integration efficiency the frequency with which a foreign DNA segment is incorporated into the geno- type of a recipient bacterium, particularly with ref- erence to transformation. integrins a large family of cell-surface receptor proteins that bind to the components of the extra- cellular matrix and function as cellular “glue,” facili- tate cellular migrations (in embryological develop- ment or in cells of the immune system of adults), and activate signal transduction pathways within cells. Integrins are structurally related to one an- other, and one or more of them appear on virtually every cell type in the animal kingdom. Integrins con- sist of two protein chains (α, β). There are at least 15 variants of the α chain and 8 of the β chains, which combine into at least 20 functional dimers (e.g., α2β1, αIIβ3).
The family name is apropos be- cause of the importance of these molecules to the structural integrity of cells and tissues and because of their integrative functions among the diverse sig- nals that impinge on cells. The extracellular matrix consists primarily of gel-like chains of sugars and in- terconnected fibrous proteins (including laminin, fibronectin, and collagen). Integrins are connected intracellularly via one or more intermediary mole- cules (e.g., talin, vinculin, paxillin, tensin) in a focal adhesion complex to actin molecules of the cy- toskeleton. Most integrins interact with the extracel- lular matrix, but some participate in adhesion be- tween cells. The molecules responsible for most cell- to-cell adhesions belong to such groups as the cad- herin, selectin, and immunoglobulin families. Some microbes enter cells at least in part by attaching to integrins. integron any transposon responsible for the hori- zontal transfer of genes between different species of bacteria.
Integrons are DNA segments that (1) en- code a site-specific recombinase, (2) include integ- rase-specific recombination sites, and (3) include a promoter that expresses one or more genes that con- fer antibiotic resistance or other adaptive traits on the host. An integron is often incorporated within a plasmid (q.v.). See R (resistance) plasmid. intein an internal protein sequence that is trans- lated in-frame in a precursor polypeptide and is ex- cised during protein splicing (q.v.).
intelligence quotient (IQ) an individual can be as- signed to a “mental age” group on the basis of perfor- mance on standardized intelligence tests. This men- tal age divided by the individual’s chronological age and multiplied by 100 is the IQ. intelligence quotient classification according to the Binet-Simon classification, intelligence quotients can be grouped as follows: genius, 140 and over; very superior, 120-139; superior, 110-119; average, 90-109; dull, 80-89; borderline, 70-79; mild retar- dation (moron), 50-69; moderate retardation (imbe- cile), 25-49; and severe retardation (idiot), 0-24. interallelic complementation referring to the change in the properties of a multimeric protein as a consequence of the interaction of subunits coded by two different mutant alleles (in contrast to the protein consisting of subunits derived from a single mutant allele). The mixed protein (heteromultimer) may exhibit more activity (positive complementa- tion) or less activity (negative complementation, q.v.) than the homomultimer.
Also known as intra- genic complementation, allelic complementation. interbands the regions between bands in a poly- tene chromosome (q.v.). The DNA concentration in interbands is only a fraction of that in bands. intercalary deletion See deletion. intercalating agent a substance (e.g., acridine dyes) that inserts between base pairs in a DNA mol- ecule, often disrupting the alignment and pairing of bases in the complementary strands. By causing ad- dition or deletion of one or more base pairs during replication, a reading frame shift (q.v.) is often in- duced. See proflavin. intercellular between cells.
interchange an exchange of segments between nonhomologous chromosomes resulting in transloca- tions. interchromosomal translocation See transloca- tion. intercistronic region the segment between the termination codon of one gene and the initiation co- don of the next gene in a polycistronic transcription unit. See spacer DNA. intercross mating of heterozygotes (a/+ × a/+). interference See positive interference. interference filter a filter used to produce a mono- chromatic light source. interference microscope like the phase micro- scope, the interference microscope is used for ob- serving transparent structures. However, with the interference microscope quantitative measurements of the relative retardation of light by various objects can be made. Such measurements can be used to de-
termine the dry mass per unit area of specimen or the section thickness. interferons (IFNs) a family of small glycoproteins produced by mammalian cells, often in response to viral infections. Type 1 IFNs are monomeric proteins that are produced by a wide variety of virus-infected cells. These IFNs induce synthesis of enzymes that inhibit viral proliferation. Type 2 IFNs consist of di- mers of identical proteins unrelated to Type 1 IFNs. Type 2 IFNs are synthesized by T lymphocytes and natural killer cells and function to destroy certain cancer cells and cells infected by parasites. See Ap- pendix C, 1977, Gilbert; leader sequence peptide, lymphocytes. intergenic suppression See suppression. interkinesis the abbreviated interphase between the first and second meiotic division. No DNA repli- cation occurs during interkinesis, unlike a premitotic interphase. interleukins a group of at least 15 soluble pro- teins, secreted by leukocytes, that function to pro- mote the growth and differentiation of cells of the immune system. The different interleukins are desig- nated IL1, IL2, etc., in order of their discovery.
Most interleukins are products of single genes. Some ILs consist of two amino acid chains, but these result from the posttranslational cleavage of a single pre- cursor protein. There is one exception, IL12, which has two chains (p35 and p40), each encoded by a different gene. intermediary metabolism the chemical reactions in a cell that transform ingested nutrient molecules into the molecules needed for the growth of the cell. intermediate filaments cytoplasmic filaments with diameters between 8 and 12 nanometers. They com- prise a heterogeneous class of cytoskeletal proteins. In general, a given class of intermediate filaments is characteristic of a specific cell type. For example, keratin filaments are characteristic of epithelial cells, neurofilaments of neurons, vimentin filaments of fi- broblasts, and desmin filaments of glial cells. intermediate host a host essential to the comple- tion of the life cycle of a parasite, but in which it does not become sexually mature. intermedin a polypeptide hormone from the inter- mediate lobe of the pituitary gland that causes dis- persion of melanin in melanophores.
Also called melanocyte-stimulating hormone or MSH. internal radiation the exposure to ionizing radia- tion from radioelements deposited in the body tis- sues. interphase the period between succeeding mito- ses. See cell cycle. interrupted genes See split genes. interrupted mating experiment a genetic experi- ment in which the manner of gene transfer between conjugating bacteria is studied by withdrawing sam- ples at various times and subjecting them to a strong shearing force in an electric blender. See Appendix C, 1955, Jacob and Wollman; Waring blender. intersex a class of individuals of a bisexual species that have sexual characteristics intermediate be- tween the male and female. See Appendix C, 1915, Goldschmidt. interspecific heterokaryons cells containing nu- clei from two different species produced by cell fu- sion (q.v.). See Appendix C, 1965, Harris and Wat- kins. interspersed elements See repetitious DNA. interstitial cells cells that lie between the testis tubules of vertebrates and secrete testosterone. intervening sequence See intron. intra-allelic complementation See allelic comple- mentation.
intrachromosomal aberration See translocation. intrachromosomal recombination sister chroma- tid exchange (q.v.). intrachromosomal translocation See translocation. intragenic complementation See interallelic com- plementation. intragenic recombination recombination between mutons of a cistron. Such recombination is charac- terized by negative interference and by nonrecipro- cality (recovery of either wild-type or double-mutant recombinants, but not both from the same tetrad). intragenic suppression See suppression. intrasexual selection See sexual selection. introgression See introgressive hybridization. introgressive hybridization the incorporation of genes of one species into the gene pool of another. If the ranges of two species overlap and fertile hy- brids are produced, they tend to backcross with the more abundant species. This process results in a population of individuals, most of which resemble the more abundant parents but that also possess some characters of the other parent species. Local
habitat modification can lead to mixing of previously distinct gene pools. Introduced species (or subspe- cies) can generate extinction of the older species by hybridization and introgression. The use of molecu- lar markers has greatly increased the ability to detect and quantify interspecific gene exchanges. For ex- ample, phylogenetic trees based on chDNAs have shown many examples of both recent and ancient exchanges of chloroplasts between sympatric spe- cies. See chloroplast DNA (chDNA), wolf. intromittent organ any male copulatory organ that implants sperm within the female. intron in split genes (q.v.), a segment that is tran- scribed into nuclear RNA, but is subsequently re- moved from within the transcript and rapidly de- graded. Most genes in the nuclei of eukaryotes contain introns.
The number of introns per gene var- ies greatly, from one in the case of rRNA genes to more than 30 in the case of the yolk protein genes of Xenopus. Introns range in size from less than 100 to more than 10,000 nucleotides. There is little se- quence homology among introns, but there are a few nucleotides at each end that are nearly the same in all introns. These boundary sequences participate in excision and splicing reactions. The first introns of some genes have been shown to contain tissue-spe- cific enhancers. The splicing reactions involving the introns of nuclear mRNAs take place within a splice- osome (q.v.).
However, the introns of mitochondrial and chloroplast DNAs are self-splicing. See Appen- dix C, 1977, Roberts and Sharp; 1978, Gilbert; 1983, Gillies et al.; alternative splicing, Caenorhab- ditis elegans, enhancers, Euglena gracilis, exon, GT- AG rule, posttranscriptional processing, R-loop map- ping, splice junctions, transcription unit. intron dynamics the lengthening or shortening of the non-coding regions of specific genes during their evolution.
A pairwise alignment of about 6,000 or- thologous genes demonstrates that the equivalent in- trons in genes of Drosophila are only half the length of Anopheles introns, whereas the exon lengths and intron frequencies are similar in both insects. There- fore a change in intron lengths has occurred during the period since the divergence of fruit flies and mosquitoes from a common dipteran ancestor (about 250 million years ago). This difference in in- tron lengths explains why the A. gambiae genome is larger than the D. melanogaster genome (278 vs 180 mbp), although both species have 13,000-14,000 genes. See Appendix C, 2002, Holt et al. intron intrusion the disruption of a preexisting gene by the insertion of an intron into a functional gene. Intron intrusion and the exon shuffling (q.v.) along with junctional sliding (q.v.), have been pro- posed as mechanisms for evolutionary diversification of genes. intron-mediated recombination See exon shuf- fling. intron origins two conflicting hypotheses have been proposed to explain the origin of introns. The introns early hypothesis assumes that the DNA mole- cules in which genes originated initially contained random sequences of nucleotides.
The random dis- tribution of stop codons permitted only short read- ing frames to accumulate. Next, a mechanism arose that allowed splicing out regions containing stop co- dons from the primary message, and so proteins of greater length and with more useful biochemical functions could be translated and selected. The orig- inal short reading frames became the exons of pres- ent-day genes, while the introns represent segments containing splice junctions originally designed to re- move deleterious stop signals. The introns late hy- pothesis assumes that genes arose from short reading frames that grew larger by duplications and fusions.
Introns arose secondarily as a result of insertions of foreign DNA into these genes. Thus, present-day in- trons are the descendants of ancient transposons (q.v.). intussusception 1. the growth of an organism by the conversion of nutrients into protoplasm. 2. the deposition of material between the microfibrils of a plant cell wall. 3. the increase in surface area of the plasmalemma by intercalation of new molecules be- tween the existing molecules of the extending mem- brane. in utero within the uterus. inv See symbols used in human cytogenetics. in vacuo in a vacuum. invagination an inpocketing or folding in of a sheet of cells or a membrane. inversion chromosome segments that have been turned through 180° with the result that the gene sequence for the segment is reversed with respect to that of the rest of the chromosome. Inversions may include or exclude the centromere. An inversion