manner. These genes are transcribed during oogen- esis, and they imprint an anteroposterior pattern on the egg. The genes are grouped phenotypically into anterior genes that effect head and thoracic struc- tures, posterior genes that define the abdominal re- gion, and terminal genes that control the develop- ment of the acron and telson. See Appendix C, 1987, Nu¨sslein-Volhard et al.; 1988, Driever and Nu¨sslein- Volhard; bicoid, cytoplasmic determinants, cyto- plasmic localization, maternal effect gene, zygotic seg- mentation mutants. mating type many species of microorganisms can be subdivided into groups (or mating types) on the basis of their mating behavior; only individuals of different mating types will undergo conjugation. In- dividuals from a given mating type possess on their surfaces proteins that will bind to complementary proteins or polysaccharides found only on the coats of individuals of opposite mating type.
See Parame- cium aurelia, cassettes. matrilinear inheritance the transmission of cyto- plasmic particles only in the female line. matrix attachment regions (MARs) particular eukaryotic DNA sequences that bind to the nuclear matrix. According to a Domain Model of Chromatin Structure, each chromosome is anchored during in- terphase to a protein matrix that occupies the pe- riphery of the nucleus. The MARs are located in se- ries at about 50-kb intervals, and each pair defines a DNA loop domain that is physically isolated from the next loop domain. MARs contain predominantly repetitive DNA. See insulator DNA. matroclinous inheritance See matrocliny. matrocliny inheritance in which the offspring re- sembles the female parent more closely than the male. In Drosophila the daughters produced by attached-X females are matroclinous in terms of their sex-linked genes. See hologynic. maturase a protein encoded by an exon-intron combination that helps catalyze the excision of the intron from its own primary transcript. A maturase is probably not an enzyme that catalyzes intron re- moval and exon splicing, but rather a factor that modifies the specificity of a preexisting splicing en- zyme.
maturation divisions a series of nuclear divisions in which the chromosome complement of the nuclei is reduced from diploid to haploid number. See mei- osis. maturation promoting factor (MPF) a complex first isolated from eggs of Xenopus laevis (q.v.). Im- mature oocytes are normally blocked in the G2 stage of the cell cycle (q.v.). However, when the cells are injected with MPF, they proceed through mitosis and stop at the transition between metaphase and anaphase. The MPF causes chromosomal condensa- tion, breakdown of the nuclear envelope, and the in- hibition of transcription during mitosis.
For these reasons, MPF is often called the mitosis promoting fac- tor. This factor is now known to contain two sub- units: a protein kinase and a cyclin (both of which see). Also see Appendix C, 1971, Masui and Smith; cell division genes, checkpoint, p34 (CDC2). Maxam-Gilbert method See DNA sequencing tech- niques. maxicells bacterial cells that have degraded chro- mosomal DNAs because they have been heavily irra- diated with ultraviolet light.
Replication and tran- scription in such cells is inhibited because of the inability of the damaged DNA to act as template. However, if the maxicells contain multiple copies of a plasmid, the plasmid molecules that did not re- ceive UV hits continue to replicate and transcribe gene products. Thus, maxicells provide a means of analyzing plasmid-encoded functions, while the products encoded by the host genome are reduced to a minimum. See minicells. maxicircles See RNA editing. maximum permissible dose the greatest amount of ionizing radiation safety standards permit to be received by a person. maze an experimental device consisting of a net- work of paths through which a test animal must find its way. The first animal maze was invented by Fran- cis Galton. See Appendix C, Galton.
maze-learning ability the speed at which an ani- mal learns to find its way through a maze without entering blind alleys. mb, mbp See megabase. MBD methyl-CpG-binding domain. See DNA methylation. McArdle disease a hereditary glycogen storage disease in humans with a frequency of 1/500,000. It arises from a deficiency of an enzyme, skeletal mus- cle glycogen phosphorylase, which is encoded by a gene on chromosome 11. M chromosome the human mitochondrial chro- mosome. See human gene maps.
MC1R gene a gene containing 954 bp which en- codes the melanocyte-1-receptor, a protein that con- tains 317 amino acids. The dominant allele (D) en- codes a protein that stimulates the production of black melanin. The recessive allele (d) encodes an isoform (q.v.) which differs from the D protein by 4 amino acid substitutions. Mice homozygous for d are light colored. In Chaetodipus intermedius (q.v.) the D allele is found in those races of mice that live among the dark rocks of ancient lava flows. Mice liv- ing in areas of light-colored rocks and sand are ho- mozygous for the d allele. See Appendix C, 2003, Nachman, Hoekstra, D’Agostino; adaptive mela- nism, melanin. MDV-1 a variant molecule derived from the RNA genome of Q beta phage (q.v.) by in vitro selection experiments. MDV-1 is the shortest replicating mol- ecule known. See in vitro evolution. mean the sum of an array of quantities divided by the number of quantities in the group. mean free path the average distance a subatomic particle travels between collisions. mean square variance (q.v.). mechanical isolation reproductive isolation due to the incompatibility of male and female genitalia. mechanistic philosophy the point of view that holds life to be mechanically determined and expli- cable by the laws of physics and chemistry.
Contrast with vitalism. medaka a common name for Oryzia latipes (q.v.). medial complex See synaptonemal complex. median the middle value in a group of numbers arranged in order of size. median lethal dose the dose of radiation required to kill 50% of the individuals in a large group of or- ganisms within a specified period. Synonymous with LD50. medium the nutritive substance provided for the growth of a given organism in the laboratory. megaannum one million years before the present; symbolized Ma. megabase a unit of length for DNA molecules, consisting of 1 million nucleotides; abbreviated mb, or mbp for megabase pairs. See base pair, gigabase. megakaryocyte a large cell with a multilobed nu- cleus present in the bone marrow, but not circulat- ing in the blood.
Megakaryocytes bud off platelets (q.v.). See von Willebrand disease. megasporangium a spore sac containing mega- spores. megaspore in angiosperms, one of four haploid cells formed from a megasporocyte during meiosis. Three of the four megaspores degenerate. The re- maining megaspore divides to produce the female gametophyte or embryo sac (q.v.). megaspore mother cell the diploid megasporo- cyte in an ovule that forms haploid megaspores by meiotic division. megasporocyte megaspore mother cell. megasporogenesis the production of megaspores. meiocyte an auxocyte (q.v.). meiosis in most sexually reproducing organisms, the doubling of the gametic chromosome number, which accompanies syngamy, is compensated for by a halving of the resulting zygotic chromosome num- ber at some other point during the life cycle. These changes are brought about by a single chromosomal duplication followed by two successive nuclear divi- sions.
The entire process is called meiosis, and it oc- curs during animal gametogenesis or sporogenesis in plants (see pages 266-267). The prophase stage is much longer in meiosis than in mitosis. It is gener- ally divided into five consecutive stages: leptonema, zygonema, pachynema, diplonema, and diakinesis. When the stages are used as adjectives, the nema suffix is changed to tene. During the leptotene stage the chromosomes ap- pear as thin threads with clearly defined chromo- meres. The chromosomes are often all oriented with one or both of their ends in contact with one region of the nuclear membrane, forming the so-called bou- quet configuration. Although each chromosome ap- pears single, it is actually made up of two chroma- tids.
However, the doubleness of the chromosome does not become obvious until pachynema. The DNA replication that doubles the diploid value oc- curs before leptonema. In diploid somatic cells, the 2N chromosomes are present as N pairs, and each chromosome is a replica of one contributed by the male or female parent at fertilization. In the somatic nuclei of most organ- isms, the homologous chromosomes do not pair. However, during the zygotene stage of meiotic pro- phase synapsis of homologous chromosomes takes place. This pairing begins at a number of points and extends “zipperlike” until complete. Pairing is ac- companied by the formation of synaptonemal com- plexes (q.v.). When synapsis is finished, the apparent number of chromosome threads is half what it was
(A) The leptotene stage during spermatogenesis in a hypothetical animal. The organism has two pairs of chromosomes (one metacentric and one submetacentric). The maternal and paternal homologous chromosomes are drawn in light and dark shades to allow them to be distinguished. The centromeres are represented by stippled circles. Note that the chromosomes are oriented with both of their ends attached to the inner surface of the nuclear envelope. The chromosomes are uncoiled and maximally extended. In this state, the chromomeres and chromonemata can be seen readily. A centrosome encompassed by astral rays is present. It contains a mother and daughter centriole, oriented at right angles to each other. (B) During zygonema, synapsis of homologous chromosomes takes place. This pairing begins at one or more points and extends ‘‘zipperlike’’ until complete. Subsequently, the cell contains two bivalents. The centrosome has divided into two daughter centrosomes, each containing a single centriole. (C) During pachynema, each chromosome can be seen to be composed of two sister chromatids, except at the region of the centromere. As a result, the bivalents are converted into tetrads. A synaptonemal complex extends the length of the bivalent. Crossing over occurs at the three places marked by the Xs. The centrosomes move apart. (D) During diplonema, the medial complex of the synaptonemal complex disappears, and in each tetrad one pair of sister chromatids begins to separate from the other pair. However, the chromatids are prevented from separating at places where interchanges have taken place. The points where the chromatids form cross-shaped
configurations are called chiasmata. As a result of the two-strand single exchange in the submetacentric tetrad, two of the four chromatids have both maternal (light) and paternal (dark) segments in their long arms. The metacentric chromosome has undergone a two-strand double exchange, so two of the four chromatids have light and dark segments in both arms. The reciprocal pattern of the exchanged segments is shown especially clearly in E and F. (E) At diakinesis, the chromatids are shorter and thicker, and terminalization is occurring. The centrosomes have reached the poles, and the nuclear envelope is beginning to disappear. (F) At metaphase I, the tetrads are arranged at the equator of the spindle. (G) At late anaphase I, the homologous chromosomes have separated and have moved to each pole.
However, the centromeres have not divided. As a consequence, maternal and paternal chromosomal material has been separated (except in regions distal to points of crossing over). (H1, H2) Secondary spermatocytes containing dyads. (I1, I2) During early anaphse II, centromeres divide and allow separation of sister chromatids. (J1, J2, J3, J4) The four spermatids containing monads. Chromosomes once again uncoil and elongate. (J1) and (J4) each contain one single and one double crossover chromatid.
before, and the visible bodies in the nucleus are now bivalents rather than single chromosomes. During the pachytene stage, each paired chromo- some separates into its two component sister chro- matids (except at the region of the centromere). As a result of the longitudinal division of each homolo- gous chromosome into two chromatids, there exist in the nucleus N groups of four chromatids lying parallel to each other called tetrads. A type of local- ized breakage followed by exchange between nonsis- ter chromatids occurs. This process, called crossing over, is accompanied by the synthesis of an amount of DNA constituting less than 1% of the total in the nucleus. The exchange between homologous chro- matids results in the production of crossover chro- matids containing genetic material of both maternal and paternal origin. During the diplotene stage one pair of sister chro- matids in each of the tetrad begins to separate from the other pair.
However, the chromatids are pre- vented from separating at places where exchanges have taken place. In such regions, the overlapping chromatids form a cross-shaped structure called a chiasma (pl. chiasmata). The chiasmata slip along laterally toward the ends of the chromatids, with the result that the position of a chiasma no longer coin- cides with that of the original crossover. This termi- nalization proceeds until during diakinesis all the chiasmata reach the ends of the tetrad, and the ho- mologs can separate during anaphase. The chromosomes coil tightly during diakinesis and so shorten and thicken to produce a group of compact tetrads lying well spaced out in the nucleus, often near its membrane. Terminalization is com- pleted, and the nucleolus disappears.
The nuclear envelope disappears during division I, and the tetrads are arranged at the equator of the spindle. The chromatids of a tetrad disjoin in such a way that there is separation of maternal from pater- nal chromosomal material with the exceptions of re- gions distal to where crossing over has occurred. Di- vision I produces two secondary gametocytes, which contain dyads surrounded by a nuclear envelope. Division II commences after a short interphase during which the chromosomes do not uncoil. The nuclear membrane disappears, and the dyads arrange themselves upon the metaphase plate. The chroma- tids of each dyad are equivalent (again with the ex- ception of regions distal to points of crossing over); the centromere divides and thus allows each chro- mosome to pass to a separate cell.
Thus during mei- otic anaphase I centromeres do not have the block to DNA replication removed, as is true for mitotic anaphase, and centromeres remain functionally sin- gle. At meiotic anaphase II, however, the block is removed, as in mitosis, and each chromatid is con- verted into a functional chromosome. In animals, di- vision II produces four spermatids (q.v.) or ootids (q.v.) which contain monads surrounded by a nu- clear membrane. Meiosis therefore provides a mech- anism whereby (1) an exchange of genetic material may take place between homologous chromosomes and (2) each gamete receives but one member of each chromosome pair. J. B. Farmer and J. E. S. Moore proposed in 1905 that the name meiosis be used for the reduction divisions that precede the for- mation of gametes. The names leptotene, zygotene, pachytene, and diplotene were coined by H. v. Wini- warter in 1900. In the third edition (1925) of his Cell in Development and Heredity, E. B. Wilson started the tradition of using these names as adjec- tives and the nema forms as nouns.
The type of meiosis described above immediately precedes gametogenesis. This type (gametic meiosis) is characteristic of all animals. Fungi are character- ized by zygotic meiosis, where meiosis immediately follows zygote formation. Haplodiplomeiosis refers to the situation seen in most plants where meiosis in- tervenes between a prolonged diploid phase and an abbreviated haploid phase. See alternation of genera- tions, centromeric coupling, cohesin, interkinesis, oo- genesis, recombination nodules, separase, sex, Sgo, sister chromatid cohesion, spermatogenesis, telo- mere-led chromosome movement, zygDNA. meiosporangium (plural meiosporangia) a sporan- gium in which meiosis occurs.
Compare with mi- tosporangium. meiospore a spore produced by meiosis. meiotic cycle a first (reductional) division, fol- lowed by a second (equational) division. See meiosis. meiotic drive any alteration in meiosis or the sub- sequent production of gametes that results in pref- erential transmission of a particular genetic variant. Examples of meiotic drive are known in a wide variety of organisms, including fungi, plants, insects, and mammals. See cheating genes, segregation dis- tortion. mei-W68 a recessive mutation in D. melanogaster that abolishes meiotic exchange in the oocytes of homozygotes. S. cerevisiae has a homolog called SPO11. The H. sapiens homolog is located at 20q13.2-q13.3. See recombination nodules. Melampsora lini a rust fungus that parasitizes flax. See Linum usitatissimum.