Melandrium album a dioecious plant with a method of sex determination similar to man (that is, maleness is determined by the Y chromosome). melanin a dark brown to black pigment responsi- ble for the coloration of skin, hair, and the pig- mented coat of the retina (q.v.). The pigments in bird feathers, reptile skins, insect exoskeletons, and the ink of cephalopods all contain melanins. The macromolecule consists of polymers of indole 5,6- quinone and 5,6-dihydroxyindole-2-carboxylic acid and is formed by the enzymatic oxidation of tyrosine or tryptophan. A segment of the structure is shown below.
Skin color variation among humans is an evolution- ary adaptation using melanin to screen out harmful ultraviolet light. The sunscreen must be dense enough to prevent the photolysis of folic acid (q.v.) but transparent enough to allow the synthesis of vi- tamin D (q.v.). For a given human population the trade-off depended on the amount of sunshine in the habitat where it evolved. See Appendix C, 2000, Jablonski and Chaplin; agouti, albinism, eumelanin, MC1R gene, phaeomelanin, SLC 24A5. melanism the hereditary production of increased melanin resulting in darker coloring. See adaptive melanism, albinism, industrial melanism. melanocytes cells located in the basal cell layer of the skin. Melanocytes form long slender processes which ramify among the adjacent keratinocytes.
Eventually about 30 of these are connected to each melanocyte. Melanosomes, the organelles that syn- thesize melanin, are in the cytoplasm of melano- cytes. The larger the melanosomes, the darker the skin pigmentation. Intermedin (q.v.) stimulates mel- anin synthesis in response to UV damage. The mela- nocytes then transfer small packets of melanin to the keratinocytes, and these move to the surface of the skin forming a sunscreen.
The result is tanning. In the mouse the melanocytes that give color to the fur are derived from 34 cells that have their fates deter- mined early during embryogenesis. See Appendix C, 1967, Mintz. melanocyte-1-receptor See MC1R gene. melanocyte-stimulating hormone intermedin (q.v.). melanoma a cancer composed of melanocytes (q.v.). Melanophus femur-rubrum a species of grasshop- per widely used for the cytological study of meiosis. melanosome an intracellular organelle containing aggregations of tyrosinase, found in melanocytes. melanotic tumor of Drosophila See pseudotumor.
Meleagris gallopavo the domesticated turkey. melphalan a mutagenic, alkylating agent. See il- lustration on page 270. melting in nucleic acid studies, the denaturation of double-stranded DNA to single strands. melting-out temperature the temperature at which the bonds joining the constituent strands of a DNA/DNA or DNA/RNA duplex are broken and the molecules dissociate. melting profile a curve describing the degree of dissociation of the strands in a DNA/DNA or DNA/ RNA duplex in a given time as a function of temper- ature. The stabilities of duplexes are a function of
their molecular weights, so that the melting profiles shift to the right as the chain lengths increase. melting proteins See helix-destabilizing proteins. melting temperature the temperature at which 50% of the double helices have denatured; the mid- point of the temperature range over which DNA is denatured. See denaturation, Tm. membrane 1. Cytology: a lipid bilayer that encloses the protoplasm (q.v.) of a cell (plasma membrane, q.v.) or divides the cell into compartments that are distinctive in morphology and function, such as the membranes of chloroplasts (q.v.), mitochondria (q.v.), lysosomes (q.v.), peroxisomes (q.v.), and the cavities of the endoplasmic reticulum (q.v.) and Golgi appa- ratus (q.v.).
See lipid bilayer model. 2. Anatomy: any thin, pliable layer of tissue that covers surfaces or separates or connects regions, structures, or organs of a plant or animal, such as the basilar membrane of the inner ear, extra-embryonic membranes, pleuro- pericardial membranes of the heart, pleuroperi- toneal membranes that line the body cavity, and sy- novial membranes of bony joints. 3. Biotechnology: nonlipid membranes of nylon, nitrocellulose, polyvi- nyladine fluoride (PVDF), or other substances that are used for blotting techniques. See northern blot- ting, Southern blotting, western blotting. memory the persistent modification of behavior that results from experience. The conversion of short- term memory (STM) to long-term memory (LTM) requires the synthesis of specific gene products. See Appendix C, 1982, Kandel and Schwartz; 1994, Tully et al.; 2003, Si et al.; Aplysia, CPEB protein, CREBs, immunological memory, neuregulins, spaced training. menarche the beginning of the first menstrual cy- cle during puberty in human females.
Compare with menopause. Mendeleev table the periodic table of chemical elements named in honor of Dmitri Mendeleev, the Russian chemist who constructed the earliest form of the periodic table in 1868. The table appears on page 330. Mendelian character a character that in inheri- tance follows Mendelian laws (q.v.). Mendelian genetics referring to the inheritance of chromosomal genes following the laws governing the transmission of chromosomes to subsequent gen- erations; also called Mendelism. Mendelian Inheritance in Man (MIM) a catalog of human genetic diseases prepared by Victor McKusick.
MIM is now in its twelfth edition and is also available electronically. Each gene is given a six digit MIM number, and this is followed by four digits that specify allelic variants. For exam- ple, the BRCA 1 mutation commonly found among Ashkenazi Jews is 113705.0003. In addition to genes that cause hereditary diseases when mutated, MIM also includes polymorphisms that influence disease resistance. The Duffy blood group gene (q.v.) (110700.0001), which confers resistance to malaria, is an example. See Appendix C, 1966, McK- usick; breast cancer susceptibility genes, human ge- netic diseases, OMIM. Mendelian laws 1. The Law of Segregation. The factors of a pair of characters are segregated. In mod- ern terms, this law refers to the separation into dif- ferent gametes and thence into different offspring of the two members of each pair of alleles possessed by the diploid parental organism. 2. The Law of Inde- pendent Assortment.
The members of different pairs of factors assort independently. A restatement of the law in modern terms is that the members of differ- ent pairs of alleles are assorted independently into gametes during gametogenesis (provided they reside on different chromosomes), and that the subsequent pairing of male and female gametes is at random. See Appendix C, 1856, 1865, 1866, Mendel; 1900, de Vries, Correns, Bateson; 1902, Sutton; 1990, Bhat- tacharyya et al. Mendelian population an interbreeding group of organisms sharing a common gene pool. mendelize to segregate according to Mendelian laws (q.v.).
menopause the cessation of menstrual cycles in human females, usually occurring between age 50 and 60. meq milliequivalent. See gram equivalent weight. mer See mers. mercaptopurine a synthetic purine analog, one of the first inhibitors of DNA synthesis, shown to sup- press the growth of cancer cells.
Mercenaria mercenaria See Pelecypoda. mericlinal chimera an organism or organ com- posed of two genetically different tissues one of which partly surrounds the other. meristems the undifferentiated, mitotically active tissues of plants. The meristems at the tips of the roots and shoots are referred to as apical meristems. See floral organ primordia. meristic variation variation in characters that can be counted, like the number of bristles, leaves, scales, etc.
merlin See neurofibromatosis. meroblastic cleavage cleavage producing cells, some of which are larger than others, because of a polarized distribution of yolk. merogony 1. an asexual replicative cycle occurring during the haploid phase of Plasmodium and other apicomplexans. Subsequently the infected hepato- cyte or erythrocyte ruptures, releasing hundreds of merozoites. See Plasmodium life cycle. 2. the develop- ment of an experimentally produced egg fragment (containing a diploid nucleus or a haploid nucleus of male or female origin) into a small-sized embryo, called a merogone. meroistic See insect ovary types. meromixis genetic exchange in bacteria involving a unidirectional transfer of a partial genome. See F factor (fertility factor).
merospermy the situation in which the nucleus of the fertilizing sperm does not fuse with the egg nu- cleus and later degenerates. Subsequent develop- ment is by gynogenesis. merotomy the dissection of cells into several por- tions, with or without nuclei, as in experimental grafting with Acetabularia (q.v.). merozoite See Plasmodium life cycle. merozygote a partially diploid bacterial zygote containing an exogenotic chromosomal fragment do- nated by the F+ mate. The exogenote may also be introduced during transduction or sexduction. See endogenote, exogenote. mers the unit that defines the number of bases in an oligonucleotide polymer. For example, oligonu- cleotides that contain 15 or 17 bases are referred to as 15 mers and 17 mers, respectively.
Mertensian mimicry See mimicry. mesenchyme an embryonic type of connective tis- sue, consisting of amoeboid cells with many pro- cesses. Populations of such cells form a loose net- work. Most mesenchyme is derived from mesoderm. The mesenchyme produces the connective tissue and the circulatory system during the development of vertebrates. Mesocricetus auratus the golden hamster, a rap- idly breeding rodent used in the laboratory. The haploid chromosome number is 22. Contrast with Cricetulus griseus. mesoderm the middle layer of embryonic cells be- tween the ectoderm and endoderm in triploblastic animals. Mesoderm forms muscle, connective tissue, blood, lymphoid tissue, the linings of all the body cavities, the serosa of the viscera, the mesenteries, and the epithelia of the blood vessels, lymphatics, kidney, ureter, gonads, genital ducts, and suprarenal cortex. See Appendix C, 1845, Remak.
mesokaryotic referring to the dinoflagellate nu- cleus, in which the chromosomes are in a con- densed, discrete condition at all times. mesophil a microorganism whose optimum growth temperature lies in the range between 20°C and 45°C. Therefore it is easily cultured in the labora- tory. mesosome one of the invaginated segments of the plasma membranes in certain bacteria to which DNA molecules are attached. Mesostigma viride a unicellular green alga that belongs to the class Prasinophyceae. Phylogenetic trees inferred from the nucleotide sequences of the RNAs of the small ribosomal subunit put the prasi- nophytes at the base of the Chlorophyta. The entire
genome of the chloroplasts of M. viride has been se- quenced and found to consist of 118,360 base pairs. This chDNA contains 135 ORFs, more genes than reported for any land plant or algal chDNA. Meso- stigma chloroplasts have a gene organization very similar to that of land plant chloroplasts. This spe- cies therefore seems to be a living fossil (q.v.) that shows close similarities to the organisms that lived about 800 million years ago and that were the ances- tors of both green algae and land plants. See Appen- dix A, Protoctista, Chlorophyta; Appendix C, 2000, Lemieux, Otis, and Turmel; Appendix F; chloro- phyte, chloroplast DNA (chDNA). mesothorax the middle of the three thoracic seg- ments of an insect.
It bears a pair of legs and (in winged insects) a pair of wings. Mesozoic the 180 myr era during which dinosaurs arose, flourished, and became extinct. See geologic time divisions. message transport organizer (METRO) a region of the Xenopus mitochondrial cloud (q.v.) in which specific RNAs are sorted into distinct spatial loca- tions for translocation to the vegetal pole of the egg. messenger RNA an RNA molecule that functions during translation (q.v.) to specify the sequence of amino acids in a nascent polypeptide.
In eukaryotes, mRNA is formed in the nucleus from premessenger RNA molecules (q.v.). See Appendix C, 1956, Volkin and Astrachan; 1961, Jacob and Monod, Brenner, Jacob, and Meselson; 1964, Marbaix and Burny; 1967, Taylor et al.; 1969, Lockard and Lingrel; exon, intron, polyadenylation, polysome, posttranscriptional modification. met methionine. See amino acid. metabolic block a nonfunctional reaction in a metabolic pathway, as a consequence of a defective (mutant) enzyme whose normal counterpart cata- lyzes the reaction. metabolic control levels a hierarchical series of biochemical levels, i.e., the level of the genome, the transcriptome, the proteome, or the metabolome (all of which See), at which metabolic controls are studied. metabolic pathway a series of stepwise biochemi- cal changes in the conversion of some precursor sub- stance to an end product, each step usually catalyzed by a specific enzyme.
The citric acid cycle, the orni- thine cycle, and the pentosephosphate pathway (all of which see) are examples of metabolic pathways. See metabolome. metabolic poison a compound poisoning a meta- bolic process. See dinitrophenol. metabolism the sum of all the physical and chem- ical processes by which living cells produce and maintain themselves and by which energy is made available for the use of the organism. See chemo- trophs, glycolysis, pentosephosphate pathway, pho- tophosphorylation, photosynthesis, phototrophs. metabolite a chemical compound that is produced or consumed during metabolism. Polymeric biologi- cal molecules are excluded from this definition. Me- tabolites include low molecular weight compounds that are produced or converted by enzymes during metabolism or the precursors or breakdown prod- ucts of biopolymers. See metabolic pathway, metabo- lome.
metabolome the sum of all small molecular weight metabolites in a biological sample of interest. The metabolome of a given cell will vary greatly de- pending on its physiological or developmental state, its age, or its response to disease or drugs. See meta- bolic control levels. metacentric designating a chromosome with a cen- trally placed centromere. metachromasy the phenomenon where one dye stains more than one color.
A substance that stains metachromatically is called a chromatrope. Chroma- tropes are high-molecular-weight structures with se- rially arranged charged groups. Acidic mucopoly- saccharides and nucleic acids are prime examples. Azure B (q.v.) is an example of a metachromatic dye. The color it produces depends on the way dye molecules stack on the chromatrope. As the amount of stacking increases, the color changes from green, to blue, to red. In an azure B-stained tissue section, chromosomes generally stain green, nucleoli and cy- toplasmic ribosomes blue, and mucopolysaccharide- containing deposits red. metachromatic dye a dye that stains tissues two or more colors. See azure B, metachromasy. metafemale in Drosophila, a female phenotype of relatively low viability in which the ratio of X chro- mosomes to sets of autosomes exceeds 1.0; pre- viously called a superfemale. See intersex, metamale. metagenesis alternation of generations (q.v.) among animals. Metagenesis is commonly seen in inverte- brates, especially coelenterates. Unlike the situation among plants, both generations are diploid.