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Icosahedron: A regular geometric polyhedron with 20 equilateral triangular faces and 12 corners. This is a particularly stable structure. The capsid of many phages and viruses are icosahedral.
Illegitimate recombination: An aberrant recombination event between non-homologous sequences that occurs in the absence of a known site-specific recombination system.
Immunological screening: The use of an antibody to detect a polypeptide encoded by a cloned gene.
Immunoassay: An immunological test to quantitate a particular protein using a antibody that binds specifically to the protein.
Incompatibility: The inability of two plasmids to stably coexist in the same cell.
Incompatibility group: A number of different types of plasmid, often related to each other, that are unable to stably coexist in the same cell.
Inducible: A regulatory system where the genes are only expressed under appropriate conditions (e.g. when the substrate is present or in specific environmental conditions).
Induction: The switching on of transcription in a repressed system due to the interaction between the inducer and a regulatory protein (e.g. the lac operon is induced by adding lactose or IPTG).
Inoculum: The initial sample of a microorganism added to a medium used to start a new culture.
Insert: A fragment of DNA integrated into a cloning vector.
Insertion sequence (IS): A transposable nucleotide sequence that only encodes the functions required for its own transposition. Insertion sequences are typically less than 5 kb.
Insertional inactivation: A cloning strategy where insertion of a piece of DNA into a vector inactivates a gene carried by the vector. Vector molecules with an insert can be identified by testing for the phenotype of this gene.
In situ hybridization: A technique for gene mapping involving hybridization of a labelled sample of a cloned gene to a large DNA molecule, usually a chromosome.
Intercalating agent: A planer molecule that can insert between two adjacent base pairs in a molecule of double-stranded DNA, distorting the architecture of the double helix. Intercalating agents often cause frameshift mutations. Examples include acridine orange and ICR19 1.
Intercalating dye: A dye which can insert between the bases of nucleic acids (e.g. ethidium bromide and acridine dyes. Intercalating dyes may be used to stain DNA or to induce frameshift mutation.
Intercalation: The insertion of flat polycyclic molecules between nucleotides in a DNA duplex. See Intercalating agent.
Intramolecular: Within the same molecule. For example, intramolecular transposition is the movement of a transposon from one site on a DNA molecule to a different site on the same DNA molecule.
Intron: A sequence of a gene which is transcribed but which is excised by a splicing reaction before the mature mRNA is translated. Common in eukaryotes, but rarer in prokaryotes.
Inversion: A DNA rearrangement where a sequence of nucleotides is in the reverse orientation relative to the rest of the molecule.
Inverted repeats: A DNA or RNA sequence where the sequence of nucleotides along one a strand of DNA is repeated in the opposite physical direction along the other strand; inverted repeats are commonly separated by a tract of non-repeated DNA. For example: 3' CTAG . . . GTAC 5' 5' GATC . . . CATG 3'

In vitro: Reactions that take place outside of the cell; in a test tube. Literally translated from Latin the term means 'in glass' but nowadays many reactions are carried out in plastic vessels but there is no term 'in plastico' since plastic wasn't known to the Romans!
In vitro mutagenesis: A method for mutating DNA outside of a host cell. Examples include, hydroxylamine mutagenesis of DNA packaged into phage particles or site-directed mutagenesis of plasmid DNA.
In vitro packaging: Synthesis of infective phage particles from a preparation of phage capsid proteins and a concatamer of phage DNA molecules. Commonly used to package DNA cloned onto a lambda vector (separated by cos sites) into infectious lambda particles.
In vivo: Reactions that take place inside the cell. Literally translated from Latin the term means 'in life', i.e. inside a living cell.
Isogenic: Two strains that are genetically identical except for a single mutation.