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Biology Definitions

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Secondary StructureThe 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 MarkerDuring 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 ProteinsNative 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 ReplicationWhen 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
SerumThe 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 TraitA 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-LimitedA 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-linkedA 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 SequencingA 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 SequenceA 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 ProteinA 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 PathwayA biochemical pathway that controls one or more cell functions. Also known as a "signaling cascade." For example, two transmembrane receptors may bind to the same ligand, causing the intercellular domains of the two receptors to phosphorolate 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 PolymorphismA 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
Single-Stranded DNA virusA type of virus that has only a single strand of DNA. 5 Virus families have this genomic structure (e.g. Circovirus, Parvovirus). After the virus enters the cell, it must first create the complementary strand of DNA so that RNA polymerase can then transcribe RNA (RNA Polymerase requires a double-stranded DNA for transcription). Single-stranded DNA viral genomes can be linear (Parvovirus) or circular (Circovirus). These viruses tend to be small and have small genomes.Baltimore Scheme, Virus Classification, Viral Genomes, DNA
Single-Stranded, Negative-Polarity RNA virusesSince the cell does not have machinery to create positive-polarity RNA from negative-polarity RNA, the virion must include an RNA-dependent RNA polymerase. Swine flu, Measles, Rabies virus, and Influenza virus are examples of viruses with a single-stranded, negative-polarity RNA viral genome.Baltimore Scheme, Viral Genomes, Virus Classification, RNA
Single-Stranded, Plus-Polarity RNA with DNA intermediate virusesRetroviruses that use Plus-Polarity, single-stranded RNA as their genome. Although the genome theoretically could be directly translated by a ribosome (and these RNA can be translated in-vitro), instead the virus produces DNA using reverse transcriptase (which it carries in its capsid), and then integrates this DNA into the host's DNA using integrase (which it also carries in its capsid).Baltimore Scheme, Viral Genomes, Virus Classification, RNA
Single-Stranded, Plus-Polarity RNA virusesA type of virus which uses a single-stranded RNA genome which is plus-polarity (can be directly translated by a Ribosome). Many of these viruses do not carry proteins in the capsid, since their genome can be translated directly by the host cell. Picornavirus, Poliovirus, Rhinovirus, Coronavirus, Togavirus, the Sars virus, West Nile virus, Yellow Fever, Rubella, and Hepatitis C virus are examples of a plus-polarity, single-stranded RNA virus.Baltimore Scheme, Viral Genomes, Virus Classification, RNA
SNPSingle 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
SomaAll of the cells of the body, excluding the germ cells (which are used for reproduction). 
SpeciesA group of organisms capable of inter-breeding and producing fertile offspring. Note that this species definition does not always work for viruses, as some virus species can exchange genetic material (e.g. some Polio virus species can recombine with Coxsackie virus). Some viruses are more "plastic" than others. For example, influenza virus can handle many more mutations in its genome than can Measles virus.Virus Classification, Linnaean Classification (Taxonomy)
SpliceosomeA 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
SporeA haploid cell (having only unpaired chromosomes) that can grow into an organism without uniting with another cell.Gametes, Haploid, Diploid, Yeast
SRCThe 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 CodonA 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
SubstrateThe 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
SupernatentThe soluble liquid left over after a solution has been centrifuged. The supernatent is distinct from the pellet (settled precipitate)Centrifuge, Pellet
SV40A 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
SyndromeA 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 PhaseAlso 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

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