MIC2 a gene located distally on the short arm of the X chromosome (at Xp22.23) in the human pseudoautosomal region (q.v.). It therefore can undergo crossing over with an allele on the Y (at Yp11.3). MIC2 encodes a glycoprotein named CD99. This is an integral membrane protein found in hema- topoietic cells and skin fibroblasts. See XG. micelle a spherical array of amphipathic molecules in which the nonpolar tails form a hydrocarbon mi- crodroplet enclosed in a shell composed of the polar heads.
Michaelis constant (Km) the substrate concentra- tion at which the reaction rate of an enzyme is half maximal; the concentration at which half of the en- zyme molecules in the solution have their active sites occupied by a substrate molecule. Michurinism the genetic theories expounded by the Russian horticulturist I. V. Michurin, particu- larly those dealing with the modification of the ge- netical constitution of a scion by grafting. Michurin’s theories were subsequently incorporated into Lysen- koism (q.v.). micro- 1. a prefix meaning one millionth; used with units of measurements in the metric system. 2. a prefix meaning microscopic or minute. microarray technology See DNA microarray tech- nology.
microbeam irradiation the use of a beam of mi- croscopic diameter to selectively irradiate portions of a cell with ionizing radiation or ultraviolet light. microbial genetics the genetic study of microor- ganisms. microbiology the scientific study of microorgan- isms. microbody an organelle, generally less than 1 mi- crometer in diameter found in the cytoplasm of eukaryotic cells. Microbodies contain a variety of functioanlly related enzymes surrounded by a mem- branous sac. Examples of microbodies are the glyco- some, the glyoxysome and the peroxisome (all of which see).
Microbracon hebetor a wasp (called Habrobracon juglandis or Bracon hebetor in the early genetic litera- ture). It is the arrhenotokous species in which the genetic control of sex determination was elucidated. See Appendix A, Arthropoda, Insecta, Hymenoptera. microcarriers microscopic beads or spheres, com- posed of dextran or other substances, that are used in tissue culture to attract and hold cells that must be anchored before they can proliferate. See anchor- age-dependent cells. microcinematography See time-lapse microcine- matography. micrococcal nuclease an endonuclease used to cleave eukaryotic chromatin preferentially between nucleosomes. microconidia See conidium. microdeletion a chromosomal deletion too short to be identified under the light microscope, but de- tectable by comparing the base sequences of the normal and deleted DNA segments.
microevolution an evolutionary pattern usually viewed over a short period of time, such as changes in gene frequency within a population over a rela- tively few generations (industrial melanism, for ex- ample). See macroevolution. microfilaments elongated intracellular fibers 5-7 nanometers in diameter, containing polymerized ac- tin, and thought to function in maintenance of cell structure and movement. Micrographia a book published in 1665 by R. Hooke in which the first description of cells is given. micromanipulator an instrument allowing surgery on and injection into microscopic specimens. It is also useful in isolating single cells.
micrometer (µm) one-millionth (10−6) meter. The preferred length unit for describing cellular dimen- sions. The micrometer replaces the micron (µ), an equivalent length unit, found in the earlier litera- ture. See nanometer. micron ( ) See micrometer. micronucleus the smaller reproductive nucleus as distinguished from the larger, vegetative macronu- cleus (q.v.) in the cells of ciliates. Micronuclei are diploid and are transcriptionally inactive. They par- ticipate in meiosis and autogamy. See nuclear dimor- phism, Tetrahymena. microorganism an organism too small to be ob- served with the unaided eye.
micropyle 1. a canal through the coverings of the nucellus through which the pollen tube passes dur- ing fertilization. In a mature seed, the micropyle serves as a minute pore in the seed coat through which water enters when the seed begins to germi- nate. 2. the pore in the egg membranes of an insect oocyte that allows entry of the sperm. microRNAs (miRNAs) small, noncoding RNAs, approximately 21-25 nucleotides long, which si- lence gene expression either by mediating the degra- dation of or by repressing the translation of protein- coding messenger RNAs (mRNAs) containing com- plementary or partially complementary sequences.
miRNAs are derived from larger precursor RNA transcripts that form hairpin structures, which are processed such that only one strand of these struc- tures gives rise to a mature miRNA molecule. Sin- gle-stranded miRNA is bound by a complex and rec- ognizes partially or fully complementary sequences in the target mRNA. Some miRNAs, such as small temporal RNAs (q.v.), bind to partially complemen- tary 3′ untranslated regions of target mRNAs to in- hibit protein synthesis. Others bind to perfectly complementary sequences in the target mRNA and, like small interfering RNAs (q.v.), mediate the cleavage of these mRNAs. Several hundred different miRNAs have been identified in a variety of organ- isms, including yeast, fungi, plants, nematodes, flies, and mammals. However, the function of most miRNAs is not known. Sequenced miRNAs are stored in a public database.
See Appendix E, Individ- ual Databases; RNA interference (RNAi). microsatellites chromosomal sites that contain re- peats of a small number of nucleotides arranged one after the other. The tandemly repeated motif usually contains between one and six nucleotides. Microsat- ellites are also called short tandem repeats (STRs). Microsatellites have been found in every species studied so far. They are evenly distributed along the chromosomes, but are rare within coding sequences.
During the replication of chromosomal segments containing tandem repeats, the nascent DNA strand may separate from the template strand and then re- anneal so that it is out of phase with the template. When replication resumes, the newly completed strand will be longer or shorter than the template depending on whether the looped-out bases oc- curred in the template or the nascent strand. Such slippage strand mispairing during the replication of microsatellites is the main mechanism for increasing the length of microsatellites. Some human diseases are caused by expaning the length of trinucleotide repeats (q.v.).
See genetic instability, STR analysis. microscopy See Nomarski differential interference microscope, phase contrast microscope, photographic rotation technique, photomicrography, polarization microscope, SEM, TEM, time-lapse microcinematog- raphy, ultraviolet microscope. microsome fraction a cytoplasmic component, obtained upon centrifugation of homogenized cells, consisting of ribosomes and torn portions of the en- doplasmic reticulum. See Appendix C, 1943, Claude. microspectrophotometer an optical system com- bining a microscope with a spectrophotometer.
With this apparatus one can determine the amount of light of specified wavelength passing through a given cytoplasmic region relative to some standard area. From such measurements estimates can be made of the concentrations of dye-binding or ultra- violet-light-absorbing materials present in the cyto- plasmic or nuclear area in question. See Appendix C, 1950, Swift; flying spot cytometer.
microspore the first cell of the male gametophyte generation of seed plants. Each becomes a pollen grain. microspore culture See Appendix C, 1973, De- bergh and Nitsch; anther culture. microsporidia parasitic species belonging to the phylum Microspora (see Appendix A, Protoctista). Microsporidian species of genus Nosema are para- sites of silkworms, honeybees, and Drosophila. A unique characteristic of microsporidians is that the unicellular resting spore contains a coiled hollow tube, the polar filament, which is a specialized prod- uct of the Golgi apparatus. During infection, the po- lar filament turns inside out to form a hollow tube through which the infectious part of the spore is ex- truded into the new host. While microsporidia lack
conventional mitochondria, they have some genes for mitochondrial proteins. On the basis of a reanal- ysis of the sequence data from their rRNAs, it ap- pears that microsporidia are specialized fungi, and their structural simplicity is now attributed to re- gressive evolution (q.v.). See Encephilitozoon cuni- culi, ribosome. microsporocyte the pollen mother-cell, which un- dergoes two meiotic divisions to produce four mi- crospores. microsporogenesis the production of micro- spores. microsporophyll the stamen. See flower. microsurgery surgery done while viewing the ob- ject through a microscope and often with the aid of a micromanipulator. See Appendix C, 1952, Briggs and King; 1967, Goldstein and Prescott; 1980, Ca- pecchi.
microtome a machine for cutting thin (1 to 10 mi- crometer thick) slices of paraffin-embedded tissue. These sections are later stained and examined with the light microscope. See Appendix C, 1870, His; ul- tramicrotome. microtomy the technique of using the microtome in the preparation of sections for study under the microscope. microtrabecular lattice a network of thin fila- ments interconnecting the three major cytoskeletal elements (microtubules, microfilaments, and inter- mediate filaments). This three-dimensional lattice can be visualized in freeze-etched preparations viewed with the electron microscope.
microtubule organizing centers (MTOCs) the structures or loci that give rise to microtubular arrays. Centrioles and kinetosomes function as MTOCs in some organisms. In others, the MTOC merely consists of an amorphous collection of fibrils and granules. MTOCs contain RNA, and RNA repli- cation accompanies their multiplication. See Appen- dix C, 1995, Moritz et al. microtubules long, nonbranching, thin cylinders with an outside diameter about 24 nanometers and a central lumen about 15 nm diameter. The lengths are at least several micrometers. The tubules are composed of strands called protofilaments, and there are usually 13 of these. Each protofilament in turn is composed of a linear array of subunits, and each subunit is a dimer containing an alpha and a beta tubulin molecule. Microtubules play key roles in cell division, secretion, intracellular transport, morpho- genesis, and ciliary and flagellar motion. See axo- neme, centriole, tubulin.
Microtus the genus of meadow mice and voles subjected to extensive cytogenetic study. M. agrestis, the field vole, has giant sex chromosomes. microvillus a fingerlike projection of the plasma- lemma of a cell. mictic referring to an organism or species capable of biparental sexual reproduction. middle lamella the outermost layer of a plant cell wall that connects it to its neighbor. midget an abnormally small adult human with normal proportions.
See ateliosis, human growth hor- mone. Contrast with chrondroplasia, dwarf. midparent value the mean of two parental values for a quantitative trait in a specific cross.
See quanti- tative inheritance. midpiece the portion of the sperm behind the head containing the nebenkern. midspindle elongation the elongation during ana- phase of the midregion of the mitotic spindle. This elongation serves to draw the chromosomes pole- ward once the movement of chromosomes along traction fibers has ceased. migrant selection selection based on the different migratory abilities of individuals of different geno- types. If, for example, individuals carrying gene M found new colonies more often than those bearing gene m, then gene M is said to be favored by migrant selection.
migration in population genetics, the movement of individuals between different populations of a species, resulting in gene flow (q.v.). migration coefficient the proportion of a gene pool represented by migrant genes per generation. migration inhibition factor a lymphokine that in- hibits the movement of macrophages under in vitro culture conditions. Miller spreads chromosomal whole mounts spread for electron microscopy by a method devel- oped by O. L. Miller.
In this method, chromosomes from ruptured nuclei are centrifuged through a solu- tion of 10% formalin in 0.1 mole sucrose and onto a membrane-coated grid. The grid is then treated with an agent that reduces the surface tension as the grid dries. After being stained with phosphotungstic acid, the preparation is ready for viewing under the elec- tron microscope.