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habituation  The phenomena that, after a number of sub-cultures, cells can grow, without the addition of specific factors, such as no longer needing exogenous growth regulators in the tissue culture medium. Such cells are autonomous.

haemoglobin  Conjugated protein compound containing iron, located in erythrocytes of vertebrates; important in the transportation of oxygen to the cells of the body.

haemolymph  The mixture of blood and other fluids in the body cavity of an invertebrate.

hair  A single or multicellular, absorptive (root hair) or secretory (glandular hair) and sometimes only a superficial outgrowth (covering hair) of the epidermal cells. The term trichome is often used but includes outgrowths from tissue below the epidermal layer. Distinguishing between hairs and trichomes can be difficult. Trichomes are usually connected to the vascular system, whereas hairs lack a vascular connection.

hairy root culture  A fairly recent development in plant culture, consisting of highly branched roots of a plant. A plant tissue is treated with a culture of the bacterium Agrobacterium rhizogenes, which transfers part of its own plasmid DNA to the cells of an infected plant. This alters the plant's metabolism, including alterations in hormone levels, which in turn cause the explant to grow highly branched roots from the sites of infection. The roots branch much more frequently than the usual root system of that plant, and are also covered with a mass of tiny root hairs. Their most significant feature is that they produce secondary metabolites at levels similar to those made in the original plant. Thus they can be used as replacement plants for making such compounds as food flavours or fragrances.

hairy root disease  A disease of broad-leaved plants, where a proliferation of root-like tissue is formed from the stem. Hairy root disease is a tumorous state similar to crown-gall, and is induced by the bacterium Agrobacterium rhizogenes, containing an Ri plasmid. See Agrobacterium, crown gall.

halophyte  A plant that can tolerate a high concentration of salt in the growing medium.

halothane  A volatile anaesthetic.

hanging droplet technique  See microdroplet array.

haploid  (Gr. haploos, single + oides, like)  A cell or organism containing only one representative from each of the pairs of homologous chromosomes found in the normal diploid cell. Having a single complete set of chromosomes, or referring to an individual or generation containing such a single set of chromosomes per cell. Usually a gamete. See monoploid.

haploid cell  A cell containing only one set, or half the usual (diploid) number, of chromosomes.

haplotype  1. A group of alleles, each from a different locus in the same region of a chromosome, that exist in the same double helix.

haplozygous  See hemizygous.

haptoglobin  A serum protein, alpha globulin, that interacts with haemoglobin during recycling of the iron molecule of haemoglobin.

hardening off  Adapting plants to outdoor conditions by gradually withholding water, lowering the temperature, increasing light intensity, or reducing the nutrient supply. The hardening-off process conditions plants for survival when transplanted outdoors. The term is also used for gradual acclimatization to in vivo conditions of plants grown in vitro, e.g., gradual decrease in humidity. cf acclimatization; free-living conditions.

Hardy-Weinberg equilibrium  The frequencies of genotypes at a locus resulting from random mating at that locus; for two alleles, A1 and A2, with respective frequencies p and q, the Hardy-Weinberg equilibrium frequencies are p2 A1A1, 2pq A1A2, q2 A2A2. Despite the simplifying assumptions required to predict these frequencies, most loci in most populations are in Hardy-Weinberg equilibrium. Thus the Hardy-Weinberg law, which predicts these frequencies, is one of the great unifying themes of biology.

harvesting  1. The process involved in gathering in ripened crops.

    2. The collection of cells from cell cultures or of organs from donors for the purpose of transplantation.

heat pump  An apparatus that extracts heat from a fluid or gas that is marginally above ambient temperature. Heat pumps are commonly used to heat (or cool) greenhouses and laboratories.

heat therapy  See thermotherapy.

helix  Any structure with a spiral shape. The Watson and Crick model of DNA is in the form of a double helix.

helminths  Parasitic worms, especially internal parasites of man and animals.

helper cells   T cells that respond to an antigen displayed by a macrophage by stimulating B and T lymphocytes to develop into antibody-producing plasma cells and killer T cells, respectively.

helper plasmid  A plasmid that provides a function or functions to another plasmid in the same cell.

helper T cells   See helper cells.

helper T lymphocytes  See helper cells.

helper virus  A virus that provides a function or functions to another virus in the same cell.

hemicellulase  An enzyme that degrades hemicellulose to galactose; hemicellulase is available as a commercial product.

hemicellulose  (Gr. hemi, half + cellulose)  Any cellulose-like carbohydrate, but with differing chemical composition. Together with pectin and lignin, hemicelluloses form the cell wall matrix.

hemizygous; haplozygous  The condition in which only one allele of a pair is present, as in sex linkage or as a result of deletion. Genes present only once in the genotype and not in pairs; as in haploids, in differential segments of sex chromosomes, or in diploids as a result of aneuploidy or loss of chromosome segments.

hemoglobin  See haemoglobin.

hemolymph  See haemolymph.

hemophilia  See haemophilia.

HEPA filter (high efficiency particulate air filter)  A filter capable of screening out particles larger than 0.3 µm. HEPA filters are used in laminar air flow cabinets (hoods) for sterile transfer work. See prefilter.

herbicide  Any substance that is toxic to plants; usually applied to agrochemicals intended to kill specific unwanted plants, such as weeds.

herbicide resistance  The ability of a plant to withstand herbicide. Herbicide resistance has been one of the early targets of plant genetic engineering. If a herbicide is sprayed onto a field planted with such resistant crops, then all the plants except the crop would be killed, thus providing an effective method of weed control without having to develop herbicides specific to each weed type. There is substantial concern in some quarters about the widespread use of this technology, which is essentially giving the plant kingdom the ability to evade man's most effective herbicides. The concern are that, firstly, such engineering will lead to increased use of the herbicides, at a time when it is generally accepted that the use of chemicals should be kept as low as possible, and that, secondly, there is the possibility that resistant crop plants will escape to become weeds, or that their resistant genes could be transferred to other species, including weeds.

heredity  Resemblance among individuals related by descent; transmission of traits from parents to offspring.

heritability  In the narrow sense: 1. the proportion of phenotypic superiority of parents that is seen in their offspring;

    2. the proportion of the total phenotypic variation due to variation in breeding values.

    In the broad sense: the proportion of the total phenotypic variation due to genetic variation. The degree to which a given trait is controlled by inheritance.

    See broad-sense heritability; narrow-sense heritability.

hermaphrodite  1. Animal that has both male and female reproductive organs, or a mixture of male and female attributes.

    2. A plant whose flowers contain both stamen and carpels.

heteroalleles  Mutations that are functionally allelic but structurally non-allelic; mutations at different sites in a gene.

heterochromatin  Regions of chromosomes that stain darkly even during interphase; thought to be for the most part genetically inactive. cf euchromatin.

heteroduplex  A double-stranded DNA molecule or DNA-RNA hybrid, where each strand is of a different origin, and consequently containing one or more mismatched (non-complementary) base pairs. A DNA duplex is prepared by the hybridization of single-stranded DNA molecules derived from two different sources. Where the two DNAs have identical or very similar sequences, a double-stranded molecule will be established, whereas where the two DNAs differ in sequence, single-stranded regions will remain. A heteroduplex will be revealed as single-strand "bushes" when DNA is observed electron microscopically. A map of homologous and non-homologous regions of the two molecules may thereby be constructed. This process is known as heteroduplex mapping.

heterogametic sex  Producing unlike gametes with regard to the sex chromosomes. In mammals, the XY male is heterogametic, and the XX female is homogametic.

heterogeneity  See genetic heterogeneity.

heterogeneous nuclear RNA (hnRNA)  Large RNA molecules, which are unedited mRNA transcripts, or pre-mRNAs found in the nucleous of a eukaryotic cell. See RNA.

heterokaryon  Cell with two or more different nuclei as a result of cell fusion. Opposite: homokaryon.

heterologous  From a different source, as in heterologous DNA.

heterologous probe  A DNA probe that is derived from one species and used to screen for a similar DNA sequence from another species.

heterologous protein  See recombinant protein.

heteroplasmy  A cellular condition in which two genetically different types of organelles are present. cf homoplasmy.

heteroploid  Term given to a cell culture when the cells comprising the culture possess nuclei containing chromosome numbers other than the diploid number.

heteropyknosis (adj: heteropyknotic)  Property of certain chromosomes, or of their parts, to remain more dense during the cell cycle and to stain more intensely than other chromosomes or parts.

heterosis   (Gr. heteros, different + osis, suffix for "a state of") See hybrid vigour.

heterozygote (adj: heterozygous)  (Gr. heteros, different + zygon, yoke) An individual that has different alleles at the same locus in its two homologous chromosomes.

Hfr  High-frequency recombination strain of Escherichia coli; in such strains, the F episome is integrated into the bacterial chromosome.

HGH  Human growth hormone, q.v.

high efficiency particulate air filter  See HEPA.

histocompatibility  The degree to which tissue from one organism will be tolerated by the immune system of another organism.

histocompatibility complex; histocompatibility system  The collection of genes coding for peptides present on the surface of nucleated cells; these peptides are responsible for the differences between genetically non-identical individuals that cause rejection of tissue grafts between such individuals. These peptides were originally called histocompatibility antigens. They are now called histoglobulins, reflecting their structural similarity to immunoglobulins.

histoglobulin  See histocompatibility complex.

histology  (Gr. histos, cloth, tissue + logos, discourse)  Science that deals with the microscopic structure of animal and plant tissues.

histone  Group of water-soluble proteins rich in basic amino acids, closely associated with DNA in plant and animal chromatin. Histones are involved in the coiling of DNA in chromosomes and in the regulation of gene activity.

HIV  Human immunodeficiency virus. The retrovirus that causes AIDS in humans.

HLA  Human-leukocyte-antigen system. See major histocompatibility antigens.

hnRNA  See heterogeneous nuclear RNA.

holoenzyme See apoenzyme.

holometabolous  An insect that undergoes complete metamorphosis to the adult from a morphologically distinct larval stage.

hollow fibre  A tube of a porous material, having an internal diameter of a fraction of a millimetre, and so its ratio of surface area to internal volume is very large. This has had two types of application. Firstly, hollow fibres can be used as filters. Because they have a huge surface area, they take much longer to clog up than normal filters. Secondly, they are used in the hollow fibre bioreactor, in which cells are kept inside the hollow, porous fibres, and the culture medium is circulated outside the reactor. The fibres let nutrients in and products out (as they are in solution), but do not allow the passage of cells. Hollow fibre bioreactors are very effective for maintaining mammalian cells in culture because they have a very large surface area for the cells to grow on without needing a large reactor to hold them, and because the nutrient reaching the cells can be kept fresh. The reactor also provides an easy way of removing the product that the cells are making: such as monoclonal antibodies. Hollow fibre reactors are less use when the cells themselves have to grow, because it is hard to get at the inside of the fibre to remove surplus cells.

homeobox  A DNA sequence found in several genes that are involved in the specification of organs in different body parts in animals; they are characteristic of genes that influence segmentation in animals. The homeobox corresponds to an amino acid sequence in the polypeptide encoded by these genes; this sequence is called the homeodomain.

homeodomain  See homeobox.

homeotic mutation  A mutation that causes a body part to develop in an inappropriate position in an organism, such as the mutation in Drosophila that causes legs to develop on the head in place of antennae.

homoalleles  Mutations that are both functionally and structurally allelic; mutations at the same site in the same gene.

homodimer  A protein with two identical polypeptide chains.

homogametic sex  Producing similar gametes with regard to the sex chromosomes. In mammals, the XY male is heterogametic, and the XX female is homogametic. See heterogametic sex.

homogenotization  A genetic technique used to replace one copy of a gene, or other DNA sequence within a genome, with an altered copy of that sequence. The DNA is first cloned and then altered in some way, e.g., a transposon is inserted into a gene. The mutated gene copy can be used to replace the original gene by recombination in vivo. The incorporation of the mutated gene is usually selected, for example, by virtue of its containing a transposon-encoded antibiotic resistance. See replacement.

homokaryon  Cell with two or more identical nuclei as a result of fusion. Opposite: heterokaryon.

homologous  From the same source, or having the same evolutionary function or structure.

homologous chromosomes  Chromosomes that occur in pairs and are generally similar in size and shape: one comes from the male parent and the other from the female. Such chromosomes contain the same linear array of genes.

homologous recombination  The exchange of DNA fragments between two DNA molecules or chromatids of paired chromosomes (during crossing over) at the site of identical nucleotide sequences.

homology  The degree of identity between individuals, or characters. The degree of identity between the nucleotide sequences of two nucleic acid molecules or the amino acid sequences of two protein molecules. Although sequence determination is the ultimate test of homology, useful estimates can be provided by either DNA-DNA or DNA-RNA hybridization.

homomultimer   See homopolymer.

homoplasmy  A cellular condition in which all copies of an organelle are genetically identical. See heteroplasmy.

homopolymer  A nucleic acid strand that is composed of one kind of nucleotide, e.g., GGGGGGGG. See polymer.

homopolymeric tailing  See tailing.

homozygote; (adj: homozygous)  (Gr. homos, one and the same + zygon, yoke) An individual that has two copies of the same allele at a particular locus in its two homologous chromosomes. See allele; genotype; heterozygote.

hormone (Gr. hormaein, to excite)  A specific organic product, produced in one part of a plant or animal body, and transported to another part where, at low concentrations, it promotes, inhibits or quantitatively modifies a biological process.

host-specific toxin  A metabolite produced by a pathogen which has a host specificity equivalent to that of the pathogen. Such toxins are utilized for in vitro selection experiments to screen for tolerance or resistance to the pathogen.

host  An organism that contains another organism, or a cloning vector.

humoral immune response  The production of antibody by B cells of the immune system in response to the presence of a foreign antigen. See antibody-mediated immune response.

Hup+  See hydrogen-uptake positive.

hybrid (L. hybrida)  1. The offspring of two parents that are genetically different. A cross between two genetically unlike individuals.

    2. A heteroduplex DNA or DNA-RNA molecule.

hybrid arrested translation  A method used to identify the proteins encoded by a cloned DNA sequence. A crude cellular mRNA preparation, composed of many individual types of mRNA, is hybridized with cloned DNA. Only mRNA molecules homologous to the cloned DNA will anneal to it. The rest of the mRNA molecules are put into an in vitro translation system and the protein products are compared with the proteins obtained by use of the whole mRNA preparation. cf hybrid released translation.

hybrid cell  The mononucleate cell which results from the fusion of two different cells, leading to the formation of a synkaryon.

hybrid dysgenesis  A syndrome of abnormal germ-line traits, including mutation, chromosome breakage, and sterility, which results from activity of transposable elements.

hybrid released translation  A method used to detect the proteins encoded by cloned DNA. The cloned DNA is bound to a nitrocellulose filter and a crude preparation of mRNA is hybridized to the filter-bound DNA. Only mRNA sequences homologous to the cloned DNA will be retained on the filter. These mRNA molecules can then be removed by high temperature or by using formamide. The purified mRNA is then placed in an in vitro translation system and the proteins encoded by the message can be analysed by electrophoresis through a polyacrylamide gel. See hybrid arrested translation.

hybrid seed  1. Seed produced by crossing genetically dissimilar parents.

    2. In plant breeding, used colloquially for seed produced by specific crosses of carefully selected pure lines, such that the F1 crop displays hybrid vigour. As the F1 crop is heterozygous, it does not breed true and so new seed must be purchased each season.

hybrid selection  The process of choosing individuals possessing desired characteristics from among a hybrid population.

hybrid vigour; heterosis  The extent to which the performance of a hybrid in one or more traits is better than the average performance of the two parental populations.

hybridization  1. Interbreeding of species, races, varieties and so on, among plants or animals; a process of forming a hybrid by cross pollination of plants or by mating animals of different types.

    2. The production of offspring of genetically different parents, normally from sexual reproduction, but also asexually by the fusion of protoplasts or by transformation.

    3. The pairing of two polynucleotide strands, often from different sources, by hydrogen bonding between complementary nucleotides. See northern hybridization; Southern hybridization.

hybridoma  A hybrid cell, derived from a B (antibody producing) lymphocyte fused to a tumour cell, which grows indefinitely in tissue culture and is selected for the secretion of the specific antibody produced by that B cell.

hydrate  A compound formed by the incorporation of water.

hydrogen bond  A relatively weak bond formed between a hydrogen atom (which is covalently bound to a nitrogen or oxygen atom) and a nitrogen or oxygen atom with an unshared electron pair. Weak interactions between electro-negative atoms and hydrogen atoms (electro-positive) that are linked to other electro-negative atoms.

hydrogen-uptake positive (Hup+ A term describing a micro-organism that is capable of assimilating (or taking up) hydrogen gas.

hydrolysis  A reaction in which a molecule of water is added at the site of cleavage of a molecule to two products.

hydrophobic interactions  Association of non-polar groups with each other when present in aqueous solutions because of their insolubility in water.

hydroponics  The growing of plants in aerated water containing all the essential mineral nutrients, with no soil. Also called soilless gardening or cultivation.

3´-hydroxyl end  The hydroxyl group that is attached to the 3´ carbon atom of the sugar (ribose or deoxyribose) of the terminal nucleotide of a nucleic acid molecule.

hyperploid  A genetic condition in which a chromosome or a segment of a chromosome is over-represented in the genotype. Opposite: hypoploid.

hypersensitive sites  Regions in the DNA that are highly susceptible to digestion with endonucleases.

hypertonic  A solution with an osmotic potential greater than that of living cells, leading to water loss from, shrinkage or plasmolysis of cells in a hypertonic situation. Opposite: hypotonic.

hypervariable region  The parts of both the heavy and light chains of an antibody molecule that enable it to bind to a specific site on an antigen.

hypervariable segment  A region of a protein that varies considerably between strains or individuals.

hypochlorite  Generic term for aqueous solutions of sodium hypochlorite, potassium hypochlorite or calcium hypochlorite, which are oxidizing agents and used for disinfecting surfaces and surface-sterilizing tissues, and for bleaching.

hypocotyl (Gr. hypo, under + kotyledon, a cup-shaped hollow) Portion of an embryo or seedling below the cotyledons, which is a transitional area between stem and root.

hypomorph  A mutation that reduces but does not completely abolish gene expression.

hypoplastic  Reduction in plant growth or development (dwarfing, stunting) resulting from an abnormal condition associated with a disease or nutritional stress.

hypoploid  A genetic condition in which a chromosome or segment of a chromosome is underrepresented in the genotype. Opposite: hyperploid.

hypostatic; hypostasis  See epistasis.

hypothalamic peptides  Peptides generated in the vertebrate forebrain and concerned with regulating the body's physiological state.

hypothesis (Gr. hypothesis, foundation)  A tentative theory or supposition provisionally adopted to explain certain facts and to guide in the investigation of other facts. Once proven by rigorous scientific investigation, it becomes a theory or a law.

hypotonic  Osmotic potential less than that of living cells. Cells placed in a hypotonic solution display swelling and turgidity.

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