Icosahedron
A regular geometric polyhedron with 20 equilateral trangular 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.

Immunity
(i) The resistance of a lysogen to superinfection by a phage with a similar regulatory mechanism (also called homoimmunity). For example, expression of the cI repressor by a lambda prophage prevents expression of genes in a secondary infection if the cI protein can bind to the regulatory region of the superinfecting phage.
(ii) An adaptive antibody or cellular response against specific microbial infections.

Immunity region
The region of a lysogenic phage that encodes the major regulator of lysogeny and the cognate DNa binding sites (e.g. lambda cI protein and the leftward and rightward operator sites).

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.

Indel
A region of DNA that is present on the chromosome of an organism (Insertion) but absent from closely related organisms (Deletion).

Inducer
A chemical or physical agent that turns on gene expression. Usually refers to an agent that alters repressor-operator interactions, often by decreasing DNA-binding.

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
(i)The switching on of transcription in a repressed system due to the interaction between the inducer and a regulatory protein. For example, the lac operon is induced by adding lactose or IPTG.
(ii) Also used to describe a condition that causes a lysogen to begin lytic growth, as occurs when the cI repressor of phage lambda is inactivated following DNA damage.

Initiation factors
Proteins that promote the binding of ribosomes and the initiator tRNA to mRNA to begin the process of translation.

Inoculum
The initial sample of a microorganism added to a medium used to start a new culture.

Informational suppressor
A second mutation that restores function of a primary mutatioin by altering the way the gene is expressed. Examples of this class of suppressors include mutations that alter a tRNA allowing it to misread a nonsense (stop) codon or a frameshift mutation, or mutations in ribosomal genes that affect the fidelity of translation.

Insert
A fragment of DNA integrated into a cloning vector.

Insertion element (IS)
A transposable nucleotide sequence that only encodes the functions required for its own transposition. Insertion elements are typically less than 5 kb. Synonymous with "insertion sequence".

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. Synonymous with "insertion element".

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
A Latin phrase meaning "in the original place". Commonly used to describe a process that visualizes the position of a biological molecule in a cell.

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), often within a cell.

Integron
DNA fragments that serve as sites for insertion of a variety of other DNA fragments (especially antibiotic resistance genes) and facilitate their transfer into other cells.

Intein
Selfish DNA elements located within coding regions that are translated with the interrupted protein, but then catalyze their own excision and the formation of a peptide bond between the flanking protein regions.

Interaction suppressor
A mutation at a second site that restores an interaction that was disrupted by a primary mutation. Interaction suppression can occur due to intragenic suppressor mutations (within the same gene that restore interactions of between domains of the protein) or intergenic suppressor mutations (mutations in two different genes that restore protein-protein interactions).

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 ICR191.

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.

Intragenic complementation
The ability of two mutant forms of a gene, neither of which produce an active gene product, to produce a functional or partially functional gene product. Intragenic complementation is rare, typically only observed for specific missense alleles of a multimeric protein.

Intragenic suppressor
Restoration of partial or complete wild-type phenotype by second mutation in the same gene as a primary mutation which resulted in a negative phenotype. For example, a +1 frameshift mutation located a short distance downstream from a -1 frameshift mutation can restore the reading frame of a protein and thereby allow production of a functional product. Intragenic suppression can also occur via two missense mutations in a single reading frame.

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.

Intergenic suppressor
Restoration of partial or complete wild-type phenotype by second mutation in a different gene than a primary mutation which resulted in a negative phenotype. Intergenic suppression can occur in many ways, for example: A nonsense mutation that inactivates a gene may be suppressed by a second mutation that produces a nonsense suppressor tRNA. A mutation in a gene encoding a protein which must interact with a second protein may disrupt the interaction resulting in a negative phenotype; a compensating mutation in the gene encoding the second protein may restore the protein-protein interactions. Alternatively, the primary mutation may make a protein very sensitive to proteolysis, so there is a not sufficient amount of the protein available to fulfill its function; a mutation that inactivates a specific protease may allow accumulation of the first function, restorying the phenotype.

Intron
A sequence of a gene which is transcribed but which is excised by a splicing reaction before the mature mRNA is translated. Found in Eukarya, Archae, Bacteria, eukaryotic viruses, and phage. Introns are 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.

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.

IPTG
An abbreviation for isopropyl--D-thiogalactoside. A gratuitous inducer of the lac operon. That is, it can induce expression of the lac operon but it is not a substrate for the lac gene products.

IS element
An insertion sequence. A transposable element that only carries the genes required for its own replication. IS elements are usually less than 5 Kb. Also called an IS sequence.

Island
A large region of DNA that is present on the chromosome of an organism but absent from closely related organisms.

Islet
A smaller genetic island that only encodes one or a few gene products.

Isogenic
Two strains that are genetically identical except for a single mutation.

Iteron
Repetitive sequences in the origin region of some plasmids that are involved in the control of replication .