that includes the centromere is called pericentric or heterobrachial, whereas an inversion that excludes the centromere is called paracentric or homobrachial. Paracentric inversions are found more often in na- ture than pericentric inversions. A paracentric inver- sion heterozygote forms a reverse loop pairing con- figuration during pachynema. inversion heterozygote an organism in which one of the homologs has an inverted segment while the other has the normal gene sequence. The results of single and double exchanges within an inversion het- erozygote are shown on page 237. Note that no mo- nocentric, single-crossover chromatids are produced. For this reason, inversions give the impression of be- ing crossover suppressors, and it was their action on crossing over that led to their discovery.
See Appen- dix C, 1926, Sturtevant; 1933, McClintock; 1936, Sturtevant and Dobzhansky. invertebrate an animal without a dorsal column of vertebrae; nonchordate metazoans. inverted repeats (IR) two copies of the same DNA sequence orientated in opposite directions on the same molecule. IR sequences are found at oppo- site ends of a transposon (q.v.). See palindrome. inverted terminal repeats short, related, or identi- cal sequences oriented in opposite directions at the ends of some transposons (q.v.). in vitro designating biological processes made to occur experimentally in isolation from the whole or- ganism; literally “in glass,” i.e., in the test tube. Ex- amples: tissue cultures, enzyme-substrate reactions. Contrast with in vivo, ex vivo. in vitro complementation See allelic complemen- tation. in vitro evolution experiments designed to study the evolution of self-duplicating nucleic acid mole- cules outside of living cells.
A classic example is a study that involved the synthesis of RNA molecules using Qβ replicase and the RNA genome of Qβ phage. Serial transfer experiments were performed in which the intervals of synthesis were adjusted to select the earliest molecules completed. As the ex- periment progressed, the rate of RNA synthesis in- creased, while the product became smaller. By the 74th transfer, an RNA molecule had evolved that was only 17% of its original size and constituted the smallest known self-duplicating molecule. While it had a very high affinity for the replicase, it was un- able to direct the syntheses of viral particles. Ribo- zymes (q.v.) have also been shown to undergo in vitro evolution. See Appendix C, 1967, Spiegelmann, Mills, and Peterson; 1973, Mills, Kramer, and Spie- gelmann; 1995, Wilson and Szostak. in vitro fertilization experimental fertilization of an egg outside the female body.
In humans, this is usually done because the woman’s Fallopian tubes are blocked. The resulting embryo can then be in- serted into the uterus for implantation. See embryo transfer. in vitro marker a mutation induced in a tissue cul- ture that allows subsequent phenotypic detection. Human in vitro markers include genes conferring re- sistance to various viruses, aminopterin, and purine analogs. in vitro mutagenesis experiments in which seg- ments of genomic DNA are treated with reagents that produce localized chemical changes in the mol- ecule. The subsequent ability of the mutated mole- cules to function during replication, transcription, etc., is assayed either by using cell-free systems or in vivo, after splicing the fragment into an appropriate plasmid. in vitro packaging the production of infectious particles from naked DNA by incapsidation of the DNA in question after supplying lambda phage packaging proteins and preheads. in vitro protein synthesis the incorporation in a cell-free system of amino acids into polypeptide chains.
See Appendix C, 1976, Pelham and Jackson. in vivo within the living organism. Contrast with in vitro, ex vivo. in vivo culturing of imaginal discs the technique developed by Hadorn in which an imaginal disc is removed from a mature Drosophila larva, cut in half, and the half organ implanted into a young larva. Here regenerative growth occurs, and once the host larva has reached maturity the implant is removed once again, bisected, and one of the halves trans- planted to a new host.
By multiple repetition of this procedure, the cells are subjected to an abnormally long period of division and growth in a larval envi- ronment. If the regenerated disc is finally allowed to undergo metamorphosis, it shows an abnormally high probability of producing structures characteris- tic of different discs. A regenerated genital disc may produce antennae, for example. Hadorn terms such differentiation allotypic. Since the allotypic organs appear in the offspring of cells that were previously determined to form genital structures, a change in determination must be postulated. This event is called transdetermination. See Appendix C, 1963, Hadorn.
in vivo marker a naturally occurring mutant mam- malian gene that allows phenotypic detection of the tissue-cultured cells bearing it. Examples are the genes causing galactosemia and glucose-6-phosphate dehydrogenase deficiency in man, and the genes pro- ducing certain cell surface antigens in the mouse. iodine a biological trace element. Atomic number 53; atomic weight 129.9044; valence 1−; most abun- dant isotope 127I, radioisotopes 125I (half-life 60 days) and 131I (half-life 8 days), radiations: beta particles dehydrogenase deficiency in man, and the genes pro- monly used in radioimmunoassay (q.v.).
iojap a mutant nuclear gene in maize that induces changes in chloroplast characters. The mutant plas- tids behave autonomously thereafter. ion exchange column a column packed with an ion exchange resin. See chromatography, column chromatography. ion exchange resin a polymeric resin that has a higher affinity for some charged groups than it has for others. For example, resins with fixed cation groups will bind anions and thus can be used in col- umn separation procedures. See molecular sieve. ionic bond electrostatic bond (q.v.).
ionization any process by which a neutral atom or molecule acquires a positive or negative charge. ionization chamber any instrument designed to measure the quantity of ionizing radiation in terms of the charge of electricity associated with ions pro- duced within a defined volume. ionization track the trail of ion pairs produced by an ionizing radiation during its passage through matter. ionizing energy the average energy lost by an ionizing radiation in producing an ion pair in a given gas. The average ionizing energy for air is about 33 eV. ionizing event the occurrence of any process in which an ion or group of ions is produced.
ionizing radiation electromagnetic or corpuscular radiation that produces ion pairs as it dissipates its energy in matter. ionophores a class of antibiotics of bacterial origin that facilitate the movement of monovalent and di- valent cations across biological membranes. Some of the major ionophores and the ions they transport are valinomycin (K+, Rb+), A 23187 (Ca++, 2H+), nigeri- cin (K+, H+), and gramicidin (H+, Na+, K+, Rb+). ion pair the electron and positive atomic or molec- ular residue resulting from the interaction of ioniz- ing radiation with the orbital electrons of atoms. IPTG isopropylthiogalactoside; a gratuitous in- ducer for the E. coli lac operon (q.v.). See ONPG. IQ intelligence quotient (q.v.).
I region one of the central regions of the major histocompatibility complex (H-2) of the mouse. It contains genes coding for Ia antigens and controlling various immune responses. It has five subregions (A, B, J, E, C) and may be the equivalent of the D/DR region of the human major histocompatibility com- plex. See HLA complex. IR inverted repeat (q.v.). Ir gene See immune response gene. iron a biological trace element. Atomic number 26; atomic weight 55.847; valence 2,3+; most abun- dant isotope 56Fe; principal radioisotope 59Fe, half- life 46 days, radiation emitted—beta particle. isauxesis See allometry, heterauxesis. IS element See insertion sequences. islets of Langerhans clusters of hormone-secre- ting cells located in the pancreas of vertebrates. Two types of cells are found: alpha cells, which secrete glucagon (q.v.), and beta cells, which secrete insulin (q.v.).
isoacceptor transfer RNA one of a group of differ- ent tRNAs that accept the same amino acid but pos- sess different anticodons. Higher organisms contain two to four isoacceptor tRNAs for certain amino acids. See amino acid. isoagglutinin an antibody directed against anti- genic sites on the red blood corpuscles of the same species and that causes agglutination. isoagglutinogen an antigenic factor on the surface of cells that is capable of inducing the formation of homologous antibodies (isoagglutinins) in some members of the same species. isoallele an allele whose effect can only be distin- guished from that of the normal allele by special tests. For example, two + alleles +1 and +2 may be indistinguishable (i.e., +1/+1, +2/+2, and +2/+1 individ- uals are phenotypically wild type).
However, when compounded with a mutant allele a, +1 and +2 prove to be distinguishable (i.e., a/+1 and a/+2 individuals are observably different). isoanisosyndetic alloploid an allopolyploid in which some chromosomes derived from both spe- cies are homoeologous and undergo a limited synap- sis. See isosyndetic alloploid. isoantibody an antibody formed in response to immunization with tissue constituents derived from an individual of the same species as the recipient. isocapsidic viruses See segmented genome. isochore a segment of DNA that has a uniform base composition that is different from adjacent seg- ments. The DNA of vertebrates and plants are mosa-
ics of such isochores. In humans, isochores are about 300 kb in length and consist of five classes. The AT- rich isochores are called L1 and L2, and the GC-rich isochores are H1, H2, and H3. Although H3 makes up only 3% of the total DNA, it contains over 25% of the ORFs. See major histocompatibility complex (MHC). isochromatid break an aberration involving breaks in both sister chromatids at the same locus, followed by lateral fusion to produce a dicentric chromatid and an acentric fragment. isochromosome a metacentric chromosome pro- duced during mitosis or meiosis when the centro- mere splits transversely instead of longitudinally.
The arms of such a chromosome are equal in length and genetically identical. However, the loci are posi- tioned in reverse sequence in the two arms. isocoding mutation a point mutation that alters the nucleotide sequence of a codon but, because of the degeneracy of the genetic code, does not change the amino acid that the codon specifies. isoelectric point the pH at which the net positive and negative charge on a protein is zero. isoenzymes isozymes (q.v.).
isofemale line a genetic lineage that began with a single inseminated female. isoforms families of functionally related proteins that differ slightly in their amino acid sequences. Such proteins may be encoded by different alleles of the same structural gene. The D and d isoforms encoded by the MC1R gene (q.v.) are examples. Other proteins are encoded by genes that are now located at different chromosomal positions but are believed to be derived from a single ancestral gene. Isoforms may also be generated by mRNAs tran- scribed from different promoters located in the same gene or by alternative splicing (q.v.).
See actin, actin genes, fibronectin, multigene family, muscular dystro- phy, myosin, myosin genes, tropomyosin, tubulin. isogamy that mode of sexual reproduction involv- ing sex cells of similar size and morphology but op- posite mating types. See anisogamy. isogeneic referring to a graft involving genetically identical donor and host; an isograft. isogenic genetically identical (except possibly for sex); coming from the same individual or from a member of the same inbred strain. isograft a tissue graft between two individuals of identical genotype. isohemagglutinin isoagglutinin (q.v.).
isoimmunization antibody formation in reaction to antigens of the same species. isoionic point isoelectric point (q.v.). isolabeling labeling of both, or parts of both, daughter chromatids at the second metaphase after one replication in tritiated thymidine, as a result of sister chromatid exchange. In the absence of sister chromatid exchange, both daughter chromatids are labeled at metaphase I, but only one is labeled at metaphase II. isolate a segment of a population within which as- sortative mating occurs. isolating mechanism a cytological, anatomical, physiological, behavioral, or ecological difference, or a geographical barrier that prevents successful mat- ing between two or more related groups of organ- isms. See postzygotic isolation mechanism, prezygotic isolation mechanism, Wallace effect. isolecithal egg one in which the yolk spheres are evenly distributed throughout the ooplasm. See cen- trolecithal egg, telolecithal egg. isoleucine See amino acid. isologous synonymous with isogeneic (q.v.). isologous cell line cell lines derived from identi- cal twins or from highly inbred animals.
isomerases a heterogeneous group of enzymes that catalyze the transfer of groups within molecules to yield isomeric forms. An example would be ra- cemase, which interconverts D-lactic acid and L-lac- tic acid. isomers compounds with the same molecular for- mula but with different three-dimensional molecular shapes or orientations in space. isometry isauxesis. isomorphous replacement a technique that allows specific atoms in a complex molecule to be replaced with atoms of higher atomic number. Dif- ferences can then be seen in the intensities of spe- cific spots on diffraction patterns when crystals of these molecules are x-rayed. See x-ray crystallog- raphy. isonymous marriage marriage between persons with the same surname. Isonymous marriages are used as indications of consanguinity in population genetics.
isophene a line on a map which connects points of equal expression of a character that varies clinally. isoprenoid lipid a family of lipid molecules made up of linear arrays of multiple isoprene units. Iso- prene has the formula
The fat-soluble vitamins (A, D, K, and E) contain multiple isoprene units. isopropylthiogalactoside a gratuitous inducer of the lac operon (q.v.); abbreviated IPTG. isopycnic having the same density; used to refer to cell constituents having similar buoyant densities. See centrifugation separation. isopycnotic referring to chromosomal regions or entire chromosomes that are not heteropycnotic, that is, are the same in appearance as the majority of the chromosomes. See heteropycnosis. isoschizomers two or more restriction endonucle- ases (q.v.) isolated from different sources that cleave DNA within the same target sequences.
isosyndetic alloploid an allopolyploid where syn- apsis is restricted to the homologs derived from one species. See isoanisosyndetic alloploid. isotonic solution a solution having the same os- motic pressure as another solution with which it is compared (usually blood or protoplasm).
isotope one of the several forms of a chemical ele- ment. Different isotopes have the same number of protons and electrons, but differ in the number of neutrons contained in the atomic nucleus. Hence they have identical chemical properties, but differ in atomic weights. See Appendix C, 1942, Schoen- heimer; radioactive isotope. isotopic dilution analysis a method of chemical analysis for a component of a mixture. The method is based on the addition to the mixture of a known amount of labeled component of known specific ac- tivity, followed by isolation of a quantity of the com- ponent and measurement of the specific activity of that sample.
isotopically enriched material material in which the relative amount of one or more isotopes of a constituent has been increased. isotropic See anisotropy. isotype exclusion synthesis of only kappa or lambda light chains by a given plasma cell as a conse- quence of allelic exclusion (q.v.). See immunoglob- ulin. isotypes antigenic determinants shared by all indi- viduals of a given species, but absent in individuals of other species. Compare with allotypes, idiotypes. isozymes multiple forms of a single enzyme. While isozymes of a given enzyme catalyze the same reaction, they differ in properties such as the pH or substrate concentration at which they function best. Isozymes are complex proteins made up of paired polypeptide subunits.
The lactic dehydrogenases, for example, are tetramers made up of two polypeptide units, A and B. Five isozymes exist and can be sym- bolized as follows: AAAA, AAAB, AABB, ABBB, and BBBB. Isozymes often have different isoelectric points and therefore can be separated by electropho- resis. The different monomers of which isozymes like lactic dehydrogenase are composed are specified by different gene loci. The term allozyme is used to refer to variant proteins produced by allelic forms of the same locus. See allozymes. iteroparity repeated periods of reproduction dur- ing the life of an individual. Compare with semel- parity. IVS intervening sequence. See intron.