| Term | Description | SeeAlso |
| Acid | There 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 base | Base, Alkali |
| Adapter Hypothesis | The 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 |
| Agar | an algae extract, rich in nutrients, used as a food source when growing various microbes (bacteria, yeast, etc.). | rich media, minimal media, algae |
| Allele | one of of the alternate forms of a single gene | Gene |
| Alpha Helix | A 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 |
| Amino Acid | an 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 proteins | Residue, Carbohydrate, Amino Acid. Protein, Lipid, Nucleic Acid |
| Antibody | An 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 |
| Antigen | A molecule or substance that triggers an immune response when recognized by the immune system. Examples of antigens include viruses and foreign bacteria. | Antigen, Antibody, Epitope |
| Apoptosis | Programmed cell death; some cells are genetically programmed to have a limited life span. Other cells undergo apoptosis if they are damaged. | Epithelial cells, Erythrocytes |
| Archaea | Single-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 |
| Autoclave | A 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 |
| Autosomal Dominant | A 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 Recessive | A 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 |
| Autosome | A chromosome that is not a sex chromosome. A chromosome that is not an X or Y chromosome. | X Chromosome, Y Chromosome |
| Auxotroph | An 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 |