Biology Dictionary – S

Meanings of biological terminology

EUGENE M. MCCARTHY, PHD GENETICS

Definitions:

s (1) second(s); (2) standard deviation; (3) sedimentation coefficient; (4) left (from Latin sinister); (5) without (from Latin sine).

S (1) Silurian Period; (2) sulfur; (3) serine; (4) S phase; (5) Svedberg unit.

S Sedimentation coefficient.

³⁵S Radioisotope of sulfur often used experimentally in radioactively labeling proteins.

sac (1) a bubblelike, pouch-like or dilated structure within a plant or animal; (2) a diverticulum.

saccharides /SAK-ə-rides/ n. Another name for carbohydrates.

saccharose /SAK-ə-rōs, -rōz/ n. An alternative name for sucrose.

saccharuria /sak-ə-RUR-ee-yə/ n. Sugar in the urine.

saccule /SACK-yool/ (1) a small sac; (2) the lesser of two sacs within the vestibule of the inner ear.

sacrad /SAY-krad/ adj. Toward the sacrum.

sacral /SAY-krəl/ adj. Pertaining to the sacrum.

sacroiliac /SAY-krow-ILL-ee-ak/ adj. Pertaining to the sacrum and the ilium.

sacrolumbar /SAY-krow-LƏM-bar/ adj. Pertaining to the sacrum and the loins (or lumbar vertebrae).

sacrum /SAY-krəm/ n. In vertebrates with pelves, the fused portion of the vertebral column to which the pelvis is attached. PICTURE

sagittal /SAJ-ət-əl/ adj. (1) of or relating to a plane extending from front to back through the body's axis of symmetry, or any plane parallel to that plane; (2) of or relating to the suture between the parietal bones, which runs from front to back down the middle of the top of the skull; (3) arrow-shaped.

sagittal crest /SAJ-ət-əl/ n. A ridge of bone running from front to back along the center line of the top of a skull; found in apes and some robust australopithecines. PICTURE OF SAGITTAL CREST ATOP A GORILLA SKULL

sahel /sə-HILL or SAH-hill/ n. The region between the Sahara desert and savannas to the south; characterized by extended dry seasons, which alternate with relatively brief rainy seasons. PICTURE

salinity /sə-LIN-ə-tee, sal-/ n. The salt concentration of a solution, particularly of a body of water.

saliva (common names: spit, spittle) /sə-LIE-və/ n. The liquid present in the mouth and secreted by the salivary glands. Saliva contains enzymes that begin the digestive process.

salpinx (pl salpinges) /SAL-pinks, sal-PIN-jeez/ n. (1) fallopian tube; (2) eustachian tube.

salt Sodium chloride (NaCl); more broadly, any chemical compound composed of a positive ion other than hydrogen, and a negative ion, other than the hydroxyl group.

saltation (also saltatory evolution) /sawl-TAY-shən/ n. The production of new types of organisms via rapid, discontinuous processes; used in opposition to the term gradualism. MORE INFORMATION.

saltationist /sawl-TAY-shən-əst/ n. A biologist who believes evolution is a saltatory process. See saltation.

SAM S-adenosylmethionine.

sanguiferous /sang-GWIF-er-əs/ adj. Carrying or conducting blood.

saprophyte (also saprophite) /SAP-rə-fite/ n. A plant, fungus, or microorganisms that feeds on the dead or decaying remains of once living matter; adj: saprophitic /sap-rə-FIT-ik/. Saprophites are often called "decomposers."

sarcoma /sar-KŌM-ə/ n. Cancer of muscle or connective tissue. Specific types of sarcomas are named for the types of tissue from which they arise (e.g., angiosarcoma, chondrosarcoma, hemangiosarcoma, leiomyosarcoma, liposarcoma, lymphangiosarcoma, osteosarcoma, rhabdomyosarcoma).

sarcomere /SARK-ə-meer/ n. The sarcomere is the fundamental unit of muscle structure. Its capacity for contraction is the essential trait that makes muscles work. MORE INFORMATION

satellite DNA /SAT-ə-lite/ n. Any fraction of DNA that forms a separate band from the main body of DNA during isopycnic CsCL gradient centrifugation ("satellite" refers to the subordinate or minor status of such bands). The DNA in such bands is either denser (GC-rich) or less dense (AT-rich) than the DNA in the main band. Generally, satellite DNA appears as separate bands because each band contains many copies of a specific highly repetitive sequence with a specific density due to its particular GC-to-AT ratio.

saurian /SAW-ree-ən/ (1) adj. lizardlike; (2) n. a lizardlike reptile.

saurophagus /saw-RAWF-ə-gəs/ adj. Feeding on lizards.

sauropod /SAW-rə-pod/ n. A member of the dinosaur infraorder Sauropoda.

savanna (or savannah) /sə-VAN-nə/ n. Open tropical or semitropical grassland, usually with a scattering of small trees and bushes; a biome found in regions where heavy rain seasons alternate with lengthy dry seasons.

Sb Chemical symbol for the element antimony.

scaffold /SKAF-əld/ n. In genomic mapping, a series of contigs, in the correct order but not necessarily assembled into a single continuous sequence.

Scala Naturae (also Great Chain of Being) /SKAL-ə nat-T(Y)OOR-eye/ n. A medieval system that ordered the various types of organisms existing in nature on a linear scale of perfection. The scala continues to have significant influence on various aspects of modern biological thought. For example, biologists still speak of "higher" and "lower" organisms, and think of birds as more "complex" than fish. MORE INFORMATION

scapula /SKAP-yə-lə/ n. A large, triangular bone forming part of the shoulder girdle; commonly known as the shoulder blade. PICTURES

Scholastics (also schoolmen) Christian medieval philosophers. Certain aspects of their worldview, based on religious dogmas, have carried over into modern biological thought, for example, their ideas concerning continuity, gradualism, and ideal forms.

sclera /SKLIR-ə/ n. The outermost coat of the eyeball, extending from the optic nerve to the edges of the cornea. MORE INFORMATION

scleriasis /sklir-EYE-ə-səs/ n. See: scleroderma.

sclerocornea /SKLIR-ō-CORN-ee-yə/ n. The cornea and sclera, taken together.

scleroderma (also scleriasis) /SKLIR-ō-DERM-ə/ n. A disorder involving a progressive thickening and hardening of the skin. Calcium deposits on the skin are also seen in association with this disease.

scleronyxis (pl scleronyxes) /SKLIR-ō-NIX-əs; pl: -NIX-seez/ n. Surgical puncture of the sclera.

sclerosis /sklir-RŌ-sis/ n. A pathological hardening of tissue — sclerotic /sklir-RAW-dick/ adj.

scolecoid /skō-LEE-koid, SKŌ-lə-/ adj. Wormlike.

scolex /SKŌ-leks/ (pl scolices /skō-LEE-seez/ or scoleces /SKŌ-lə-seez/ or scolexes /SKŌ-lex-ehz/) n. The head of an adult tapeworm (see picture). A tapeworm attaches itself to the wall of the small intestine with its scolex.

scrotum /SKRŌ-təm/ n. The external pouch containing the testes.

scutal /SKYOO-təl/ adj. Of or pertaining to scuta.

scute /skyoot/ n. A scutum.

scutellum /skyoo-TELL-əm/ n. (pl scuta /skyoo-TELL-ə/) n. A small scutum.

scutiferous /skyoo-TIF-er-əs/ adj. Bearing scutes.

scutiform /SKYOO-tə-form/ adj. Shield-shaped.

scutum /SKYOO-təm/ (pl scuta /SKYOO-tə/) n. (1) a hard, external, shieldlike plate; in vertebrates scuta may be composed of horn or bone; in invertebrates they are generally chitinous; (2) the largest of the four parts covering the upper surface of the thorax of an insect.

SDS Sodium dodecyl sulfate.

second messenger A molecule that relays a message — carried by a hormone from elsewhere in the body to the surface of a cell — to some point within the cell. See: signal transduction

secrete /sə-KREET/ v. To produce a secretion.

secretion /sə-KREE-shən/ n. (1) a substance produced and emitted by a gland; (2) the action wherein a gland produces and emits a substance or substances; secretion can be of two types, external and internal. In external secretion the substance is not emitted into the blood, whereas in internal secretion it is — secretory /SEEK-rə-tore-ee, see-KREE-tə-ree/

sediment /SED-ə-mənt/ n. Any solid material that settles out of a suspending liquid or a gas.

sedimentary /sed-ə-MENT-er-ee, British: sed-ə-MEN-tree/ adj. Pertaining to or produced from sediment.

sedimentary rock /sed-ə-MENT-er-ee, British: sed-ə-MEN-tree/ n. Rock produced by the consolidation of sediment.

sedimentation /SED-ə-mənt/ n. Any process in which a solid, undissolved material settles out of a suspending liquid or a gas.

sedimentation coefficient (S) /sed-ə-men-TAY-shən co-ə-FISH-ənt/ n. A value indicating the rate at which a particular type of molecule moves through a solution during centrifugation as it settles toward its equilibrium position in the centrifugation gradient.

seed n. An encapsulated plant embryo in an arrested state of development and, usually, surrounded by endosperm.

seed plant n. A plant that produces seeds.

segregation /seg-rə-GAY-shən/ n. The normal biological process whereby the chromatids of each chromosome pair are separated during meiosis and randomly distributed to the germ cells.

self To self-fertilize or self-pollinate.

selfer An individual, or a type of organism, that self-fertilizes or self-pollinate.

Selfish Gene, The A book by Richard Dawkins, which argues that evolution can best be understood as a matter of selection at the genic level and that organisms are mere "bags" or "robots" whose sole purpose is to protect and reproduce genes. This is the opposite view from that taken in the theoretical portion of this website, which argues that evolution is typically a matter of selection among distinct, stable types of organisms. MORE INFORMATION.

self-fertilization /fert-əl-eye-ZAY-shən/ n. Union of male and female gametes from the same individual.

self-pollination /pawl-ə-NAY-shə/ n. Pollinating a stigma with pollen from the same plant.

semelparity /SEM-əl-PAIR-ə-tee/ n. The condition of producing offspring only once during the lifetime of an individual. Compare: iteroparity.

semen /SEE-mən/ n. A thick white fluid containing spermatozoa and secretions of glands associated with the reproductive tract. Semen is produced by males and injected into the female during copulation, an necessary prerequisite of fertilization.

semicircular canals /sem-ee-SIR-kyə-lər/ n. Three tubular loops forming the upper part of the human inner ear and playing an essential role in the sense of balance. PICTURE

seminiferous /sem-in-NIF-ər-əs/ adj. Sperm-bearing.

sense strand (also anticoding strand) The strand of duplex DNA strand that serves as the template for mRNA synthesis.

sepsis /SEP-sis/ n. Presence of pathogenic microorganisms or their toxic products in the bloodstream.

septum /SEP-təm/ n. A wall separating two cells, cavities or bodily regions (e.g., the nasal septum).

sequence See: base sequence.

sequence assembly A process whereby the order of multiple sequenced DNA fragments is determined.

sequence tagged site (STS) A short (200-500 base-pair) DNA fragment, with a known location and sequence, that occurs only once in a genome. STSs are useful for correlating mapping and sequence data reported from different laboratories since they are unique and detectable by polymerase chain reaction. Expressed sequence tags (ESTs) are STSs derived from cDNAs.

sequencing Determination of the order of nucleotides (base sequences) in a DNA or RNA molecule or the order of amino acids in a protein.

serine (ser or S) /SAIR-een/ n. One of the nonessential amino acids. CODONS | MOLECULAR STRUCTURE | SERINE BIOSYNTHESIS

serous fluid /SIR-əs/ n. A fluid, resembling blood serum, secreted into the cavities of the body by the serous membranes, which line them.

serous membranes /SIR-əs/ n. Thin, lining membrane that secrete a serous fluid. Specifically: the three membranes lining the pleural, pericardial, and peritoneal cavities.

Serpentes /sir-PEN-teez/ n. The suborder of Squamata comprised of the snakes. READ ABOUT THE DIET OF SNAKES

sessile /SESS-əl/ adj. Permanently attached; non-mobile.

setaceous /sə-TAY-shəs/ adj. Having bristles, bristly.

setose /SEE-tose/ adj. Having bristles, bristly.

setous /SEE-təs/ adj. Having bristles, bristly.

sex chromosomes Chromosomes that determine the sex of an individual. There are two sex chromosome systems (1) the XX /XY system, where XY individuals are male, and XX individuals are female (the usual system, for example, in mammals and butterflies); and (2) the WW/WZ system; where WW individuals are male, and WZ individuals are female (the normal system in birds, for example). In humans the sex chromosomes comprise the 23rd chromosome pair. See also: autosome.

sex factor See: F factor.

sex linkage A linkage involving a locus on the X or Z chromosome. In certain crosses sex-linked traits manifest themselves only in the heterogametic sex.

sexual reproduction Reproduction that involves the fusion of gametes.

sialagogue /sigh-AL-ə-gawg/ n. A stimulant of salivation.

Siberia /sigh-BEER-ee-yə/ n. A region of modern Russia that formerly existed as a separate continent of the same name. The paleocontinent Siberia came into existence as a separate entity in the Cambrian. It fused with another, smaller paleocontinent, Kazakhstania, in the Carboniferous. This composite continent later joined with Baltica to form Pangea in the Permian.

side chain n. A chemical group, often with a chainlike structure, attached to a main chain or ring. See: amino acid.

siderocytes /SID-er-ə-sights/ n. Abnormal erythrocytes containing iron granules in the mitochondria.

sigmoid colon /SIG-moid/ n. An S-shaped portion of the large intestine, about. It connects the descending colon with the rectum. Also known as the sigmoid flexure of the descending colon.

signal transduction /SIG-nəl trans-DUCK-shən/ n. Any process in which a cell changes one type of stimulus into another. Most signal transduction processes are ordered sequences of intracellular reactions ("signal transduction pathways") mediated by enzymes and activated by second messengers. OVERVIEW OF SIGNAL TRANSDUCTION PATHWAYS

Silurian Period (S) /sə-LURE-ee-ən/ n. A geologic period of the Paleozoic Era lasting from 443.7 ± 1.5 to 416.0 ± 2.8 mya. During the Silurian, the earliest known vascular plants appeared on land. The first coral reefs formed in the oceans, and fish with movable jaws made their appearance and eurypterids were abundant. PICTURE

simian /SIM-ee-ən/ adj. Apelike.

Sinanthropus pekinensis A name formerly assigned to remains of early hominids found near Beijing (then Peking), which are now assigned to Homo erectus.

single-gene disorder Hereditary disorder caused by a mutant allele of a single gene (e.g., Duchenne muscular dystrophy, retinoblastoma, sickle cell disease).

single nucleotide polymorphism (SNP) /pawl-ee-MORE-fiz-əm/ n. DNA sequence variations that occur when a single nucleotide (A, T, C, or G) in the genome sequence is altered. See also: mutation, polymorphism, single-gene disorder.

single variations An obsolete term much in use in evolutionary discussion during the 19th century, for example in Darwin's writings. It was used to refer to saltatory changes producing new forms in an abrupt, non-gradual manner. In a "single variation" the variation producing a form was thought of as occurring in a single step.

sire The male parent; used in connection with domestic animals.

Sirenia /sigh-REEN-ee-yə/ n. A mammalian order that includes the dugong and the manatees.

sister chromatids The two identical nucleoprotein strands of a single replicated chromosome, which are joined at the centromere. MORE INFORMATION

Sitophilus /sigh-TAWF-ə-ləs, sə-/ n. An economically important genus of weevils, which are highly destructive of grains.

sitology /sigh-TAWL-ə-jee/ n. The study of nutrition and dietetics — sitologist /sigh-TAWL-ə-jist, -jəst/

sitosterol (also β-sitosterol) /SIDE-ō-STAIR-awl/ n. A plant sterol with a molecular structure similar to that of cholesterol. MOLECULAR STRUCTURE

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skull The bony structure that serves as the underlying supporting framework of the head.

skullcap Within the context of fossil human remains, a fragment composed of the upper portion of the skull.

SLS Sodium lauryl sulfate.

Smilodon /SMILE-ə-dawn/ n. Genus of the prehistoric American saber-toothed tiger, or saber-toothed cat. PICTURE

snRNA Small nuclear RNA.

snRNP Small nuclear ribonucleoprotein.

sodium (Na) /SŌD-ee-əm/ n. Chemical element; atomic number 11, atomic weight 22.98976928. Sodium salts are found in body fluids (blood, serum, and lymph) and in the tissues (in lower concentrations). They are required to maintain the balance between calcium and potassium required for normal heart action. They also regulate osmotic pressure of cells and protect against excessive water loss from tissues. PERIODIC TABLE

sodium bicarbonate (NaHCO₃ also bicarbonate of soda) /SODE-ee-əm by-KAR-bə-nate/ n. A white, soluble compound used as an antacid and found in carbonated drinks and baking powders.

sodium carbonate (Na₂CO₃) /SŌD-ee-əm KAR-bə-nate/ n. A sodium salt of carbonic acid; used in making detergents, paper, and glass.

sodium chloride (NaCL) /SŌD-ee-əm CLORE-ide/ n. The most common form of salt, also known as table salt or common salt; in its mineral form it is called halite. In salty solutions water is extracted from cells by osmosis, a process that kills many microorganisms. For this reason, salting preserves foods. Salt will also detach feeding leeches and disinfect wounds.

sodium dodecyl sulfate (SDS) also sodium lauryl sulfate (SLS) /SŌD-ee-əm dō-DECK-əl SƏL-fate/ n. An anionic surfactant commonly used in biological experimentation, particularly in preparing proteins for electrophoresis in the SDS-PAGE technique.

sodium fluoride (NaF) /SŌD-ee-əm FLOOR-ide/ n. A white, crystalline, water-soluble powder with a saline taste often added to drinking water for the prevention of dental caries.

solute /SAW-loot/ n. The substance dissolved in a solvent.

solution /sə-LOO-shən/ n. A homogeneous mixture, usually a liquid mixture, of two or more substances.

solvent /SAWL-vənt, -vent/ n. The dissolving medium of a solution.

soma /SOAM-ə/ n. The cells of the body taken as a whole, in opposition to germ cells.

somatic /sō-MAT-ək, -ik/ adj. Relating to the body.

somatic cell /sō-MAT-ək, -ik/ n. Any cell in the body except gametes and their precursors. See also: gamete.

somatic cell gene therapy /sō-MAT-ək, -ik/ n. The incorporation new genetic material into somatic cells for therapeutic purposes. The new genetic material cannot be passed to offspring. See also: gene therapy.

somatic mutation (also somatic cell mutation) /sō-MAT-ək, -ik/ n. A mutation in a non-reproductive cell. Compare: germ line mutation.

somites /SŌ-mites/ n. Blocks of mesoderm along the sides of a chordate embryo.

southern blotting (also southern blot) A method developed by E. M. Southern for transferring DNA fragments, separated in electrophoretic gels, onto membrane filters. Used to detect specific fragments by complementary radioactive probes.

spawn (1) to lay eggs in water (said of an aquatic animal); (2) eggs laid in water by an aquatic animal.

species (pl species) /SPEE-seez/ n. In taxonomy, a division of a genus. A given type of organism is treated as a species if it is assigned a binomial name. However, there is no general consensus among scientists concerning how to decide whether any given group of organisms should be so treated, since there is no general agreement among biologists on the definition of the word species. MORE INFORMATION | CHART OF RELATIVE TAXONOMIC RANKS

spectral karyotype (SKY) /KARE-ee-ō-type/ n. A graphic of all an organism's chromosomes, each labeled with a different color. PICTURE

spectrophotometer /SPECK-trō-fə-TOM-ə-ter/ n. An instrument that measures the intensity of a light beam of a particular wavelength, both before and after passing through a light-absorbing medium.

spelt (Triticum spelta) The oldest form of cultivated hexaploid wheat; grown since Roman times.

sperm (1) spermatozoa; (2) semen; (3) a spermatozoon.

spermatids /SPERM-ə-tidz/ n. During spermatogenesis, the immature products of the second meiotic division. During spermiogenesis, each haploid spermatid develops, without further division, into a functionally mature spermatozoon.

spermatocyte /sperm-MAWD-ə-site/ n. Either of two types of cells that originate from the spermatogonium during spermatogenesis and that develop, via division into spermatids. The first of these two types is the primary spermatocyte, which is a mature sex cell that develops from the spermatogonium without division. The second is the secondary spermatocyte, which is produced from the primary spermatocyte by division.

spermatogenesis /sperm-awd-ə-JEN-ə-səs/ n. The process producing spermatozoa. See also: spermatogonium.

spermatogonium (pl spermatogonia) /sperm-awd-ə-GŌ-nee-əm, pl -nee-ə/ n. During spermatogenesis, one of the primordial, undifferentiated sex cells that give rise, via maturation and growth, first to a primary spermatocyte, then via division, to two secondary spermatocytes that in turn divide to form four spermatids, which then mature without further division into four fully functional spermatozoa — spermatogonial /sperm-awd-ə-GŌ-nee-əl/

spermatozoon (pl spermatozoa) /sper-mat-ə-ZŌ-ən; pl: -ZŌ-ə/ n. A mature male haploid gamete capable of active movement by means of a undulipodium. During spermatogenesis, spermatozoa form in huge quantities within the seminiferous tubules of the testes. In shape, a spermatozoon resembles a tadpole. It has an oval, flattened head containing a haploid nucleus. A human spermatozoon is about 0.005 mm (0.002 in) in length. When a spermatozoon pierces an ovum it loses its tail as the two cell fuse. See also: acrosome reaction.

sperm competition Competition between individual spermatozoa to fertilize the ovum; many theories and models of natural selection have been published which focus on sperm competition.

spermiogenesis /sperm-ee-ə-JEN-ə-səs/ n. The process during which spermatids mature into fully functional spermatozoa.

S phase The portion of interphase during which chromosomes are replicated.

Spheniscidae /sfə-NISS-ə-dee/ n. The family of birds comprised of the penguins.

Sphenisciformes /sfə-NISS-ə-FORM-eez/ n. The order of birds comprised of the penguins.

Spheniscus /sfə-NISS-cəs/ n. The genus of birds comprised of the banded penguins. MORE INFORMATION | PICTURE

sphincter /SFING-ter/ n. A ring of muscle controlling passage of an orifice.

spina bifida /SPĪ-nə BIF-ə-də/ n. A developmental defect characterized by failure of fusion of vertebral arches, with or without protrusion and dysplasia of the spinal cord or its membranes.

sphygmic /SFIG-mick/ adj. Pertaining to the pulse.

spinal cord /SPĪ-nəl/ n. The column of nervous tissue that in vertebrates runs along the back, and that in bony animals is enclosed within the vertebral column. In humans, it gives rise to all the nerves of the trunk and limbs.

spinous process /SPIGH-nəs/ n. The prominence at the posterior extremity of a vertebra. PICTURE

Spirillum /spigh-RILL-əm/ n. A genus of helical microorganism belonging to the family Pseudomonadacea (Spirillum minus is the causative agent of rat-bite fever).

spirochete (also spirochaete) /SPIGH-rō-keet/ n. Any member of Spirochaetes, a phylum of helical bacteria. Three spirochete genera, Borrelia, Leptospira, and Treponema, contain organisms that are important causative agents of human disease

spiroscope /SPIGH-rō-skope/ n. A device for measuring respiratory capacity.

splanchnology /splank-NAWL-ə-jee/ n. The study of the internal organs.

splenic flexure /SPLEH-nik/ n. An angular bend in the large intestine between the transverse and descending colons.

splice sites Locations where RNA splices exons together to form a continuous gene transcript.

splicing The process of joining adjacent exons after the removal of an intervening intron.

spondylitis /spawn-dull-EYE-təs/ n. Inflammation of, and resulting damage to, the vertebrae.

spontaneous generation n. A supposed process — that has never actually been observed — in which living things arise from nonliving matter.

sporangium /spə-RANJ-ee-əm, spō-/ n. A case containing developing spores.

spore (1) in a plant or fungus, an asexual reproductive cell that does not participate in fertilization; (2) in prokaryotes, a dormant, relatively impervious cell that is resistant to destruction by heating.

sport n. A new type of organism arising in a single generation. The term is usually applied to plants, but sometimes, especially in older literature, also to animals (for example, Darwin called the Ancon sheep a sport). The name is derived from the Latin term lusus naturae, "sport of nature," which expressed the idea that nature was in some way play a game and entertaining itself when it made new organisms in this way.

sporulation (also sporogenesis) /spore-yə-LAY-shən/ n. The production of spores.

sputum /SPYOOT-əm/ n. Ejected saliva mixed with mucous and sometimes pus.

Squamata /skwə-MAWD-ə/ n. The most diverse order of Class Reptilia; includes the lizards, snakes, and worm lizards.

squamous /SKWAM-əs/ adj. (1) (also: squamate) scale-covered, scaly; (2) scale-like (as in the cells of squamous epithelial tissue); (3) (also: squama) the anterior portion of either temporal bones in humans and many other mammals (PICTURE 1 | PICTURE 2).

SRP Signal recognition particle.

ssDNA Single-stranded DNA.

stable isotope A nonradioactive isotope.

stamen /STAY-mən/ n. The pollen-producing organ of a flower.

standard deviation (s) /dee-vee-AY-shən/ n. The positive square root of the variance; a commonly used measure of variability.

staphyloangina /STAF-ə-lō-an-JINE-ə/ n. Sore throat caused by staphylococcus.

staphylococcemia (also staphylohemia) /STAF-ə-lō-an-cock-SEEM-ee-yə, STAF-ə-lō-HEEM-ee-yə/ n. Presence of staphylococcus in the blood.

Staphylococcus /staf-(ə)-lō-KAWK-əs/ n. A genus of gram-positive cocci usually present on human skin and mucous membranes. Under the microscope it can be seen that staphylococci are round in shape (i.e. they are cocci) and form clusters like bunches of grapes. Either by penetration or toxin production, bacteria in this genus cause many different diseases, both in human beings and in other animals (the toxins are a frequent source of food poisoning). PHOTOMICROGRAPH

staphylopharyngeus /staf-ə-lō-fə-RINJ-ee-əs/ n. A muscle of the soft palate. It narrows the fauces and blocks off the nasopharynx.

starch The storage polysaccharide (a polymer of α-D-glucose molecules) of most plants. Starch polymers are cleaved by the enzyme amylase.

stasis /STAY-səs, -sis/ n. The persistence of a fossil form unchanged over geological time.

steatorrhea /stee-əd-ə-REE-ə/ n. Presence of excessive fat in the stools.

stem cell Undifferentiated, primitive cells with the ability both to multiply and to differentiate into various types of cells.

stenosed /stə-NOAST, -NOZED/ adj. Affected by stenosis.

stenosis /stə-NŌ-səs/ n. A narrowing of a passage, space, or orifice.

stenostomia /STEN-nō-STŌM-ee-yə/ n. Narrowing of the mouth.

stenothermal /STEN-nō-THERM-əl/ adj. Able to tolerate only a narrow range of temperatures.

sternum /STER-nəm/ n. The flat bone at the front of the chest that connects the ribs on one side with those on the other.

sternebra (pl sternabrae) /STER-nə-brə/ n. An individual segment of the sternum.

sterols /STAIR-alls, STIR-/ n. A class of complex cyclic alcohols with a tetracyclic structure (MOLECULAR STRUCTURE), which occur in plants, animals, and fungi. Cholesterol is the most widely known sterol.

stigma (pl stigmata or stigmas) /STIG-mə, stig-MAWT-ə/ n. The sticky tip of a carpel; the stigmata are the parts of a flower that receive pollen.

stoma (pl stomata) /STŌ-mə, stō-MAWT-ə/ n. Microscopic pores in the epidermis of plants; stomata allow gas exchange with the atmosphere.

stomach /STƏM-ək/ n. A baglike, elastic portion of the digestive tract following the esophagus. Lying beneath the diaphragm. It secretes acidic gastric juices that convert proteins into peptones. PICTURE OF STOMACH | HUMAN DIGESTIVE SYSTEM

stool Excrement discharged from the bowels.

stratified /STRAT-ə-fide/ adj. Lying in layers.

stratiform /STRAT-ə-form/ adj. Lying in layers.

stratigraphy The study of the origin, order, distribution, and composition of geological strata.

stratum /STRAT-əm/ (pl strata /STRAT-ə/) (1) 1n geology: a layer of rock or earth, with a characteristic composition and fossil content, lying between other layers differing from it with respect to composition and content; (2) in biology: a layer of an organ or other living structure.

stratum corneum /STRAT-əm KORN-ee-əm/ (pl strata cornea /STRAT-ə KORN-ee-ə/) The outermost, horny layer of the skin.

stratum germinativum /STRAT-əm jer-men-ə-TEE-vəm/ (pl strata germinativa /STRAT-ə jer-men-ə-TEE-və/) The skin's inmost layer, which is composed of columnar epithelial cells cells that divide to replace the outer layers as they wear away.

stratum granulosum /STRAT-əm gran-yə-LŌ-səm/ The layer of the skin lying between the stratum germinativum and stratum lucidum.

stratum lucidum /STRAT-əm loo-SEED-dəm/ (pl strata lucida /STRAT-ə loo-SEE-də/) A transparent layer of the skin. It lies between the stratum granulosum and stratum corneum.

strepticemia /strep-tə-SEEM-ee-yə/ (also streptococcemia /STREP-tə-cock-SEEM-ee-yə/) Presence of streptococci in the blood.

streptococcal /STREP-tə-cock-əl/ adj. Caused by or related to streptococci.

Streptococcus (pl streptococci) /STREP-tə-cock-əs/ n. A genus of gram-positive cocci, of which most strains are harmless. However, some are both common and among the most dangerous of human pathogens.

streptodermatitis /STREP-tō-derm-ə-TIGHT-əs/ n. Streptoccocal inflammation of the skin.

stroma /STRŌ-mə/ (pl stromata /strō-MAWT-ə/) (1) The portion of a chloroplast where the synthesis of organic molecules from carbon dioxide and water occurs; (2) the healthy tissue surrounding a tumor.

stromatolites /strō-MAT-ə-lites/ n. Large mineral structures formed in shallow water by microorganisms, especially cyanobacteria. Some stromatolites are among the most ancient fossils known, dating to about 3.5 billion years ago. PICTURE

structural gene A gene that encodes a product (e.g., a polypeptide, tRNA, or rRNA).

structural genomics /jə-NOAM-iks/ n. The determination, by means of both experimental techniques and computer simulation, of the three-dimensional structures of proteins.

structural isomers /ICE-ə-mers/ n. Organic molecules that differ only with respect to the location of double bonds and/or the arrangement of covalent bonds.

structural protein n. A protein that contributes to cell or tissue structure.

styles Column-shaped structures in flowers, through which the pollen tubes grow.

styloid process /STIGH(-ə)-loid/ n. A slender, pointed process on a bone at the attachment point of certain muscles (in particular, the styloid process of the temporal bone). PICTURE 1 | PICTURE 2

stylomandibular /STIGH-lō-man-DIB-yə-ler/ adj. Pertaining to or connecting the lower jaw (mandible) and the styloid process.

subclass (also infraclass, superorder) In taxonomy, a division of a class; specifically, a category ranking beneath a class, but above an order.

subcutaneous /sub-kyə-TAY-nee-əs, -kyoo-/ adj. Beneath the skin.

suberin /SOOB-ə-run/ n. A waxy, waterproof, air-proof substance found in the layers of dead cork cells that sheathe woody stems and mature roots.

suberization /soob-ə-righ-ZAY-shən/ n. The infiltration of plant cells with suberin.

suberized /SOOB-ə-rized/ adj. Impregnated with suberin.

subkingdom (also infrakingdom, superdivision, superphylum) n. In taxonomy, a division of a kingdom; specifically, a category ranking beneath an kingdom, but above a phylum. CHART OF RELATIVE TAXONOMIC RANKS

suborder (also infraorder, superfamily) In taxonomy, a division of an order; specifically, a category ranking beneath an order, but above a family. CHART OF RELATIVE TAXONOMIC RANKS

subphylum (also superclass) In taxonomy, a division of a phylum; specifically, a category ranking beneath an phylum, but above a class. CHART OF RELATIVE TAXONOMIC RANKS

sub-Saharan /sub-sə-HAR-ən/ n. South of the Sahara Desert, in Africa.

subsidence /sub-SIDE-əns/ n. Sinking or settling of the Earth's surface.

substitution n. See: base-pair substitution.

substrate n. (1) an artificial medium on which a microorganism is grown; (2) the molecule on which an enzyme acts.

sucrase /SOO-kraze/ n. An enzyme that hydrolyzes sucrose into fructose and glucose.

sucrose /SOO-krōs, -rōz/ (Suc also saccharose /SACK-ə-rōs, -rōz/) Table sugar (also: cane or beet sugar). Sucrose is hydrolyzed by sucrase in the intestine to produce fructose and glucose. MOLECULAR STRUCTURE

sulfhydryl /səlf-HIDE-rəl/ n. A functional group of the form —C—SH or —R—SH, where R is an alkane, alkene, or some other group of atoms containing carbon.

sulfonamides /səl-FAWN-ə-mides/ n. Synthetic substances containing the sulfonamide group, many of which are used as antimicrobial agents. The first antibiotics discovered were sulfonamides. MOLECULAR STRUCTURE

sulfur Chemical element; atomic weight 32.064; atomic number 16. An essential component of all living cells, sulfur is present in the amino acids cysteine and methionine. Within the human body, sulfur occurs in many bacterial defense molecules. It is also present in many antibiotics, and, in its elemental form is an important fungicide, particularly within the context of organic agriculture.

superclass See: subphylum.

superior /soo-PEER-ee-er/ adj. (1) adv. above; (2) adj. top.

supercoiling See: linking number.

superfamily See: suborder.

superfecundation /SOOP-er-FEK-ən-DAY-shən/ The fertilization of two or more ova from the same ovulation. Heteropaternal superfecundation occurs when separate fertilizations result from two or more inseminations administered by two or more different males.

superfetation /SOOP-er-fee-TAY-shən/ The fertilization of ova from different ovulations, that is, conception occurring while the mother is already pregnant (characteristic of lagomorphs, but abnormal in nearly all other mammals).

supernumerary /SOOP-er-NOOM-er-rare-ee/ adj. In addition to the usual number. PICTURE OF SUPERNUMERARY FINGER | PICTURE OF SUPERNUMERARY RAINBOW | PICTURE OF SUPERNUMERARY NIPPLE

superorder See: subclass.

superoxide /SOO-pər-AWK-sīd/ n. A compound that contains the superoxide anion.

superoxide anion /SOO-pər-AWK-sīd AN-ī-awn/ n. The anion with the formula O₂^-.

superoxide dismutase (SOD) /SOO-pər-AWK-sīd DIS-myoo-tāz/ n. An enzyme that catalyzes the conversion of superoxide into hydrogen peroxide and oxygen.

suprarenal /SOO-prə-REE-nəl/ adj. Above or on top of the kidney.

suprarenal gland /SOO-prə-REE-nəl/ n. See: adrenal gland.

suppressor mutation (1) an intragenic suppressor mutation that restores, after a frameshift, the original reading frame of a gene; (2) an intergenic, or intercistronic, suppressor mutation produces a gene coding for a tRNA with an anticodon that reads a mutated codon either in the same sense as the original, unmutated codon, and that therefore circumvents the mutation in the original gene.

suppressor T cell A kind of T cell that causes B cells, and other cells, to ignore antigens.

surgery /SER-jer-ee/ n. (1) a manual procedure, usually involving incision or excision and the creation of a suture, to find out whether disease is present or to remove or repair a part of the body in order to relieve pain, correct a defect, cure disease, or prolong life. An operation; (2) the branch of medicine dealing with such procedures; (3) the room in which such procedures are carried out (4) (British) the room where a doctor sees patients.

suture (1) a line of union forming an immovable joint (as in the skull or between the segments of a gastropod shell) SUTURES OF HUMAN SKULL | SUTURES IN A DEER SKULL (2) surgical stitches uniting two parts (or the line of union so formed). SUBCUTICULAR SUTURES | SURGEON'S KNOT

Svedberg unit (S) A sedimentation coefficient of 1 x 10^-13 sec.

symbiosis /SIM-bye-ō-səs/ n. A relationship, between two distinct types of organisms, in which they live together. Symbiosis is of three types: parasitism, mutualism, and commensalism.

sympatric /sim-PAT-rick/ adj. Of two populations: Occupying overlapping geographic regions.

symphalangia /SIM-fə-LAN-jee-ə/ n. Webbing or fusion between two or more fingers.

symphysis /SIM-fə-səs/ n. (1) a line of fusion between bones that were once separate; (2) a joint, lined by cartilage, between two bones (as between two vertebrae).

synapomorphy /sin-AP-ə-more-fee/ (also synapomorphic character /sin-ap-ə-MORE-fik/) In cladistics, a trait that is: (1) shared by a group of two or more taxa deemed more closely related to each other than to any other taxon under consideration, and (2) not shared with any taxon outside the group. Under such circumstances, it is generally claimed that the trait arose after the divergence of the most recent common ancestor of the taxa in the related group, but before the divergence of those taxa. Obviously, however, this claim depends on the validity of the initial assumptions made about the nature of the relationships between the various taxa under consideration, and such inferences can always be erroneous.

synapse /SĪ-naps/ n. The junction between neurons that permits a one neuron to pass a signal to another.

synapse /sin-APS/ v. To join or fuse; within the context of biology this term is used primarily with regard to meiotic synapsis.

synapsids /sin-NAP-sidz/ n. Mammal-like reptiles (or reptile-like mammals) that flourished in the late Paleozoic and early Mesozoic. MORE INFORMATION

synapsis /sin-NAP-sis/ n. The intimate fusion of homologous chromosomes during prophase I of meiosis. MORE INFORMATION

synaptic /sə-NAP-tick/ adj. Of or pertaining to a synapse.

synaptic knobs /sin-NAP-tick/ n. The bulbous structures on the end of an axon, each of which contains many synaptic vesicles.

synaptic vesicle /sin-NAP-tick VES-ə-kəl/ n. One of the many minute vesicles in a synaptic knob where neurotransmitters are stored.

syncytial /sin-SISH-ul/ adj. Pertaining to or being a syncytium.

syncytium (pl syncytia) /sin-SISH-ee-əm/ n. A single cell containing multiple nuclei.

syndrome The group or recognizable pattern of symptoms or abnormalities that indicate a particular trait or disease.

synergy /SIN-er-jee/ n. Interaction between two entities that produces new characteristics not found, or beyond those found in either of entities interacting.

synergistic /sin-er-JIST-ik/ adj. Involving, or characterized by synergy.

syngamy /SIN-gam-ee/ n. The union of two nuclei during fertilization.

syngeneic /sin-jə-NAY-ik/ adj. Genetically identical.

syntenic group /sin-TEN-ik/ n. A block of genes occurring in the same order in two different types of organisms.

synthesis /SIN-thə-sis, -səs/ n. During the eukaryotic cell cycle, a substage of interphase when each of the chromosomes replicate. When this occurs, each forms a duplicate of itself and the resulting two structures, called sister chromatids, are joined at the centromere.

syringe /sə-RINJ/ n. An instrument used to accurately administer small amounts of fluid; when a needle is attached to a syringe it can be used to make injections. PICTURE

syringitis /sə-rin-JIGHT-əs/ n. Inflammation of the eustachian tube.

systematics (also taxonomy) The study of the classification of living things.

systematist One who engages in the practice of systematics.

systole /SIST-ə-lee/ n. The portion of the cardiac cycle during which the heart is contracting. Compare: diastole.

syzygy /SIZ-ə-jee/ n. The fusion of two or more organs or portions of the body.

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