V

val

GUU, GUC, GUA, GUG

free AMP (adenosine monophosphate). See adenosine phosphate.

amino group a chemical group (—NH2) which with the addition of a proton can form —NH+.

p amino benzoic acid a component of folic acid (q.v.).

aminopeptidase an enzyme (in both prokaryotes and eukaryotes) that removes the formylated methi-onine (fMet) or methionine from the NH2 terminus of growing or completed polypeptide chains.

aminopterin See folic acid.

aminopurine 2-aminopurine (2-AP) is a fluorescent analog of adenine (6-aminopurine). The fluorescence of 2-AP ia quenched once it is incorporated into the base-stacked structure of dsDNA. However, if the base stacking or base pairing of DNA is locally perturbed, the fluorescence of 2-AP is enhanced. Therefore the intensity of fluorescence emissions by 2-AP molecules can be used to monitor pertur bations in DNA structure caused by interactions between DNA and proteins, such as DNA polymer-ases, helicases, repair enzymes, and methyl transfer-ases (all of which see).

amino terminal end the end of a polypeptide chain that has a free amino group.

Amish a human population descended from a limited number of founders who emigrated from southwestern Germany to the United States during the eighteenth century. The population is highly inbred, since marriage is allowed only within the community. Beneficial collaboration between geneticists and religious leaders has led to discoveries concerning certain genetic diseases that occur at unprecedent-edly high frequencies among the Amish. See cartilage-hair hypoplasia (CHH), consanguinity, Ellis-van Creveld syndrome, inbreeding.

Amitochondriates a subkingdom of protoctists that includes the Archaeprotista and the Microspora (see Appendix A). These phyla contain anaerobic microorganisms that lack mitochondria and presumably were without them from the outset of their evolution.

amitosis the division of a nucleus into two parts by constriction without the participation of a mi-totic apparatus. Accessory nuclei (q.v.) grow by ami-tosis.

amyotrophic lateral sclerosis (ALS) 23

amixis a reproductive cycle lacking meiosis and fertilization. Asexual reproduction. Contrast with amphimixis, apomixis.

AML1 gene See acute myeloid leukemia 1 gene.

amniocentesis sampling of amniotic fluid for the prenatal diagnosis of fetal disorders. During the procedure, a hollow needle is inserted through the skin and muscle of the mother's abdomen, through the uterus, and into the amniotic sac that surrounds the fetus. Cells that have sloughed from the fetus are suspended in the fluid. Cells in the sample are cultured for about three weeks to raise their numbers to the point where chromosomal and biochemical analyses can be made. Amniocentesis cannot be done until about 16 weeks from the last menstrual period, since the sac containing the embryo is not large enough to permit safe withdrawal of the fluid until this time. See Appendix C, 1967, Jacobson and Barter; chorionic villi sampling, informed consent, genetic counseling, prenatal genetic testing.

amniocytes cells obtained by amniocentesis (q.v.).

amnion A fluid-filled sac within which the embryos of reptiles, birds, and mammals develop. The wall of this sac has a two-layered epithelium. The inner epithelium of the wall is the amnion, although the term is sometimes applied to the whole sac. The outer epithelium is usually called the chorion. Am-niotic fluid within the sac provides a liquid environment for the embryo.

amniote a land-living vertebrate (reptile, bird, or mammal) whose embryos have an amnion and allan-tois.

Amoeba proteus a common species of rhizopod; a giant protozoan used for microsurgical nuclear transplantations. See Appendix C, 1967, Goldstein and

Prescott.

amoeboid movement cellular motility involving cytoplasmic streaming into cellular extensions called pseudopodia.

amorphic mutation a mutation in which the altered gene product fails in its molecular function. Also called a loss of function mutation or a null mutation.

AMP adenosine monophosphate. See adenosine phosphate.

amphidiploid an organism that is diploid for two genomes, each from a different species; synonymous with allotetraploid. See Appendix C, 1925, Good-speed and Clausen.

amphimixis sexual reproduction resulting in an individual having two parents: synonymous with mixis. Contrast with amixis, apomixis. The adjective form is amphimictic.

Amphioxus See Branchiostoma.

amphipathic descriptive of a molecule that has distinct polar and nonpolar segments (e.g., membrane phospholipids).

amphoteric compound (also ampholyte) a substance that can act both as an acid and a base. Thus a protein is amphoteric because it tends to lose protons on the more alkaline side of its isoelectric point and to gain protons on the acid side of its isoelectric point.

ampicillin See penicillin.

ampR a selectable gene which encodes the enzyme P-lactamase, which inactivates ampicillin (q.v.). Cells containing a plasmid vector (q.v.) which expresses ampR can be selected from those that do not by growth in an ampicillin-containing medium. See penicillin, R (resistance) plasmid.

amplicon a segment of the genome that forms multiple linear copies after exposure of the organism to a compound that inhibits the functioning of a gene in the segment. For example, in mammals the enzyme dihydrofolate reductase (q.v.) is inhibited by methotrexate (q.v.). Exposure to this inhibitor causes amplification of the DHFR gene. More generally, the term amplicons is used for DNA fragments that have been generated in experiments utilizing the polymerase chain reaction (q.v.).

amplification See gene amplification, polymerase chain reaction, RNA amplification.

amplified RNA See RNA amplification.

amylase an enzyme that hydrolyzes glucosidic bonds in polyglucosans such as glycogen.

amyloid-beta-precursor protein (ApPP), amyloid-beta-peptides (ApPs), amyloid plaques. See Alzheimer disease (AD).

amyloplast a starch-rich plastid.

amyotrophic lateral sclerosis (ALS) a disease in humans resulting from the degeneration of motor neurons in the lateral columns of the spinal cord. The disease begins with an asymmetric weakness in the limbs and progresses to complete paralysis and death. ALS is sometimes called Lou Gehrig disease after the famous American baseball player who suffered from ALS. In familial cases of ALS, the condi tion was first shown to be due to mutations in a gene (ALS1) located at 21q22.1 which encoded the enzyme superoxide dismutase (SOD) (q.v.). Next ALS2 was mapped to 2q33, and it encoded a protein (alsin) thought to be a GTPase regulator protein. Familial ALS due to ALS1 is an adult-onset disease, and it shows dominant inheritance; whereas FALS due to ALS2 causes a disease with juvenile onset which shows recessive inheritance. Two other forms of FALS occur: one caused by mutations in ALS3 at 18q21 and the other by mutations in ALS4 at 9q34. However, the cause of 90 percent of ALS cases (sporadic or non-familial) is unknown. See Appendix C, 1993, Rosen, Siddique et al.

anabolism the metabolic synthesis of complex molecules from simpler precursors, usually requiring the expenditure of energy and specific anabolic enzymes. Contrast with catabolism.

anaerobe a cell that can live without molecular oxygen. A strict anaerobe cannot live in the presence of oxygen. See Appendix C, 1861, Pasteur.

anagenesis phyletic evolution within a single lineage without subdivision or splitting; the opposite of cladogenesis.

analog a compound related to, but slightly different structurally from a biologically significant molecule, such as an amino acid (see azaserine), a pyrimi-dine or purine (see base analogs), or a hormone (see ZR515).

analogous referring to structures or processes that have evolved convergently, as opposed to the term homologous (q.v.). Analogous structures have similar functions but are different in evolutionary origin: e.g., the wing of a butterfly and of a bat. See homoplasy.

analysis of variance a statistical technique that allows the partitioning of the total variation observed in an experiment among several statistically independent possible causes of the variation. Among such causes are treatment effects, grouping effects, and experimental errors. Checking the absence of an effect due to the treatment is often the purpose of the inquiry. The statistical test of the hypothesis that the treatment had no effect is the F test, or variance-ratio test. If the ratio of the mean square for treatments to the mean square for error exceeds a certain constant that depends on the respective degrees of freedom of the two mean squares at a chosen significance level, then the treatments are inferred to have been effective. Analysis of variance is particularly useful in judging which sources of uncontrolled variation in an experiment need to be allowed for in testing treatment effects.

anamnestic response See immune response, immunological memory.

anaphase See mitosis.

anaphase lag delay in the movement of one or more chromosomes from the metaphase plate during anaphase, often resulting in chromosome loss (q.i>J.

anaphylaxis a systemic allergic or hypersensitivity response leading to immediate respiratory and/or vascular difficulties.

Anas platyrhyncha the mallard duck, ancestor to the domestic or Pekin duck, A.p. domestica.

anastomosis the joining of two or more cell processes or tubular vessels to form a branching system.

anastral mitosis the type of mitosis characteristically found in plants. A spindle forms, but no centri-oles or asters are observed.

anautogenous insect an adult female insect that must feed for egg maturation. See autogenous insect.

anchorage-dependent cells cells (or in vitro cell cultures) that will grow, survive, or maintain function only when attached to an inert surface such as glass or plastic; also known as substrate-dependent cells. The only normal animal cells that are designed to survive without attachment and spreading are cells that circulate in the blood. Some tumor cells acquire this ability to be anchorage-independent and leave their original tissue sites to form metastases. See microcarriers, suspension culture.

Anderson disease See Fabry disease.

androdioecy a sexual dimorphism in plants having bisexual and separate male individuals.

androecious referring to plants having only male flowers.

androeci um the aggregate of the stamens in a flower.

androgen any compound with male sex hormone activity. In mammals, the most active androgens are synthesized by the interstitial cells of the testis. See testosterone.

androgenesis 1. development from a fertilized egg followed by disintegration of the maternal nucleus prior to syngamy. The resulting individual possesses only paternal chromosomes and is haploid. 2. production of an embryo having a diploid set of paternal chromosomes by nuclear transfer (q.v.). Compare with gynogenesis.

Angelman syndrome (AS) 25

androgenic gland a gland found in most crustaceans belonging to the subclass Malacostraca. When implanted into maturing females, the gland brings about masculinization of primary and secondary sex characters.

androgen insensitivity syndrome a condition in which XY individuals develop as normal-appearing, but sterile, females. Spermatogenesis does not occur in the testes, which are generally located inside the abdomen. Also called testicular feminization. See Appendix C, 1988, Brown et al.; androgen receptor, androgen receptor gene.

androgenote a cell or embryo produced by andro-genesis. Compare with gynogenote.

androgen receptor (AR) a protein belonging to a subfamily of steroid hormone receptors within a larger family of DNA-binding proteins. The human AR is made up of 919 amino acids and is subdivided into three domains. The N-terminal domain has a regulatory function, and the C-terminal domain binds dihydrotestosterone. The central domain, which binds to DNA, contains zinc fingers. The receptor binds to DNA as a homodimer. See androgen, testosterone, vitamin D receptor, zinc finger protein.

androgen receptor gene a gene symbolized AR that is located on the X chromosome at q12. It contains eight exons and is 180,245 bp long. AR encodes the androgen receptor (q.v.). Homologous genes occur in the rat and mouse. Mutations in critical portions of AR cause the loss of receptor activity, and this results in abnormalities in sexual phenotype referred to as androgen insensitivity syndrome (q.v.) or testicular feminization. See Appendix C, 1988, Brown et al.

androgynous 1. being neither distinguishably masculine nor feminine in appearance or behavior. 2. bearing staminate and pistillate flowers on distinct parts of the same inflorescence. See flower.

andromonecy a sexual condition in which plants develop both staminate flowers (that do not develop fruit) and hermaphroditic flowers.

androphages "male-specific" bacteriophages that absorb on the surface of F pili. Examples are MS2, R17, and Qß (all of which see). See F factor (fertility factor).

anemia a disorder characterized by a decrease in hemoglobin per unit volume of blood. In the case of hemolytic anemia, there is a destruction of red blood cells. In the case of hypochromic anemia, there is a reduction in the hemoglobin content of the erythro-cyte.

anemophily pollination by the wind.

anergy the lack of an expected immune response.

aneucentric referring to an aberration generating a chromosome with more than one centromere.

aneuploidy the condition in which the chromosome number of the cells of an individual is not an exact multiple of the typical haploid set for that species. The nomenclature employs the suffix somic, as the following examples illustrate. Down syndrome (q.v.) and Turner syndrome (q.v.) are examples of a human trisomic and monosomic, respectively. Nulli-somics result from the loss (2N - 2) and tetrasomics from the gain (2N + 2) of a chromosome pair. If more than one different chromosome is lost or gained, the condition is described as doubly monosomic (2N -1 - 1) or doubly trisomic (2N + 1 + 1). Early studies of aneuploids led to the conclusion that genes carried by specific chromosomes controlled morphological traits. For example, in Datura stramonium (q.v.) extra doses of chromosome G broaden and reduce the seed capsule and increase the size of the spines (see page 26). See Appendix C, 1934, Blakeslee; hy-perploid, hypoploid, symbols used in human cytoge-netics, polyploid.

aneurin vitamin Bj; more commonly known as thiamine.

aneusomy the condition in which an organism is made up of cells that contain different numbers of chromosomes. Aneusomy is widespread in flowering plants possessing B chromosomes (q.v.). In animals, the term generally refers to a diploid organism with subpopulations of aneuploid, somatic cells. The term aneusomy has been misused in the recent literature of human cytogenetics to refer to a genetic imbalance within a chromosome pair. For example, an individual heterozygous for a deficiency including one or more genes is hemizygous for those genes on the normal homolog. To call such an individual a segmental aneusomic is confusing, since aneusomy traditionally implies mosaicism. See aneuploidy.

Angelman syndrome (AS) children with this condition are hyperactive and are unable to develop normal speech. Because they show impaired motor control and tend to laugh excessively, the condition is sometimes called "happy puppet syndrome." The British pediatrician Harry Angelman gave the first description of children with the disease in 1965. Later the condition was found to be the result of a deletion in the long arm of chromosome 15. See Prader-Willi syndrome (PWS).

2N diploid

4N tetraploid

2N diploid

4N tetraploid

trisomic diploid

tetrasomic diploid

trisomic diploid

pentasomic tetraploid hexasomic tetraploid heptasomic tetraploid

tetrasomic diploid

pentasomic tetraploid

hexasomic tetraploid

heptasomic tetraploid

Aneuploidy in the Jimson weed, Datura stramonium. Extra doses of G chromosomes make the seed capsules smaller and broader and their spines larger.

angiosperm a flowering plant. Any species in the Superclass Angiospermae (see Appendix A, Kingdom Plantae) characterized by having seeds enclosed in an ovary. Almost all agriculturally important plants (apart from conifers) belong to the Angiospermae.

Angstrom unit a unit of length equal to one ten-thousandth of a micron (10-4 micron; a micron being 10-6 meter); convenient for describing atomic dimensions; also equivalent to 10-1 nanometers (nm) or 10-10 meter. Abbreviated A, A°, A, A.U., or A.U. Named in honor of the Swedish physicist Anders Jonas Angstrom.

Animalia the kingdom containing animals (heterotrophic organisms developing from a blastula). See Appendix A, Kingdom 4; opisthokonta.

animal pole that pole of an egg which contains the most cytoplasm and the least yolk.

anion a negatively charged ion. Contrast with cation.

Aniridia a dominant mutation of the Pax-6 gene located at 11p13, which causes defects in the iris, lens, cornea, and retina of humans. The Pax-6 gene of humans, the Sey gene of mice and rats, and the ey gene of Drosophila are homologous. See eyeless.

anisogamy that mode of sexual reproduction in which one of the sex cells, the egg, is large and nonmobile, whereas the other (the sperm) is small and motile. In most anisogamous eukaryotes, the centri-

ole is paternally inherited. See isogamy, parthenogenesis.

anisotropy a directional property of crystals and fibers having a high degree of molecular orientation. Anisotropic substances have different physical properties when tested in different directions. When a ray of plane polarized light passes through aniso-tropic material, it is split into two rays polarized in mutually perpendicular planes. This property of ani-sotropic material is called birefringence. Muscle fibers and the metaphase spindle are examples of living materials exhibiting birefringence. Materials showing no birefringence are said to be isotropic. See polarization microscope.

ankylosing spondylitis an arthritic disease resulting in a stiffening and bending of the spine; inherited as an autosomal dominant with reduced penetrance. Over 90% of patients with this disease carry the B27 HLA antigen. See histocompatibility.

ankyrin a protein that binds to P-spectrin as well as to the cytoplasmic domains of a variety of integral membrane proteins, and that is thought to interconnect the spectrin-based membrane cytoskeleton and the overlying lipid bilayer. Based on sequence similarity, ankyrins have been identified in various tissues and cell types from a variety of organisms. These proteins generally have three structural domains: a conserved, N-terminal region containing membrane-binding properties, a highly conserved

Antennapedia 27

spectrin-binding region, and a variable, regulatory C-terminal domain. In humans, three forms of ankyrin have been characterized. Ankyrin-1 (also called ankyrin-R) is encoded by the ANK1 gene at chromosomal map position 8p11 and expressed in erythrocytes and the brain. Ankyrin-2 (also called an-kyrin-B) is encoded by the ANK2 gene at position 4q25-q27 and expressed primarily in the brain. A third protein, ankyrin-3 (also known as ankyrin-G), is encoded by the ANK3 gene mapping to 10q21, and alternatively spliced isoforms are expressed in nervous tissue, muscle, and other tissues. Mutations in the erythrocytic ankyrin gene, ANK1, are associated with hereditary spherocytosis (q.v.). See fu-some, integral protein, spectrin, spectrosome.

anlage the embryonic primordium from which a specific part of the organism develops.

anneal to subject first to heating then to cooling. In molecular genetics experiments, annealing is used to produce hybrid nucleic acid molecules containing paired strands, each from a different source. Heating results in the separation of the individual strands of any double-stranded, nucleic-acid helix, and cooling leads to the pairing of any molecules that have segments with complementary base pairs.

annidation the phenomenon where a mutant is maintained in a population because it can flourish in an available ecological niche that the parent organisms cannot utilize. A wingless mutant insect, for example, might be poorly adapted in its ancestral habitat but able to live in tunnels and crevices that a winged form could not occupy.

annotation See genome annotation.

annulate lamellae paired membranes arranged in stacks and possessing annuli resembling those of the nuclear membrane. Annulate lamellae may serve to transfer nuclear material to the cytoplasm by the replication of the nuclear envelope and may be a mechanism for storing gene-derived information to be used for cytoplasmic differentiation during early embryogenesis. During insect oogenesis, annulate lamellae occur alongside nurse cell nuclei and germinal vesicles, and they are abundant in the ooplasm.

annulus a ring. Applied to any of a number of ring-shaped parts of animals and plants. Used in cytology to refer to the ring-shaped nuclear pores.

anode the positive electrode; the electrode to which negative ions are attracted. Contrast with cathode.

anodontia the congenital absence of teeth. Hypo-dontia is currently the preferred term.

anonymous DNA a segment of DNA of unknown gene content that has been localized to a specific chromosome.

Anopheles a genus containing about 150 species of mosquitoes, many of which are of medical importance. Africa's principal malaria vector is A. gam-biae. Other vector species are A. funestus, A. quadri-maculatus, A. atroparvus, A. nili, A. moucheti, and A. pharoensis. Polytene chromosomes occur in both larval salivary gland cells and adult ovarian nurse cells. Sibling species can often be separated by differences in the banding patterns of their polytene chromosomes. The genome of A. gambiae has been shown to contain 278 mbp of DNA and about 13,700 genes. See Appendix A, Arthropoda, Insecta, Diptera; Appendix C, 1898, Ross; 1899, Grassi; 2002, Holt et al.; Appendix E; intron dynamics, malaria, mariner elements, Minos element, shotgun sequencing.

anosmia the absence or loss of the sense of smell. Anosmia may be caused by injury to or loss of olfactory receptor neurons (q.v.) or by injury to region(s) in the brain or elsewhere where olfactory signals are processed. Anosmia may also result due to defects in any element of the olfaction signaling pathway. For example, mice and nematodes with defects in G proteins (q.v.) found in olfactory receptor neurons exhibit olfactory and chemosensory defects, respectively.

Anseranser the Gray Lag goose, a favorite experimental organism for students of animal behavior and its hereditary components.

antagonist a molecule that bears sufficient structural similarity to a second molecule to compete with that molecule for binding sites on a third molecule. See competition.

antagonistic pleiotropy a phenomenon in which alleles (that are detrimental late in life) improve fitness earlier in life.

antenatal before birth; during pregnancy.

antennae the first paired appendages on the head of arthropods.

Antennapedia a gene residing at 47.9 on the genetic map and within segment 84B of the salivary map of Drosophila melanogaster. The Antp gene is one of a cluster of three genes that specify the type of differentiation that cells in the segments from the

Arg Lys Arg Gly Arg Gin Thr Tyr Thr Arg Tyr Gin Thr Leu Glu Leu Glu Lys Glu Phe His Phe Asn Arg Tyr Leu Thr Arg Arg Arg 1 30

Helix

Turn

Recognition helix

Arg lie Glu lie Ala His Ala Leu Cys Leu Thr Glu Arg Gin lie Lys lie Trp Phe Gin Asn Arg Arg Met Lys Trp Lys Lys Glu Asn 31 60

Antennapedia homeobox head to the anterior portion of the second thoracic segment will undergo. Mutations in the Antp gene cause the transformation of the segment that normally produces the antenna into one that produces a middle leg. The gene encodes a protein characterized by a homeobox (q.v.). This is a segment of 60 amino acids that lies close to the C terminus of the Antp protein. The amino acid sequence of this segment is shown above. It binds to target DNA sequences by its helix-turn-helix motif (q.v.). The complete three-dimensional structures of the Antp homeodomain as well as of the homeodomain-target DNA complex have been determined using NMR spectroscopy (q.v.) and x-ray crystallography (q.v.). See Appendix C, 1983, Scott et al.; 1989, Qian et al. ; 1990, Maliki, Schughart, and McGinnis; bithorax, homeotic mutants, Hox genes, Polycomb, probos-cipedia, segment identity genes.

anther the terminal portion of a stamen bearing pollen sacs.

anther culture a technique that utilizes anthers or pollen cells to generate haploid tissue cultures or even plants. See Appendix C, 1973, Debergh and Nitsch, haploid sporophytes.

antheridium the male gametangium of algae, fungi, bryophytes, and pteridophytes. Contrast with oogonium.

anthesis the time of flowering.

anthocyanins the red, violet, or blue glycosidic pigments that give color to flowers, fruits, seeds, stems, and leaves of plants. The common structural unit is a 15-carbon flavone skeleton to which sugars are attached. An example is pelargonidin, a scarlet pigment produced by geraniums. Unlike the carot-enoids and chlorophylls, which are lipid-soluble pigments of plastids, anthocyanins are water-soluble and are found dissolved in the vacuoles (q.v.) of plant cells. A primary function of anthocyanins is to attract insect pollinators to plants. See kernel, pelar-gonidin monoglucoside, R genes of maize.

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