A small, highly conserved protein which derives its name from its widespread distribution – it is present in all eukaryotic cells and is located in both the nucleus and the cytoplasm. Ubiquitin becomes covalently attached to other intracellular proteins, thereby tagging them for breakdown.

The reflex that links an unconditional stimulus and an unconditional response, e.g. a series of neurons that links the presence of food in the mouth to secretion by the salivary glands.

The response evoked by an unconditional stimulus. An example is salivation triggered by the presence of food in the mouth.

A stimulus that evokes a response even in the absence of a history of conditioning, e.g. food in the mouth with regard to salivation.

One of the four nucleotide bases that occur in RNA. Uracil is not present in DNA (thymine (T) occurs in its place).