Degeneracy: In reference to the genetic code, the condition in which more than one codon specifies the same amino acid.

Degenerate: With reference to the genetic code, having more than one codon that specifies a certain amino acid.

Dephosphorylation: Removal of phosphate from a molecule by hydrolysis. The process of protein dephosphorylation causes an alteration in activity, often through change in shape.

Depolarization: A change to a less negative membrane potential. It is often used in reference to neurons. When their resting potential goes from -70 mV to 0 mV, for instance, the cell is considered depolarized. Depolarization of neurons often generates action potentials that can lead to the release of neurotransmitter.

Desmosomes: Electron-dense cell membrane junctions often found between epithelial cells such as keratinocytes. They are found in tissues where tensile strength is important. Desmosomes often have keratin filaments that connect them to interior parts of the cell as well as indirectly to other desmosomes. A hemidesmosome connects cells to the basal lamina.

Determination: A term often used in the study of development. It refers to the ability of a cell to commit to a specific pathway of differentiation.

Diacylglycerol (DAG): A molecule consisting of glycerol linked to two fatty acid chains. It is a lipid created by the splitting of inositol phospholipids in response to an extracellular stimulus. DAG then activates protein kinase C. Phorbol esters are chemical mimics of DAG and can cause cancer in animal cells.

Differentiation: A process that occurs during development whereby a precursor or stem cell undergoes gene activation, resulting in a highly specialized cell.

Dipoles: Small charge separations within the same molecule due to a large difference of electronegativity between two atoms that make up the molecule.

Disulfide Bond: A covalent linkage between cysteine residues in two different proteins or in different parts of the same protein.

DNA polymerases: A family of enzymes that polymerize DNA. They use the template strand to form a new complementary strand. Deoxyribonucleotides are used as precursors and added one at a time in the proper base-pairing sequence. A pyrophosphate is released as a consequence.

Domains: Areas of common function found in a number of proteins or other molecules (e.g., the protein kinase domain). The term is useful in comparing similar or homologous domains between molecules.

Double Bond: A type of covalent bond formed by equal sharing of two pairs of electrons between two atoms.