Online Biology Dictionary - S

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

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 labeling proteins.

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

saccharides /SAK-uh-rides/ Another name for carbohydrates.

saccharose (SAC or sucrose) /SACK-uh-rohs, -roze, SOO-krohs, -kroze/ Sucrose (cane or beet sugar).

saccharuria /sak-uh-RUR-ee-yuh/ Sugar in the urine.

sacrad /SAY-krad/ Toward the sacrum.

sacral /SAY-kruhl/ Pertaining to the sacrum.

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

sagittal /SAJ-uht-uhl/ (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-uht-uhl/ 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 /suh-HILL or SAH-hill/ The region between the Sahara desert and savannas to the south; characterized by extended dry seasons, which alternate with relatively brief rainy seasons. PICTURE

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

salt (1) sodium chloride (NaCl); (2) Any chemical compound composed of a positive ion other than hydrogen, and a negative ion, other than the hydroxyl group.

saltation (also saltatory evolution) /sall-TAY-shun/ The production of new types of organisms via rapid, discontinuous processes; used in opposition to the term gradualism. More information.

saltationist A biologist who believes evolution is a saltatory process. See saltation.

SAM S-adenosylmethionine.

sanguiferous /sang-GWIF-er-uhs/ Carrying or conducting blood <~ vessels>.

saprophyte (also saprophite) /sap-ruh-FITE/ A plant or fungus feeding on decaying organisms; adj: saprofitic /sap-ruh-FIT-ick/.

sarcoma /sar-KOAM-uh/ 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-uh-meer/ 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-uh-lite/ 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-uhn/ (1) lizardlike; (2) a lizardlike reptile.

savanna (or savannah) /suh-VAN-nuh/ 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.

scaffold /SKAFF-uhld/ 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) /SKALL-uh nat-TYOOR-eye/ 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. READ MORE ABOUT THIS TOPIC

scapula /SKAHP-yuh-luh/ 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 /SKLAIR-uh/ The outermost coat of the eyeball, extending from the optic nerve to the edges of the cornea.

scolex /SKO-leks/ (pl scolices /SKO-lee-seez/ or scoleces or /SKO-luh-seez/ scolexes /SKO-lex-ez/) The head of an adult tapeworm (see picture). A tapeworm attaches itself to the wall of the small intestine with its scolex.

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

sediment /SED-uh-muhnt/ Any solid material that settles out of a liquid.

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

sedimentation coefficient /sed-uh-men-TAY-shun co-uh-FISH-uhnt/ 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.

segregation /seg-ruh-GAY-shun/ The normal biological process whereby the 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. READ MORE ABOUT THIS TOPIC.

self-fertilization /fert-uhl-EYE-zay-shuh/ Union of male and female gametes from the same individual.

self-pollination /PAWL-uh-NAY-shuh/ Pollinating a stigma with pollen from the same plant.

semelparity /SEM-uhl=PAIR-uh-tee/ The condition of producing offspring only once during the lifetime of an individual. Compare: iteroparity.

semen /SEE-men/ 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.

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

sequence See: base sequence.

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

sequence tagged site (STS) Short (200 to 500 base pairs) DNA fragment, with a known location and sequence, that occurs only once in a genome and whose location. 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/ One of the nonessential amino acids. CODONS | MOLECULAR STRUCTURE | SERINE BIOSYNTHESIS

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

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

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

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.

Siberia 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 See: amino acid.

sigmoid colon /SIG-moid/ 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. PICTURE

signal transduction /SIG-nuhl trans-DUCK-shuhn/ 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

Mixopterus kiaeri — An extinct sea scorpion Photo: Ghedoghedo

simian /SIM-ee-uhn/ Apelike.

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-uhm/ 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.

Bones in the skull

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

SLS Sodium lauryl sulfate.

Smilodon (click to enlarge) Photo: Wallace63

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

snRNA Small nuclear RNA.

snRNP Small nuclear ribonucleoprotein.

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

sodium dodecyl sulfate (SDS) also sodium lauryl sulfate (SLS). /doe-DECK-uhl/ An anionic surfactant commonly used in biological experimentation, particularly in preparing proteins for electrophoresis in the SDS-PAGE technique.

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

solution /suh-LOO-shun/ A homogeneous mixture, usually a liquid mixture, of two or more substances.

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

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

somatic /soh-MAT-uhk/ Relating to the body.

somatic cell /soh-MAT-uhk/ Any cell in the body except gametes and their precursors. See also: gamete.

somatic cell gene therapy /soh-MAT-uhk/ 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 cell mutation /soh-MAT-uhk/ A mutation in a non-reproductive cell. See also: germ line mutation.

somites /SO-mites/ 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.

species (pl species) /SPEE-seez/ In taxonomy, a division of a genus. A form is treated as a species if it is assigned a binomial name. However, there is no general concensus among scientists concerning how to decide whether any given group of organisms should be so treated since there is no general agreement on the definition of the word species. READ A DETAILED DISCUSSION OF THIS TOPIC | CHART OF RELATIVE TAXONOMIC RANKS

Human Spectral Karyotype (SKY)

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

spectrophotometer /SPECK-troh-fuh-TOM-uh-ter/ 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) spermatazoa; (2) semen; (3) a spermatazoan.

spermatid /SPERM-uh-tid, -tuhd/ One of the four haploid cells produced by meiosis that subsequently develop into spermatozoa without further division.

spermatogenesis /sperm-awt-uh-JEN-uh-suhs/ (1) the process producing spermatozoa; (2) the conversion of a spermatid into a spermatozoan.

spermatozoan (pl spermatozoa) /sper-mat-uh-ZOH-uhn; pl: -ZOH-uh/ A male haploid gamete capable of active movement by means of a undulipodium. STRUCTURE OF A SPERMATOZOAN

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.

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

sphygmic /SFIG-mick/ Pertaining to the pulse.

spinal cord /SPIGH-nuhl/ 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.

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

spirochete (also spirochaete) /SPIGH-roe-keet/ 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-roe-skope/ A device for measuring respiratory capacity.

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

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

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-tuhs/ Inflammation of, and resulting damage to, the vertebrae.

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

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.

sporulation (also sporogenesis) /spore-yuh-LAY-shun/ The production of spores.

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

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

squamous /SKWAM-uhs/ (1) (also: squamate) scale-covered, scaly; (2) scale-like (as in the cells of 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-muhn/ The pollen-producing organ of a flower.

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

staphyloangina /STAF-uh-low-an-JINE-uh/ Sore throat caused by staphylococcus.

staphylococcemia (also staphylohemia) /STAF-uh-low-an-cock-SEEM-ee-yuh, STAF-uh-low-HEEM-ee-yuh/ Presence of staphylococcus in the blood.

Staphylococcus aureus

Staphylococcus /staf-(uh)-low-KAWK-uhs/ A genus of Gram positive bacteria 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).

staphylopharyngeus /staf-uh-low-fuh-RINJ-ee-uhs/ 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-suhs, -sis/ The persistence of a fossil form unchanged over geological time.

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

sternum /STER-nuhm/ 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-nuh-bruh/ An individual segment of the sternum.

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

stoma (pl stomata /STOH-muh, stoh-MAWT-uh/ Microscopic pores in the epidermis of plants; stomata allow gas exchange with the atmosphere.

stool Excrement discharged from the bowels.

stratum (sing strata) In geology, a layer of rock or earth, with a characteristic composition and fossil content, lying between other layers differing from it with respect to their compositions and contents.

stroma (pl stromata) /stroh-MAWT-uh/ The portion of a chloroplast where the synthesis of organic molecules from carbon dioxide and water occurs.

stromatolites /stroh-MAT-uh-lites/ Large mineral structures formed in shallow water by microorganisms, especially cyanobacteria. Certain stromatolites are among the most ancient fossils known, dating to about 3.5 billion years ago. PICTURE 1 PICTURE 2

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

structural genomics /juh-NOAM-iks/ The effort to determine the 3D structures of large numbers of proteins using both experimental techniques and computer simulation.

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

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

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

styloid process /STIGH(-uh)-loid/ 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

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

subfamily In taxonomy, a division of a family; specifically a category ranking beneath a family, but above a genus. 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 /sug-suh-HAR-uhn/ South of the Sahara Desert, in Africa.

substitution See: base-pair substitution.

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

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

sucrose (Suc or saccharose) /SOO-krohs, -kroze; SACK-uh-rohs, -roze/ Table sugar (also cane or beet sugar). Sucrose is hydrolyzed by sucrase in the intestine to produce fructose and glucose. MOLECULAR STRUCTURE

sulfur Chemical element; atomic weight 32.064; atomic number 16.

superclass See: subphylum.

superfamily See: suborder.

supernumerary In addition to the usual number (e.g., supernumerary finger).

superorder See: subclass.

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/ (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 shell); (2) surgical stitches uniting two parts (or the line of union so formed).

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

symbiosis /SIM-bye-oh-suhs/ 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/ Of two populations: Occupying overlapping geographic regions.

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

synapsids Mammal-like reptiles (or reptile-like mammals) that flourished in late Paleozoic and early Mesozoic. MORE INFORMATION

synapsis /sin-NAP-sis/ The pairing of homologous chromosomes during prophase I of meiosis.

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

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

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

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

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

syngamy /sin-GAM-ee/ The union of two nuclei during fertilization.

syngeneic /sin-juh-NAY-ik/ Genetically identical.

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

synthesis /SIN-thuh-sis, -suhs/ 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

syringe /suh-RINJ/ 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.

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

systematist One who engages in the practice of systematics.

systole /SIST-uh-LEE/ The portion of the cardiac cycle during which the heart is contracting. Compare: diastole.

syzygy /SIZ-uh-gee/ The fusion of two or more organs or portions of the body.