Biology Dictionary

Diastole: Relaxation of heart muscles.

Discontinuous variation: Variation which falls neatly into categories; one thing or the other, e.g. ABO blood groups. Tends to be controlled by single alleles, which are present or not.

Disease: Disfunction of the normal state of an organism, i.e. an irregularity from the normal structure / function of any part of an organism, as recognised by a characteristic set of signs and symptoms.

Dissociation: The splitting of one molecule into two or more molecules e.g. Oxyhameoglobin into oxygen and haemoglobin. Comes from ‘dis-association’ i.e coming apart.

DNA ligase: An enzyme which joins DNA strands, such as those cut by restriction enzymes

DNA polymerase: Enzyme which controls DNA replication by causing the addition of complementary bases on the exposed strands. 

Dominant: An allele which, if present, is shown in the phenotype

Down’s syndrome: A genetic condition cause by three chromosome 21s. It usually arises due to a fault in meiosis so that the eggs have both chromosome 21s instead of one.