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P

p53
Tumor suppressor gene
p arm
[of chromosome]
Packaging cell
for retroviral gene vectors
Paleozoic era
Pancreas
[hormones of] [digestive enzymes]
Pandemic
caused by antigenic shift of the influenza virus
Para-aminobenzoic acid (PABA)
Paracrine
Refers to the influence on a cell of chemical signals released by neighboring cells. Compare autocrine and endocrine.
Parasite
Organism living on or in another organism from which it derives its food and which it harms to some extent.
Parasympathetic nervous system
in humans
Parathyroid hormone (PTH)
[and calcium]
Parenchyma
Plant tissue consisting of thin-walled cells, often loosely-packed, that function in photosynthesis and/or food storage. [View]
Parthenogenesis
Development of an unfertilized egg into a new individual. Often occurs naturally in certain plants and animals (e.g., aphids).
Patch test
[for contact dermatitis]
Pathfinder
[Mission to Mars]
Pathogen
Disease-causing organism or virus.
PCR
polymerase chain reaction
Penicillin
Pentose
A sugar containing 5 carbon atoms. Deoxyribose and ribose are pentoses.
[structure]
Pepsin
Peptide bond
Covalent bond that links amino acids in a polypeptide. [More]
Peptidoglycan
Peripheral membrane protein
[More] [Schematic (48K)]
Peroxisomes
PERV
porcine endogenous retrovirus
Peyote
pH
Negative logarithm of the hydrogen ion concentration (in moles per liter) of a solution, which thus provides a measure of acidity and alkalinity. [More] [Effect on enzyme action]
Phagocytosis
Engulfing of solid particles by a cell.[More]
Phencyclidine (PCP)
Phenotype
Appearance of an organism, resulting from the interaction of its genotype and its environment.
Phenylalanine
[structure]
Phenylketonuria (PKU)
Pheromone
Substance secreted externally by an animal which influences the behavior of other members of the same species.
Phloem
Complex vascular tissue of plants that translocates food throughout the plant. [More]
Phosphate group
[Structure]
Phospholipid
[Discussion]
Photoperiodism
Developmental or behavioral response of an organism to the duration of daylight or darkness.
Photophosphorylation
[More]
Photosynthesis
[Calvin cycle][light reactions]
Photosystems I & II
[Discussion]
Phototropism
Phylogenetic tree
Diagram of the deduced evolutionary (genealogical) relationships (history) of a group of organisms. A dendrogram.
Phylogeny
Evolutionary history of a species.
Phylum
Major taxonomic category comprising one or more classes. In plant classification the term division is often used instead.
Physiology
Study of the processes occurring in living organisms.
Pigment
Substance that absorbs light, often selectively.[antenna]
Pineal gland
and melatonin
Pinocytosis
Pistil
PISTILLATA (PT)
Pituitary gland
hormones of
Placenta
[functions] [hormones] [as an allograft]
Plankton
Floating, generally microscopic, protistan and animal life in a body of water.
Plasma
Fluid matrix of the blood. [Discussion]
Plasma cell
Antibody-secreting cell. [More]
Plasma membrane
Plasmid
Small molecule of DNA that is replicated within the cell (usually of a bacterium) independent of the chromosome(s) of its host cell.[More]
Plasmodesmata
Plasmolysis
Shrinkage of the cytoplasm away from the wall of a plant cell, placed in a hypertonic medium, because of the loss of water by osmosis.
Platelets
Pleiotropy
The production by a single gene of more than one effect on the phenotype.
Plumule
Terminal bud of a plant embryo, usually consisting of embryonic leaves and the epicotyl. [Picture]
Poikilothermic
Having a body temperature that fluctuates with that of the surroundings. Commonly, "cold-blooded".
Polar body
The smaller of the cells produced by meiosis I and meiosis II during oogenesis.
Pollination
Polyadenylation
[in pre-mRNA processing]
Polymer
Compound whose molecule consists of many repeated units linked together.
Polymerase chain reaction (PCR)
Polymorphism
Occurrence of several distinct phenotypes in a population (e.g., queen, drone, and worker bees). Also, occurrence of allelic versions of a gene in at least 1% of a population.
Polyp
Abnormal growth of tissue that is still benign and polyclonal. Some polyps tend in time to develop a malignant clone of cells.
Polypeptide
Molecule consisting of amino acids linked together in a single chain. Proteins are made of one or more polypeptides. [ More] [polypeptide antibiotics]
Polyploidy
Having 3 or more complete (=haploid) sets of chromosomes.
Polysaccharide
Carbohydrate (e.g., starch, cellulose) that is made up of 3 or more monosaccharides linked together. [More]
Polysome
[in protein synthesis] [Picture]
Polytene
Used to describe the multistranded ("giant") interphase chromosomes found in certain metabolically active cells of insects.
Pongid
Group of primates to which the great apes (e.g., chimpanzee, gorilla, orangutan) belong.
Pons
Population
All the members of a single species present in a defined area.
Porphyria
Inheritance
Positron-emission tomography (PET)
Potential
[action potential] [resting potential]
PPB
Parts per billion. [More]
PPM
Parts per million.[More]
PPT
Parts per trillion. [More]
Prader-Willi syndrome
An inherited disorder of genomic imprinting. [More]
Precursor
Substance from which another substance is formed.
Precursor activation
The activation of the last enzyme in a metabolic pathway by the substrate of the first enzyme in that pathway. [Discussion]
Predation
Living by devouring other organisms.
Pregnancy
[in humans] [hormones]
Pre-messenger RNA (pre-mRNA)
[Discussion]
Primary host
Host normally used by a parasite during the adult stage of its life cycle.
Primary structure
[Protein]
Primary transcript
The original RNA molecule synthesized by transcription of a eukaryotic gene. Further processing of the primary transcript produces the final mRNA or tRNA or rRNA molecule.The primary transcript of mRNA molecules is also called a pre-mRNA molecule.[More]
Primitive
Similar to that occurring in the early evolutionary history of the structure or organism in question.
Producer
An organism that can synthesize organic molecules from inorganic ones (i.e., an autotroph). Producers start food chains.
Progesterone
A steroid hormone vital for the establishment and maintenance of pregnancy. [More]
Progestin
Any steroid that mimics the effects of progesterone.
Prokaryote
An organism, one of the bacteria, whose cell contains neither a membrane-bound nucleus nor other membrane-bound organelles such as mitochondria and plastids. Sometimes spelled procaryote.
Prolactin (PRL)
Proline
[structure][in collagen]
Promoter
Region of DNA, usually on the 5' side of a gene, that is needed for the initiation of the transcription of that gene. RNA polymerase and other transcription factors bind to the promoter.[part of response element] [bacterial promoters]
Pronucleus
The sperm nucleus and egg nucleus within the fertilized egg before their fusion to form the diploid zygote nucleus.[More]
Prophase
[mitosis][meiosis I]
Prostaglandin
Any of a number of 20-carbon organic acids that are synthesized in the body from unsaturated fatty acids and are responsible for a variety of metabolic activities. [role in birth] [role in allergies plus graphic]
Prosthetic group
The nonprotein part of a conjugated protein. Metal ions and a variety of organic molecules (e.g., vitamins, sugars, lipids) can serve as prosthetic groups. Prosthetic groups are usually bound covalently to their proteins. [More]
Protease
Enzyme that hydrolyzes peptide bonds whether in proteins or peptides.
Protein
Macromolecule composed of one or more polypeptides. Often associated with other molecules (e.g., carbohydrates and lipids) and ions. [More] [Synthesis] [kinesis]
Proteinase
Enzyme that hydrolyzes the peptide bonds of proteins.
Prothallus
Protist
Eukaryotes that are either unicellular or, if multicellular, lack the variety of specialized cells and tissues characteristic of plants and animals.
Proton
Positively charged particle found in the nuclei of all atoms. The hydrogen ion (H+) is a proton. [More]
Protonema
Proto-oncogene
A normal cellular gene that, when mutated or inappropriately expressed, can cause a cell to become cancerous. Designated c-onc. They normally participate in the control of the cell cycle. [More]
Protoplast
A cell (plant or bacterial) from which the cell wall has been removed.
Protozoan
A unicellular, heterotrophic, often motile, eukaryote. The amoeba is a familiar example.
Proximal
Situated near the place of origin or attachment.
Prozac
PSA
Prostate specific antigen
Pseudoautosomal region
A region on the X and Y chromosomes where pairing (synapsis) occurs and carrying homologous genes so that two copies of the gene are present in males as well as females. There are at least two pseudoautosomal regions in humans. Genes in them are inherited like any other autosome (accounting for the name); that is, males can inherit an allele originally present on the X chromosome of their father and females can inherit an allele originally present on the Y chromosome of their father. [More]
Psychedelic drugs
Pulmonary system, human
Pupa
Stage, usually dormant, between the larva and the adult of insets having complete metamorphosis.
Pupil
of human eye
Purine
Double-ring, nitrogen-containing base that is a component of nucleic acids and several other biologically active substances. Adenine (A) and guanine (G) are purines. [Structures] [analogs used as immunosuppressants]
Pyrimidine
Single-ring, nitrogen-containing base that is a component of nucleic acids. Thymine (T), cytosine (C), and uracil (U) are pyrimidines. [Structures]
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