Harvey rat sarcoma virus
Haplopappus gracilis a species of flowering plant showing the lowest number of chromosomes (N = 2), and therefore studied by cytologists. See Appen- dix A, Dicotyledoneae, Asterales. haplophase the haploid phase of the life cycle of an organism, lasting from meiosis to fertilization. haplosis the establishment of the gametic chro- mosome number by meiosis. haplotype the symbolic representation of a spe- cific combination of linked alleles in a cluster of re- lated genes. The term is a contraction of haploid ge- notype and is often used to describe the combination of alleles of the major histocompatibility complex (q.v.) on one chromosome of a specific individual. Compare with phenogroup. hapten an incomplete antigen; a substance that cannot induce antibody formation by itself, but can be made to do so by coupling it to a larger carrier molecule (e.g., a protein). Complex haptens can react with specific antibodies and yield a precipitate; sim- ple haptens behave as monovalent substances that cannot form serological precipitates. haptoglobin a plasma glycoprotein that forms a stable complex with hemoglobin to aid the recycling of heme iron. In man this protein is encoded by a gene on chromosome 16. Hardy-Weinberg law the concept that both gene frequencies and genotype frequencies will remain constant from generation to generation in an infi- nitely large, interbreeding population in which mat- ing is at random and there is no selection, migration, or mutation. In a situation where a single pair of al- leles (A and a) is considered, the frequencies of germ cells carrying A and a are defined as p and q, respectively. At equilibrium the frequencies of the genotypic classes are p2 (AA), 2 pq (Aa), and q2 (aa). See illustration below and Appendix C, 1908, Hardy, Weinberg. harlequin chromosomes See 5-bromodeoxyuri- dine. Harvey rat sarcoma virus a virus carrying the on- cogene v-ras that was discovered in 1964 by J. J. Harvey and is homologous to its cellular proto-onco- gene c-ras. Human DNA sequences homologous to
The relationships between the frequencies of genes A or a and the frequencies of genotypes AA, Aa, and aa as predicted by the Hardy-Weinberg law.
the c-ras gene have been identified, and the homolog has been mapped to human locus 11p15.5. See ret- rovirus, T24 oncogene. HAT medium a tissue culture medium containing hypoxanthine, aminopterin, and thymidine. Mutant cells deficient in or lacking the enzymes thymidine kinase (TK−) and hypoxanthine-guanine-phosphori- bosyl transferase (HGPRT−) cannot grow in HAT medium because aminopterin blocks endogenous (de novo) synthesis of both purines and pyrimidines. Normal TK+ HGPRT+ cells can survive by utilizing the exogenous hypoxanthine and thymidine via the salvage pathway (q.v.) of nucleotide synthesis. HAT medium has been used to screen for hybridomas (q.v.) by mixing TK+ HGPRT− myeloma cells with antigen-stimulated TK− HGPRT+ spleen cells. The hybrid TK+ HGPRT+ clones that survive in HAT me- dium are then assayed for monoclonal antibodies specific to the immunizing antigen. See Appendix C, 1964, Littlefield; 1967, Weiss and Green. Hawaiian Drosophilidae about a thousand spe- cies of Drosophila living on the Hawaiian islands that have undergone a remarkable adaptive radiation (q.v.). Over one hundred of these species are much larger than familiar fruit flies and are called picture- winged because of the striking pattern of spots they have on their wings. Over 90% of them are single island endemics, and all are derived from one or two ancestral lines.
All have undergone parapatric speci- ation (q.v.). See Appendix A, 1985, Carson; courtship ritual, hot spot archipelago, mate choice, sexual selec- tion. Hayflick limit an experimental limit to the num- ber of times a normal animal cell seems capable of dividing; mouse and human cells divide 30 to 50 times before they enter the “crisis period” (q.v.). See Appendix C, 1961, Hayflick and Moorehead; marginotomy, passage number, telomerase. Hb hemoglobin. HbA symbolizes normal hemoglobin; HbF, fetal hemoglobin: HbS, sickle hemoglobin, etc. HbO2 oxyhemoglobin (q.v.). H chain See immunoglobulin. H-2 complex the major histocompatibility com- plex of the mouse lying in a segment of chromosome 17, which carries a number of polymorphic loci as- sociated with various aspects of the immune system. It consists of four regions (K, I, S, D) that contain genes coding for classical transplantation antigens, Ia antigens and complement components, as well as immune response genes. The I region is further sub- divided (see I region).
hCS somatomammotropin. See human growth hor- mone. headful mechanism a mechanism of packaging DNA in a phage head (e.g., T4) in which concatem- eric DNA is cut, not at a specific position, but rather when the head is filled. This mechanism accounts for the observations of terminal redundancy and cy- clic permutation in T4. heat in reproductive biology, that period of the sexual cycle when female mammals will permit coitus; estrus. heat-shock proteins proteins synthesized in Dro- sophila cells within 15 minutes after a heat shock. These proteins are named in accordance with their molecular weights (in kilodaltons). Hsp 70 is the major protein involved in tolerance to high tempera- tures in Drosophila. Similar proteins have been iso- lated from bacteria, fungi, protozoa, birds, and mammals, including humans.
It now appears that heat-shock proteins combine with target proteins and prevent their aggregation and denaturation at high temperatures, and they promote the normal re- folding of the target proteins when cells are returned to normal temperatures. Comparative studies of the base sequences of genes encoding heat-shock pro- teins from diverse organisms show that these genes have been highly conserved during evolution. See Appendix C, 1974, Tissiers et al.; 1975, McKenzie et al.; chaperones. heat-shock puffs a unique set of chromosomal puffs induced in Drosophila larvae exposed to ele- vated temperatures (for example, 40 minutes at 37°C). In D. melanogaster, there are nine heat-induc- ible puffs. Heat shock results in the transcription of a specific set of mRNAs by the genes that form the puffs in polytene chromosomes. In culture, heat- shocked Drosophila cells also transcribe these same mRNAs into specific heat-shock proteins. See Ap- pendix C, 1962, Ritossa. heat-shock response the transcriptional activity that is induced at a small number of chromosomal loci following exposure of cells to a brief period of elevated temperature. At the same time, other loci that were active prior to the heat shock are switched
off. This phenomenon appears to be universal, since it has been observed in Drosophila, Tetrahymena, sea urchin embryos, soybeans, chick fibroblasts, etc. See ubiquitin. heavy chain in a heteromultimeric protein, the polypeptide chain with the higher molecular weight (e.g., in an immunoglobulin molecule, the heavy chains are about twice the length and molecular weight of the light chains); abbreviated H chain; the smaller molecules are called light (L) chains. The heavy chain determines the class to which the im- munoglobulin belongs. heavy chain class switching the switching of a B lymphocyte from synthesis of one class of antibodies to another. For example, a B lymphocyte first syn- thesizes IgM, but it may later switch to the synthesis and secretion of IgG, and both antibodies will have the same antigen specificity. Thus, only the constant regions of the heavy chains differ between these two classes of antibodies.
Class switching of this type in- volves both somatic recombination prior to tran- scription and processing of transcripts by eliminating some segments and splicing the fragments. See so- matic recombination, V(D)J recombination. heavy isotope an atom, such as 15N, that contains more neutrons than the more frequently occurring isotope, and thus is heavier. heavy metals elements such as cobalt, nickel, manganese, copper, zinc, arsenic, antimony, mer- cury, lead, and bismuth, which are toxic pollutants found in the tailings left behind in gold, silver, and iron mines. The development of resistance to such heavy metals in plants growing in mine entrances is an example of recent evolution by natural selection. See Appendix C, 1952, Bradshaw. heavy-metal stain one of the elements of high atomic weight, often used as stains in electron mi- croscopy (U, Pb, Os, Mn). heavy water nontechnical term for deuterium oxide. hedgehog (hh) a segment polarity gene of Dro- sophila located at 3-81.2. This selector gene (q.v.) is responsible for the proper development of both wings and legs. Homologs of hh have been found in fish, birds, and mammals. See Sonic hedgehog (Shh).
HeLa cells an established cell line (q.v.) consisting of an aneuploid strain of human epithelial-like cells, maintained in tissue culture since 1951; originating from a specimen of tissue from a carcinoma of the cervix in a patient named Henrietta Lacks. H. Lacks eventually died from her cancer. However, her can- cer cells are subcultured in laboratories all over the world. It is estimated that the combined weight of these cells is now 400 times Henrietta’s adult body weight. HeLa cells have been used to investigate the cell cycle (q.v.), to develop the polio vaccine, and to study the behavior of human cells in the environ- ment of outer space. Because of sloppy subculturing, HeLa cells contaminate other cell lines in many lab- oratories throughout the world. See Appendix C, 1951, Gey; 1968, Gartler; 1995, Feng et al. Helianthus annuus the sunflower. helicase a DNA-binding protein that functions to unwind the double helix. Such unwinding is neces- sary at replication forks so that DNA polymerases can advance along single strands. Unwinding is also necessary for cut and patch repair (q.v.).
Loss of function mutations in genes that encode helicases can result in cancer or premature aging. For exam- ple, mutations of genes that encode enzymes belong- ing to the RecQ family of human DNA helicases cause Bloom syndrome and Werner syndrome (both of which see). See nucleolus, replisome, xeroderma pigmentosum. Helicobacter pylori a bacterium that is thought to infect the gastric mucosa of over half the adults over the age of 60 in industrial countries. Attachment of the parasite to the stomach wall is facilitated by re- ceptors on the surface of the mucous cells to which the bacterium binds. In humans most stomach ulcers are caused by H. pylori. It is but one of a dozen spe- cies of Helicobacter that inhabit the gastrointestinal tract of birds and mammals. Humans with AB anti- gens (q.v.) are less susceptible to gastric ulcers than type O individuals. See Appendix A, Bacteria, Pro- teobacteria; Appendix E; Lewis blood group. heliotropism a synonym for phototropism (q.v.). helitron a DNA segment that transposes as a roll- ing-circle transposon. Unlike other transposons (q.v.), helitrons lack terminal repeats and do not du- plicate the insertion sites of their hosts. They carry 5′ TC and 3′ CTTR termini and are always inserted between nucleotides A and T of the host DNA. Hel- itrons were uncovered by an in silico analysis of ge- nomic sequences from Arabidopsis, rice, and Caenor- habditis, and a helitron was later isolated as an insertion in a mutated Sh2 gene of Zea mays. See Appendix C, 2001, Kapitonov and Jura; rolling circle. helix a curve on the surface of a cylinder or cone that cuts all the elements of the solid at a constant angle. Applied especially to the circular helix on a