Insertion site - a unique restriction site in a vector DNA molecule into which foreign DNA can be inserted. The term is also used to describe the position of integration of a transposon or insertion site element.

Intercalary growth - a pattern of stem elongation typical of grasses. Elongation proceeds from the lower internodes to the upper internodes through the differentiation of meristematic tissue at the base of each internode.

Intercalating agent - a chemical capable of inserting between adjacent base pairs in a DNA molecule.
Intercellular space - ipore space between cells, especially typical of leaf tissues.

Isoform - a member of a family of closely related proteins that have some amino acid sequences in common and some different.

Isogamy - sexual reproduction involving the production fusion of gametes that are similar in size and structure.

Isogenic stocks - strains of organisms that are genetically identical; completely homozygous.

Isolating mechanism - any of the biological properties of organisms that prevent interbreeding (and therefore exchange of genetic material) between members of different species that inhabit the same geographical area.

Isolation medium - an optimum medium suitable for explant survival, growth and development.

Isoleucine - see amino acid.

Internode - the region of a stem between two successive nodes.

Isotope - one of two or more forms of an element that have the same, number of protons (atomic number) but differing numbers of neutrons (mass numbers). Radioactive isotopes are commonly used to make DNA probes and metabolic tracers.

Isozyme - a variant of a particular enzyme. In general, all the isozymes of a particular enzyme have the same function and sometimes the same activity, but they differ in amino-acid sequence. With the help of isozyme analysis, based on electrophoresis techniques that can separate the different variants, related species or cultivars can be distinguished.