m 1. mole (expressed in daltons or kilodaltons, q.v.). See gram molecular weight. Compare with gram equivalent weight. 2. molar. Compare with normal solution. M 1. the stage of the cell cycle (q.v.) where mitosis occurs. 2. molal (q.v.).
Compare with molar. M13 a single-stranded bacteriophage cloning vehi- cle, with a closed circular DNA genome of approxi- mately 6.5 kilobase pairs. The major advantage of using M13 for cloning is that the phage particles re- leased from infected cells contain single-stranded DNA that is homologous to only one of the two complementary strands of the cloned DNA, and therefore it can be used as a template for DNA se- quencing analysis. Ma megaannum (q.v.). Macaca mulatta the rhesus monkey. The catar- rhine primate most used in the laboratory and the original source of the Rh antigen. Its haploid chro- mosome number is 21, and about 30 genes have been located on 14 linkage groups. See Rh factor. macaroni wheat Triticum durum (N = 14). macha wheat Triticum macha (N = 21). macroconidia See conidia. macroevolution a large evolutionary pattern usu- ally viewed through the perspective of geologic time, such as the evolution of the horse from Eohip- pus to Equus. Macroevolution includes changes in taxonomic categories above the species level, and events that result in the origin of a new higher taxon.
See microevolution. macromolecule a molecule of relative molecular mass ranging from a few thousand to hundreds of millions (proteins, nucleic acids, polysaccharides, etc.). macromutation See evolution. macronucleus the larger of the two types of nuclei in the ciliates; the “vegetative” nucleus. Macronuclei contain many copies of each gene and are transcrip- tionally active. See micronucleus, nuclear dimor- phism, Tetrahymena.
macrophage a large, phagocytic, mononuclear leukocyte found in tissues, but derived from blood monocytes. Macrophages are called histiocytes in connective tissues, Kupffer’s cells in the liver, micro- glial cells in the nervous system, and alveolar macro- phages in the lung. To stimulate an immune re- sponse, most antigens must be “processed” by macrophages and presented on their surfaces to lym- phocytes in association with self-Ia molecules. See Gaucher disease, major immunogene complex. macrophage activation factor a lymphokine (q.v.) that activates macrophages. macroscopic visible to the unaided eye. macula adherans desmosome (q.v.).
MAD mutations mitotic arrest deficient mutations. The wild-type alleles of these genes encode proteins that bind to kinetochores until spindle fibers attach to them. If there is a failure of spindle microtubules to attach, the MAD protein remains bound to the kinetochore, and the entry into anaphase is blocked.
See checkpoints, mitosis. magic bullets See chemotherapy. magnesium an element universally found in small amounts in tissues.
Atomic number 12; atomic weight 24.312; valence 2+; most abundant isotope 24Mg, radioisotope 28Mg, half-life 21 hrs, radiation emitted—beta particle. See ribosome. mainband DNA the major DNA band obtained by density gradient equilibrium centrifugation of the DNA of an organism. maize See Zea mays. major gene a gene with pronounced phenotypic effects, in contrast to its modifiers; also called an oli- gogene. Compare with polygene. major histocompatibility complex (MHC) a large cluster of genes on human chromosome 6 and on mouse chromosome 17. The MHC controls many activities of immune cells, including the transplan- tation rejection process and the killing of virus- infected cells by specific killer T lymphocytes. The MHC is part of a larger major immunogene complex (q.v.) with more diverse functions. In different mam- mals different symbols have been assigned to the MHC; for example: chicken (B), dog (DLA), guinea pig (GPLA), human (HLA), mouse (H-2), and rat (Rt-1). The chromosomal segment (6p21-31) spans 3.6 Mb and contains 128 genes and 96 pseudogenes.
The MHC is the segment that contains the densest distribution of genes sequenced in the human ge- nome so far. The MHC is associated with more dis- eases than any other chromosomal region, including autoimmune conditions like diabetes and rheuma- toid arthritis. The gene map is divided into three segments: the genes of class I are in the segment closest to the p telomere, genes of class II are in the segment closest to the centromere, and class III genes lie between blocks I and II. Class II genes are in a low G + C isochore, and class III genes are in a high G + C isochore. See Appendix C, 1948, Gorer, Lyman, and Snell; 1953, Snell; 1999, MHC Se- quencing Consortium; histocompatibility molecules, HLA complex, isochore, pseudogene, sequence simi- larity estimates.
major immunogene complex (MIC) a genetic re- gion containing loci coding for lymphocyte surface antigens (e.g., Ia), histocompatibility (H) antigens, immune response (Ir) gene products, and proteins of the complement system. The genes specifying im- munoglobulins assort independently of the MIC, but the plasma cells responsible for their production are under the control of the MIC. malaria a disease caused by species of protozoa be- longing to the genus Plasmodium (q.v.) and transmit- ted by female mosquitoes belonging to the genus Anopheles (q.v.). Malaria is the single most critical infectious disease of humankind. There are about 200,000,000 people infected by the parasite, and 2,000,000 die annually.
Mortality rates are greatest in Africa, below the Sahara desert, where 90% of the deaths occur in children less than 5 years old. Ma- laria is the strongest known force in recent history for evolutionary selection within the human ge- nome. There are many lines of evidence for diverse genetic adaptions in different human populations. Since the malaria parasite spends a part of its life cycle in the red blood cell, many mutations confer- ring malaria resistance involve genes that encode erythrocyte proteins. Among these are hemoglobin S, hemoglobin C, glucose-6-phosphate dehydroge- nase deficiency, alpha thalassemia, and the Duffy blood group (all of which see).
Also see Appendix C, 1880, Laveran; 1898, Ross; 1899, Grassi; 1954, Allison; apicoplast, artemisinin, blackwater fever, het- erozygote advantage, Minos element, Plasmodium life cycle. male gametophyte See pollen grain. male pronucleus the generative nucleus of a male gamete. male symbol P the zodiac sign for Mars, the Ro- man god of war. The sign represents a shield and spear. maleuric acid C5H6N2O4, a mitotic poison. malignancy a cancerous growth. See cancer. Malpighian tubule the excretory tubule of insects that opens into the anterior part of the hind gut. The tubule was discovered by Marcello Malpighi (1628- 1694), the Italian physician who founded micro- scopic anatomy.
Malthusian having to do with the theory advanced by the English social economist, T. R. Malthus, in his An Essay on the Principle of Population, published in 1798. According to this theory the world’s popu- lation tends to increase faster than the food supply, and poverty and misery are inevitable unless this trend is checked by war, famine, etc. Malthusian parameter the rate at which a popu- lation with a given age distribution and birth and death rate will increase. mammary tumor agent See mouse mammary tu- mor virus. man See Homo, human mitotic chromosomes.
Mandibulata a subphylum of arthropods contain- ing those species possessing antennae and a pair of mandibles. See Appendix A, Animalia, Arthropoda. mantid an insect belonging to the orthopteran family Mantidae of the Dictyoptera. See Appendix A. manifesting heterozygote a female heterozygous for a sex-linked recessive mutant gene who expresses the same phenotype as a male hemizygous for the
mutation. This rare phenomenon results from the situation where, by chance, most somatic cells criti- cal to the expression of the mutant phenotype con- tain an inactivated X chromosome carrying the nor- mal allele of the gene. See Lyonization. map See genetic map. map distance the distance between genes ex- pressed as map units or centiMorgans (cM). mapping function a mathematical formula devel- oped by J. B. S. Haldane that relates map distances to recombination frequencies. The function is repre- sented graphically below. It demonstrates that no matter how far apart two genes are on a chromo- some, one never observes a recombination value greater than 50%. It also shows that the relation be- tween recombination frequencies and map distances is linear for genes that recombine with frequencies less than 10%.
map unit a measure of genetic distance between two linked genes corresponding to a recombination frequency of 1% or 1 centiMorgan (cM). See Morgan unit. mare a female horse. Marfan syndrome a human hereditary disorder in- herited as an autosomal dominant. The disease has a prevalence of 1/20,000 and is caused by mutations in the fibrillin gene (FBN1) at band 21.1 on the long arm of chromosome 15. Rupture of the aorta is the major cause of death in Marfan patients, and this is directly linked to defects in fibrillin, which serves as a substrate for elastin in the aorta.
FBN1 is relatively large, occupying 235 kilobases and containing 65 ex- ons. Forty-two of the exons encode cystein-rich, EGF domains that function in calcium binding. The condition was described in 1896 by Antoine B. Mar- fan and is often called arachnodactyly in the earlier literature. See Appendix C, 1993, Pereira et al.; elas- tin, epidermal growth factor, fibrillin. http://www.m- arfan.org marginotomy a term coined by A. M.
Olonikov for the formation of nucleic acid replicas that are shortened relative to their templates. Telomeres (q.v.) undergo marginotomy, and the maximum number of mitoses any somatic cell line can undergo is correlated with the lengths of their telomeres and the rate at which they shorten per replication. See Appendix C, 1971, Olonikov; Hayflick limit. mariner elements a family of transposable ele- ments (q.v.) found in many species of insects. The mariner element isolated from Drosophila mauri- tiana is short (1,286 base pairs), with 28-bp inverted repeats. Between the repeats is a gene that encodes a transposase of 345 amino acids. Mariner-like trans- posable elements (MLTEs) occur in Anopheles and show promise as vectors for inserting parasite-resis- tance genes into populations of mosquito species that transmit human diseases. MLTEs appear to have been passed horizontally by insects to other in- vertebrates and vertebrates.
One example of a MLTE in humans is found between bands 11.2 and 12 on the short arm of chromosome 17. This region is also a recombination hot spot. See horizontal transmis- sion, hot spot, malaria, Minos element, TC1/mariner element. marker 1. a gene with a known location on a chro- mosome and a clear-cut phenotype, used as a point of reference when mapping a new mutant. 2. anti- genic markers serve to distinguish cell types. 3. marker DNAs, RNAs, and proteins are fragments of known sizes and/or properties that are used to cali- brate an electrophoretic gel.
marker rescue the phenomenon found when bac- teria are mixedly infected with two genetically marked phages, of which only one type is irradiated. The progeny phages are predominately of the unir- radiated type. However, some recombinants are found that contain genes from the irradiated parent. Such genes have been “rescued” by recombination. MARs matrix attachment regions (q.v.). marsupial any of a group of primitive mammals, females of which have an abdominal pouch in which the young are carried and nourished (kangaroos, opossums, etc.). See Appendix A. masked mRNA messenger RNA that can be stored in large quantities because it is in some way inacti- vated and protected against digestion by nucleases
Maternal polarity mutants
(perhaps by being associated with proteins). Sea ur- chin eggs store mRNA in this masked form. mass action, law of the principle that the rate of a chemical reaction is proportional to the concentra- tions of the reacting substances, with each concen- tration raised to a power equal to the relative num- ber of molecules of each species participating in the reaction. massed training See spaced training. mass extinctions intervals in the geologic record during which between 25 and 50% of all fossil fami- lies became extinct in a geologically short time. Dur- ing the Phanerozoic Eon, mass extinctions occurred at the close of the Cambrian, Ordovician, Devonian, Permian, Triassic, and Cretaceous periods. A 50% reduction in families is estimated to be equivalent to a reduction of more than 90% in the number of spe- cies. See geological time divisions, impact theory. mass number the number of protons and neutrons in the nucleus of an atom. mass spectrograph an instrument for analyzing a substance in terms of the ratios of mass to charge of its components. It produces a focused mass spec- trum of lines on a photographic plate.
mass unit See atomic mass unit. mast cells tissue cells, thought to be the counter- parts of blood basophils, that are abundant in lymph nodes, spleen, and bone marrow, in connective tis- sues, and in the skin. However, blood and lymph do not contain mast cells. The cytoplasm of mast cells contains granules rich in heparin, serotonin, and his- tamine. Mast cells possess receptors for IgE mole- cules. When antigen becomes attached to mast-cell- bound antibodies, these vasoactive amines are re- leased and initiate an allergic reaction. mastigonemes See Chromista, Chrysomonads. mastigote See undulipodium. mate (noun) 1. an individual who has formed a bond with a member of the opposite sex for breed- ing. 2. an individual actually engaging in a sexual union. (verb) 1. to form an alliance for future breed- ing. 2. to engage in a sexual union which may result in the fertilization of eggs. mate choice the situation where an animal (gener- ally a female) chooses as a mate a specific individual from a small group within the species. Mate choice by females has been observed in insects, fish, rep- tiles, birds, and mammals. Selection involving mate choice plays a major role in the evolution of many sexually dimorphic characters seen in animals. The genetic changes induced by mate choice occur in quite small subspecific populations, and the factors that control choice behavior generally show quanti- tative inheritance (q.v.).
See arena-breeding birds, courtship ritual, lek, sexual selection. Contrast with random mating. mate killers Paramecium aurelia carrying mu par- ticles. Such ciliates kill or injure sensitives with which they conjugate. The presence of mu in a para- mecium protects it from the action of other mate killers. maternal contamination the situation where blood of maternal genotype contaminates a sample of fetal cells from amniotic fluid, chorionic villi, or umbilical blood. This can confound the interpreta- tion of the genetic analysis. maternal effect gene a gene that encodes or af- fects the spatial distribution of a maternal product (mRNA or protein) that is stored in the unfertilized egg and required for normal development in the early embryo. Mothers homozygous for recessive al- leles of such genes are viable, but lay eggs that pro- duce phenotypically abnormal embryos. The geno- type of the mother thus affects the phenotype of her offsprings. Compare with paternal effect gene, zy- gotic gene.
See Appendix C, 1987, Nu¨sslein-Volhard et al.; 1988, 1989, Driever and Nu¨sslein-Volhard; 1988, Macdonald and Struhl; 1995, Lewis et al.; bi- coid, cytoplasmic determinants, cytoplasmic localiza- tion, grandchildless genes, maternal polarity mutants. maternal inheritance phenotypic differences con- trolled by cytoplasmic genetic factors (e.g., in mito- chondria, chloroplasts, or viruses) derived solely from the maternal parent; also known as uniparental heredity, cytoplasmic inheritance, or extranuclear heredity. Contrast with paternal inheritance. See Ap- pendix C, 1923, Boycott and Diver. maternal PKU maternal phenylketonuria, a syn- drome sometimes shown by infants of mothers who are homozygous for the PKU gene. High concentra- tions of phenylalanine (PA) damage the developing brain, and over 90% of infants subjected to an in utero environment rich in PA are irreversibly re- tarded, irrespective of their genotypes. Therefore, it is essential that women with PKU who have gone off the low-phenylalanine diet return to it if they be- come pregnant. See 1954, Bickel, Gerrard, and Hick- mans; phenylketonuria. maternal polarity mutants mutations in Drosoph- ila melanogaster that are maternally inherited and af- fect the development of embryos in a polarized