-YER-ee-uh/ Excessive urination.

population genetics /juh-NET-iks/ The study of variation in genes in a population of individuals.

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 Pertaining to all parts of the skeleton other than the skull.

potassium (K) /puh-TAZ-ee-uhm/ 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-uhn/ (1) adjective: prior to the Cambrian Period, which began 543 million years ago; (2) noun: 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.

precocial (pruh-KOHSH-uhl) 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 /pruh-DAY-shun/ The act of preying upon animals.

prehensile /pruh-HEN-suhl/ Suited for grasping.

premature chromosome condensation (PCC) A method of studying chromosomes in the interphase stage of the cell cycle.

premolar /pre-MOLE-er/ One of the teeth between the molars and the canines.

primary structure The structure of a protein expressed as a simple sequence of amino acids without regard to its three-dimensional shape.

primer A short nucleotide sequence used by a DNA or RNA polymerase to initiate transcription.

primer binding site (PBS) The site on a retrovirus or retrotransposon where the tRNA primer binds to initiate reverse transcription. DIAGRAM OF REVERSE TRANSCRIPTION

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.

pro Proline.

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 /pro-buh-SID-ee-uhn or -BOSS-uhd-/ A member of the mammalian family Proboscidea (/pro-buh-SID-dee-uh or -BOSS-suhd-/), which includes the elephants and extinct related forms. ABOUT ELEPHANT EVOLUTION ABOUT THE DIET OF ELEPHANTS

product /PRAW-duhkt/ (1) the molecule or substance produced by a chemical reaction; (2) the protein or RNA produced by a gene.

proglottid (also proglottis) /pro-GLAWD-uhd, pro-GLAWD-uhs/ One of the segments of a tapeworm. SEE PICTURE

proline (pro or P) /PRO-leen/ One of the nonessential amino acids. CODONS | MOLECULAR STRUCTURE

promoter /pruh-MOTE-er/ A DNA site where RNA polymerase binds and initiates transcription.

pronucleus /pro-N(Y)OOK-lee-uhs/ The nucleus of a sperm, egg, or pollen grain.

protein /PRO-teen/ A molecule composed of one or more polypeptide chains.

proteome Proteins expressed by a cell or organ at a particular time and under specific conditions.

proteolytic /PROTE-ee-uh-LIT-ik/ Causing, or capable of causing, the cleavage of proteins into separate subunits.

proteomics /PROTE-ee-OME-iks/ The study of the full set of proteins encoded by a genome.

protist /PROTE-ist/ A eukaryote that is either unicellular, or multicellular without specialized tissues; sometimes treated as a separate kingdom, Protista.

prothallium /pro-THAL-ee-uhm/ A pteridophyte in its gametophyte stage (the term is sometimes applied to bryophytes).

protoctist /pro-TOCK-tist/ (1) a protist; (2) a member of Kingdom Protoctista in the five-kingdom taxonomic system, which is no longer generally recognized.

protozoans /PRO-toe-ZOE-uhns/ Motile, animal-like protists.

PrP Prion protein.

PS (1) Photosystem; (2) Phosphatidylserine.

pseudogene /SOO-duh-JEEN/ A sequence of genomic DNA similar to a gene at some other locus but rendered nonfunctional by mutations that prevent its normal transcription and/or translation.

PSTV Potato spindle tuber virus.

psychrophilous /SIGH-crow-FEEL-uhs/ Cold-loving, suited to cold conditions.

pteridophytes /tuh-RID-uh-fights/ Vascular plants with leaves, stems, and roots, but lacking both seeds and flowers.

puberty /PYOOB-er-tee/ The time when an organism becomes capable of sexual reproduction.

ptyalagogue /tie-AL-uh-goag/ A stimulant of salivation.

pulicide /PYOOL-uh-side/ Flea killer.

pulmonary /PUHL-muhn-NARE-ee/ Of or relating to the lungs.

pulsed-field gel electrophoresis (PFGE) /uh-lek-troh-fur-REE-suhs/ A special form of electrophoresis allowing the separation of very large DNA molecules.

purine /PYOOR-een/ 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

pygal /PIE-guhl/ (1) of or pertaining to the buttocks; (2) concerning the lower back.

pyloric valve /pie-LORE-ik, puh-/ The valve controlling the pylorus. It is normally closed but opens with building gastric pressure. PICTURE

pylorus /PIE-lore-uhs/ The exit orifice connecting the stomach with the duodenum. PICTURE

pyrimidine /puh-RIM-uh-deen/ 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 | MOLECULAR STRUCTURE OF THYMINE | MOLECULAR STRUCTURE OF URACIL

pyruvate /pie-ROOV-ate/ The carboxylate (COOH) ion of pyruvic acid; pyruvate is the end-product of glycolysis.

pyruvate kinase (PK) /pie-ROOV-ate KIGH-naze/ An enzyme catalyzing transfer of a phosphate group from PEP to ADP during glycolysis. The result is one molecule of ATP and one molecule of pyruvate.

q (1) quart; (2) coulomb; (3) symbol used to indicate the long arm of a chromosome.

Q (1) coenzyme Q₁₀; (2) glutamine.

Q10 Coenzyme Q₁₀.

QH2 Reduced coenzyme Q₁₀.

quaternary structure /QWAT-er-NAIR-ee or qwa-TURN-er-ee/ Refers to the ways that the separate polypeptide chains of a complex protein interact.

quintipara /kwin-tuh-PAR-uh/ A woman who has had five babies, either in succession or as the result of a single pregnancy.

quintuplet /kwin-TOOP-luht/ One of five babies resulting from a single pregnancy.

quotidian /kwoh-TID-ee-uhn/ Daily.