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Biology Definitions

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TermDescriptionSeeAlso
AcidThere are three different, overlapping definitions of an acid; an acid is a chemical substance that does one or more of the following: dissociates in water releasing Hydrogen ions (H+), donates a proton (H+) to a base, or accepts a pair of electrons from a baseBase, Alkali
Adapter HypothesisThe Adapter Hypothesis was proposed by Francis Crick and later verified experimentally. It describes how codons (3 base sequences) on an RNA molecule are translated into an Amino Acid by using an "adapter" molecule, Transfer RNA (tRNA), which has 3 complementary bases (so it can attach to the messenger RNA) and also the relevant protein (so that the Ribosome can attach it to the protein).tRNA, Transfer RNA, Messenger RNA, mRNA
AdsorpWhen a virus attaches to a cell, it is said to "adsorp" to the surface of the cell.Virus, AIDS Virus
Agaran algae extract, rich in nutrients, used as a food source when growing various microbes (bacteria, yeast, etc.).rich media, minimal media, algae
AIDSAcquired Immunodeficiency Syndrome. A retroviral disease which attacks the T Helper cells of the immune system. Because the T Helper cells are the lynch pins of the immune system, it has not (yet) been possible to create a vaccine for AIDS. Because T Helper cells are responsible for activating B Cells to create antibodies and also activating Cytotoxic T Cells to destroy infected cells, the immune system becomes highly compromised by HIV. Ironically, the immune system is forced to kill its own T Helper cells which have been infected with the HIV virus. The population of T Helper continues to decrease over time, and eventually the patient dies from an opportunistic infection. Because retroviruses have a great deal of antigenic variation (often caused by mistakes of the reverse transcriptase), the immune system has a difficult task in creating antibodies against the rapidly mutating virus. Some of the drugs currently used to fight AIDS include inhibitors of reverse transcription, and inhibitors of the protease which is used to break down precursors which are later used as capsid proteins.CD4, AIDS Virus (HIV), Antigenic Variation
AIDS Virus (HIV)The AIDS Virus, also known as Human Immunodeficiency Virus, is a retrovirus which contains two RNA strands (it's diploid, with two copies of its genome), surrounded by a nucleo capsid, surrounded by a lipid bilayer which is "stolen" by the virus from the cell in which it was created. The virus also contains reverse transcriptase (to convert the RNA into DNA). There are glyco proteins protruding from the lipid bilayer, which the virus uses to attach and infect new cells. The AIDS virus attaches to the CD4 receptor of T Helper Cells. Once attached, the lipid bilayer of the virus fuses with the cell's plasma membrane, and the virus's contents spill into the cell. Note that the provirus is not transcribed unless the T Helper Cell is activated, making it impossible for the immune system to detect the virus in T Helper Cells (the only evidence being the provirus in the cell's DNA) which can remain quiescent for very long periods of time.AIDS, CD4, Adsorp, Retrovirus, Provirus
Albert SabinScientist who developed a polio vaccine by passing the virus through multiple in-vitro cell cultures over an extended period of time (several years), until the virus had become attenuated. As the virus is passed from culture to culture, there is no selective pressure for the virus to maintain its pathogenic properties, and the virus gradually becomes attenuated.Virus, Jonas Salk, SV40
Alleleone of of the alternate forms of a single geneGene
Allosteric ChangeA change in the shape of a protein caused by binding with a regulatory substance at a site other than the catalytic cleft. This change in shape causes a change in the function of the protein, usually enabling or disabling its function in a biochemical pathway. For example, the LacI repressor protein can no longer repress the Lac Operon once it has bound to a Lactose molecule.Lac Operon, Catalytic Cleft
Alpha HelixA single, spiral chain of molecules which is stablized by weak Hydrogen bonds between nearby molecules. Commonly seen in the secondary structure of proteins.Secondary Structure, Beta Pleated Sheet, Protein, Polypeptide
Alternative SplicingUsing RNA Splicing, one cell may use a given gene to make one protein, while another cell might use the same gene, but different RNA splicing, to create a different protein. This is called "Alternative Splicing". In this way, Eukaryotes can produce many different proteins using a limited number of genes.RNA Splicing, Eukaryote
Amino Acidan organic compound that contains at least one amino group (NH2) and one carboxyl group (COOH); there are 20 different amino acids which are used to build proteinsResidue, Carbohydrate, Amino Acid. Protein, Lipid, Nucleic Acid
AmphipathicA molecule is ampipathic if contains both polar (hydrophilic, water-soluble) and nonpolar (hydrophobic, not water-soluble) domains in its structure.Hydrophilic, Hydrophobic
AmphitropicA virus is "amphitropic" if it can infect the cells of several different species, including the species that produces the virus.Viral Tropism, Ecotropic, Xenotropic
Ampipathic HelixA helix is ampipathic if it is charged on one side of the helix, and non-polar on the other side (as one follows the helix, around and around)Ampipathic
AnaphaseA sub phase of Mitosis in which the sister chromatin are pulled to opposite sides of the cell by the fibers which are attached to their centromeres and also centrosomes on opposite sides of the cell. In some cancer cells, this process breaks down, and one of the daughter-chromatin-pairs ends up with both daughters pulled to one side of the cell (instead of one daughter chromatin on each side of the cell.)Cell Cycle Phases, Mitosis
Anchorage IndependenceNormal cells require tethering to the bottom of a petri dish in order to grow; Cancer cells often show "Anchorage Independence", growing in suspension without tethering.Cancer Cells
Anti-CodonA 3-letter nucleotide sequence in a tRNA that complements a 3-letter nucleotide sequence (codon) in an mRNA. Because the two sequences are complementary, the Ribosome can match the correct tRNA to the current mRNA codon (because they fit), and thus get the correct amino acid from the correct tRNA and add the amino acid to the protein it is currently building.tRNA, Codon, Amino Acid, Protein, Ribosome
AntibodyAn antibody is one of the immune system's mechanisms for disabling foreign invaders (viruses, bacteria, parasites, etc.) An antibody is a protein, created by a B Cell, which binds to an antigen, thus disabling the adverse effects of the antigen. Antibodies are also refered to as immunoglobulins.Antigen, Virus, Bacteria, Parasite, Epitope, B Cell
AntigenAn agent (a chemical entity) that triggers an immune response when recognized by the immune system. Examples of antigens include viruses and foreign bacteria. Very often, the antigen is an oligopeptide. For example, there may be several different antibodies which can neutralize a single virus, with each antibody binding to a different oligopeptide on the virus.Antigen, Antibody, Epitope
Antigenic VariationViruses, particularly retroviruses, often make mistakes during replication, making it harder for the immune system to recognize their "antigens" (sub-sequences of proteins on their surface). This is called "antigenic variation", and is a type of immunoevasion (mechanism for evading the immune response).Virus, Retrovirus, AIDS
AntiserumSerum containing antibodies for a specific pathogen or antigen. Also called immune serum.Serum, Antibody, Antigen, Pathogen
ApoptosisProgrammed cell death; some cells are genetically programmed to have a limited life span. Other cells undergo apoptosis if they are damaged.Epithelial cells, Erythrocytes
ArchaeaSingle-celled organisms without a nucleus which are distinct from Prokaryotes and Eukaryotes. Some archaea live in extreme conditions (e.g. high temperature). Archaea are considerted by some to be remnants of early life forms ("living fossils")Prokaryotes, Eukaryotes
AssayAnalysis of a substance to determine the quantity of one or more components; also, a substance to be assayed 
AUGWhen a Ribosome sees this nucleotide sequence on a messenger RNA (mRNA), it begins the process of translation (tranlating the mRNA into a protein). This is similar to how a "Promoter" works on a DNA sequence. There are exceptions to this rule (some translations don't start on AUG). Note that AUG translates into Methionine.Messenger RNA, Ribosome, Promoter, DNA
AutoclaveA device used in the laboratory to sterilize its contents with high-pressure steam. For example, it can be used to kill bacteria.Bacteria, Transforming Principle
Autocrine SignalingWhen a cell produces proteins, and excretes them into the extra-cellular space, and the cell also has receptors for these same proteins, the cell is taking part in "autocrine signalling". Some cancers are caused by autocrine signalling of growth factors. Autocrine signalling is rare in normal (non-cancerous) cells. Most cells don't have receptors for the proteins they excrete.Cell Cycle, Cancer, Growth Factor
Autoimmune DiseaseEarly 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. Examples of autoimmune diseases include Type 1 Diabetes, Rheumatoid Arthritis, and Lupus.Antibody, Antigen, V Segment, J Segment, D Segment, Hypermutation
Autosomal DominantA phenotype (inherited trait) is autosomal dominant if its associated genes are on non-sex chromosomes (autosomal) and if the phenotype appears in individuals who have only one copy of the associated allele. In general, automsomal dominant traits appear in every generation, with half of all the offspring having the phenotype (on average), with equal numbers of males and females exhibiting the phenotype (on average), and individuals who do not show the phenotype also do not transmit the phenotype to their offspring.Phenotype, Allele, Dominant, Gene, Incomplete Penetrance
Autosomal RecessiveA phenotype (inherited trait) is autosomal recessive if it's associated genes are on non-sex chromosomes (autosomal) and if the phenotype only appears in organisms that have two copies of the allele. If both parents are heterozygotes, 25% of the offspring will have the phenotype (on average). Cystic Fibrosis is an example of an autosomal recessive disorder.Phenotype, Allele, Recessive, Gene, Heterozygote
AutosomeA chromosome that is not a sex chromosome. A chromosome that is not an X or Y chromosome.X Chromosome, Y Chromosome
AuxotrophAn organism which, through mutation, has lost its ability to synthesize one or more biochemicals (e.g. an amino acid), and thus won't grow on minimal media. Geneticists often compare prototrophs (wild type) with mutants (auxotrophs) as a means of studying biochemical pathways. Yeast is a good candidate for these types of experiments.Genetics, Minimal Media, Prototroph, Yeast
Avery-MacLeod-McCarty ExperimentAn experiment, announced in 1944, that showed that DNA (and not proteins) is the substance that causes Bacterial Transformation, and thus showing that DNA is the molecule that contains genetic material. Central Dogma of Molecular Biology, DNA, RNA, Transforming Principle, Hershey-Chase experiment


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