| Term | Description | SeeAlso |
| Secondary Structure | The secondary structure refers to the 3-D structure of a particular domain (or part) of a biomolecule. For example, a protein may have a domain (section) which is folded into an alpha helix or a beta sheet or a random coil. | Primary Structure, Beta Sheet, Alpha Helix, Tertiary Structure, Quaternary Structure, Protein |
| Selectable Marker | During Recombinant DNA experiments, a scientist "transforms" cells by adding new DNA to them. Due to laboratory constraints, some cells are successfully transformed, and some are not. The scientists uses "selectable markers" to weed out the cells that have not been transformed. For example, a scientist may include an ampicillan resistance gene (in addition to the gene of interest) in the vector used to transform cells, so that the scientist can then test the resultant cell cultures for ampicillan resistance, and thus know which cell colonies contain the new genes. | Recombinant DNA |
| Self Proteins | Native proteins that the immune system should not recognize as "foreign". Early in the development of the immune system, the immune system generates many different types of antibodies through gene fusion and hypermutation. B Cells which cannot produce viable antibodies (with heavy and light chains) or that produce antibodies which attack "self proteins" must be eliminated. Autoimmune diseases occur when B Cells that create antibodies that recognize "self proteins" are not elimated. | V Segment, J Segment, D Segment, Hypermutation, Antibody, Antigen |
| Semiconservative Replication | When DNA replicates, its two strands separate, and each strand is used as a template for a new DNA molecule which contains the old template strand (the old strand is conserved) and a new strand which is created via complementary base pairing with the old strand. | DNA, Replication, Base Pairing |
| Serum | The straw-colored liquid obtained after separating clotted blood into liquid and solid (containing the platelets and red blood cells) in a centrifuge. It contains Anitbodies which can be used to determine if the organism has been exposed to antigens, and also as way of giving immunity to another organism. It also contains growth-factors which can stimulate a cell to grow and divide. | Centrifuge, Cell Cycle, Antiserum |
| Sex Linked Trait | A phenotype (observable trait) that is inherited via the X or Y chromosome. Most often, these traits are recessive and carried by the X chromosome. For example, most Drosophila flies have red eyes. White eyes are a recessive trait. The white eye allele is carried on the X chromosome. The Y chromosome does not have a corresponding gene for this trait. All males with an X chromosome with the white eye allele will have white eyes. Females with only one X chromosome with a white eye allele will have red eyes because white eyes are a recessive phenotype. Females with two X chromosomes with the white eye allele will have white eyes, but females with white eyes are much rarer than males with white eyes. | Phenotype, Chromosome, X Chromosome, Y Chromosome, Allele, Genetic Map |
| Sex-Limited | A trait (pheontype) which is only expressed in either males or females, regardless of genotype. For example, male pattern baldness is a sex-linked trait. It is caused by excess testosterone. | Sex-Linked |
| Sex-linked | A trait (phenotype) is sex-linked if its genes are carried on the X or Y chromosomes. Most sex-linked traits are carried on the X chromosome as the Y chromosome has fewer genes. Statistically, sex-linked phenotypes are most often seen predominantly in males, the trait is not seen in the offspring of an affected male (in particular, it is not seen in sons), carrier females transmit to half of their sons, affected females transmit to all of their sons, and the trait often appears to skip generations. | X Chromosome, Y Chromosome, Sex-Limited |
| Shotgun Sequencing | A scientist wishing to sequence an entire genome can do so by performing variations on the following procedure: 1.) Create a library of clones via DNA Cloning; usually the scientist breaks up the library into medium-sized strands of 100,000 base pairs (perhaps cloning these sequences in BACs) 2.) Take one of the BACs from step 1, and break it up into even smaller segments which are suitable for gel electrophoresis (< 1000 base pairs). 3.) Sequence the short segments from Step 2 using Gel Electrophoresis. 4.) Use computer programs to find the overlaps in the short segments from Step 3, and thus put the medium-sized sequences in order. 5.) Put the medium-sized sequences from step 4 in order, perhaps by again using computer programs to find overlaps between the BAC sequences. This technique was developed by Sanger circa 1980. | Library, DNA Cloning, BAC |
| Signal Recognition Particle (SRP) | A particle that binds to the Signal Sequence of a partially synthesized protein as it leaves the Ribosome, and then guides the protein and Ribosome to the correct location in the Endoplasmic Reticulum where it binds to a Signal Recogition Particle Receptor ("docking protein"). | Signal Sequence |
| Signal Sequence | A sequence of amino acids (peptide) on a protein that signals to the cell that this protein is destined for a specific part of the cell. It is usually at the N terminus, and is often cleaved before reaching its final destination. Specifically, the signal sequence is cleaved for proteins that are fully exported, or are destined for the plasma membrane or mitochondrian. The signal sequence is not cleaved in proteins that are destined for the nucleus. | Protein, Amino Acid, Peptide |
| Signal Transducing Protein | A protein that receives (chemical) signals from higher up in a Signal Sequence (or "Signal Cascade") and then, having been modified in some way, passes (chemical) signals further down the cascade, perhaps by breaking a chemical bond, and/or bonding to a different molecule. | Signal Sequence |
| Signaling Pathway | A biochemical pathway that controls one or more cell functions. Also known as a "signaling cascade." For example, two transmembrane receptor may bind to the same ligand, causing the intercellular domains of the two receptors to phophorate each other, resulting in a series of subsequent chemical reactions which cause the cell to grow and divide. | Cell Cycle, Cancer Cell, Oncogene, Growth Factor Receptor |
| Single Nucleotide Polymorphism | A single base (A,T,C, or G) change in a DNA sequence that varies between members of the same species. For example, one member of the species has an "A" at a given location, while another member of the species has a "G" at the same location on the same chromosome. It is abbreviated as "SNP". | SNP, Nucleotide, Polymorphism, Chromosome |
| SNP | Single Nucleotide Polymorphism; A single base (A,T,C, or G) change in a DNA sequence that varies between members of the same species. For example, one member of the species has an "A" at a given location, while another member of the species has a "G" at the same location on the same chromosome. | Nucleotide, Polymorphism, Single Nucleotide Polymorphism, Gene, Chromosome |
| Soma | All of the cells of the body, excluding the germ cells (which are used for reproduction). | |
| Spliceosome | A protein-RNA complex that performs RNA Splicing. The Spliceosome removes introns from an immature mRNA and splices together the mRNA exons, creating a mature mRNA. | RNA Splicing, Eukaryote |
| Spore | A haploid cell (having only unpaired chromosomes) that can grow into an organism without uniting with another cell. | Gametes, Haploid, Diploid, Yeast |
| SRC | The gene in Rous Sarcoma Virus (SRV) which causes cancer. Pronounced "Sarc". Research on Rous Sarcoma Virus (RSV) has shown that an RSV virus that does not contain the SRC gene will produce offspring with the SRC gene as the cell itself contains a copy of the SRC gene. This led to research in proto-oncogenes, and the discovery that almost all vertebrates have the SRC proto-oncogene, indicating that this gene is conserved by nature because it must be essential for some biochemical pathway. | Rous Sarcoma Virus, Cancer Cells, Virus, Retrovirus, Proto-Oncogene, Oncogene |
| Stop Codon | A Ribosome stops translating an mRNA into a protein when it "sees" a 3-letter nucleotide stop signal on the mRNA. UAA, UGA, and UAG are all read as stop signals by the Ribosome. Also called a "Stop Signal". | mRNA, Ribosome, Translation |
| Substrate | The molecule or substance which is affected by an enzyme or catalyst, Many proteins are catalysts which temporarily hold a molecule (substrate) in a catalytic cleft, enabling a biochemical pathway by positioning the molecule so that it can more easily react with yet another molecule. | Enzyme, Catalyst, Protein, Catalytic Cleft |
| Supernatent | The soluble liquid left over after a solution has been centrifuged. The supernatent is distinct from the pellet (settled precipitate) | Centrifuge, Pellet |
| SV40 | A DNA tumor virus found in monkey kidney cells. Because Sabin created polio vaccine by culturing on monkey kidney cells, many people who received this virus were exposed to SV40. SV40 is a very potent virus in hamsters, mice, and rats. Fortunately, epidemiological studies have shown that exposure to SV40 from Sabin's polio vaccine have not caused higher rates of cancer. | Albert Sabin, Virus |
| Syndrome | A collection of symptoms that appear together. For example, in the early 1980s, a number of young, homosexual men were found to have Kaposi's Sarcoma, Pneumatosis Carinae, and various herpes virus infections. This collection of symptoms was named Acquired ImmonoDefiiciency Syndrome (AIDS). | AIDS |
| Synthesis Phase | Also called the "S" Phase. The cell cycle phase. between the G1 and G2 phases, in which the DNA is copied, creating pairs of identical, daughter chromatin, which are joined via Centromeres. | Cell Cycle Phases, G1 Phase, G2 Phase |