Miller trees

31 Mar

Miller trees
Miller trees transcribing rRNA genes first seen in Miller spreads of the amplified rRNA of extrachro- mosomal nucleoli from salamander oocytes. Each rRNA transcription unit (rTU) was about 2.5 mi- crometers long, corresponding to a molecule of 8,000 base pairs. The rRNA molecules attached to the chromatin fiber give rTUs a Christmas tree mor- phology (hence the term Miller tree), and such rTUs are arranged in tandem, separated by fiber-free spac- ers. Each nascent transcript ends distally with a ter- minal ball. This represents a processing complex that forms when the 5′- external transcribed spacer (ETS) region of the rRNA interacts with a group of specific polypeptides. See Appendix C, 1969, Miller and Beatty; 1976, Chooi; preribosomal RNA, ribo- somal RNA genes, 5.8S rRNA. milliequivalent See gram equivalent weight. milliliter (ml) one-thousandth of a liter; the vol- ume contained in a cube of side 1 centimeter. One ml of water weighs one gram at 4°C. millimicron See nanometer. Millipore filter a disc-shaped synthetic filter hav- ing holes of specified diameter through its surface.

The available pore sizes range between 0.005 and 8 micrometers. The discs are used to filter micro- organisms out of nutrient fluids that will not stand sterilization by autoclaving. mimicry the similarity in appearance of one spe- cies of animal to another that affords one or both protection. In Batesian mimicry, one of two species is poisonous, distasteful, or otherwise protected from predators, and often conspicuously marked.

The mimic is innocuous, gaining protection from preda- tors by its similarity to the model. In Mullerian mim- icry, both are distasteful to predators, and they gain mutually from having the same warning coloration, since predators learn to avoid both species after tast- ing one. Peckhammian mimicry is an aggressive mim- icry in which the predator is the pretender (mimic); e.g., a female firefly of one species mimics the flash- ing sexual signals of another species, thus attracting a male of that other species which becomes a meal for the pretender. In Mertensian mimicry, one species is mildly poisonous (e.g., false coral snakes) and thus serves as a model for a fatally poisonous species (e.g., true coral snakes). Obviously, a predator can learn only if it survives the encounter. See automimicry, fireflies, frequency-dependent fitness. MIM number a catalog number for an inherited human disease that is listed in Mendelian Inheri- tance in Man (q.v.).

minicells small anucleate bodies produced by the aberrant division of bacteria. Certain mutant strains of E. coli produce large numbers of minicells. Al- though minicells lack genophores, they may contain one or more copies of the plasmids present in the parent cells. This permits the analysis of the plasmid DNA or of plasmid-encoded gene products in the absence of the host genome and its products. See maxicells.

minichromosomes beaded DNA structures of cer- tain viruses, similar to the beaded nucleosomal structures characteristic of eukaryotic chromatin. The circular DNA duplexes of papovaviruses (q.v.) are bound throughout the replication cycle to host histones, except H1. minicircles See RNA editing. minigenes segments on chromosomes that code for the variable regions of immunoglobulin heavy and light chains. There are several hundred of these segments, but during the differentiation of B lym- phocytes a single minigene is attached to the gene

Mitochondrial cloud

segment coding for the constant region. See Appen- dix C, 1965, Dreyer and Bennett; immunoglobulin genes. minimal medium in microbiology a medium pro- viding only those compounds essential for the growth and reproduction of wild-type organisms. mink long, slender mammals with short legs and partially webbed toes. These carnivores are prized for their luxurious fur. They have adapted well to ranch-rearing, although they have not become tame. Skins from ranch-reared mink are the single most important source of pelts for the fur industry.

There are at least three loci controlling coat pigmentation, and mutations of these genes produce color varia- tions, some of which are popular commercially. See Aleutian mink, Mustella. minority advantage the phenomenon first ob- served in multiple choice mating experiments with Drosophila melanogaster. Males carrying certain ge- netic markers mate with relatively greater success when they are rare than when they are common. See Appendix C, 1951, Petit; frequency-dependent selec- tion. Minos element a transposon from Drosophila hy- dei that belongs to the Tc1 family of transposable elements (q.v.). Minos elements that carry foreign genes have been experimentally introduced into the germ line of Anopheles stephensi.

This mosquito is a major carrier of malaria (q.v.) in urban areas of the Indian subcontinent. The success in producing trans- genic mosquitoes using the Minos vector may even- tually lead to the construction of genetically engi- neered strains of the insect that are resistant to infection by the malaria parasite. See Appendix C, 2000, Catteruccia et al.; Anopheles, mariner ele- ments, Plasmodium. minus (−) viral strands See plus (+) and minus (−) viral strands. Minutes mutations in Drosophila that are domi- nant and lethal when homozygous. Heterozygotes have a prolonged developmental period, and the adults are small, semisterile, with short thin bristles, rough eyes, etched tergites, and abnormal wing ve- nation. Many Minutes are small deficiencies.

There are at least 60 different Minute loci scattered throughout the genome, and some represent genes encoding ribosomal proteins. Miocene the fourth epoch in the Tertiary period. Representatives of all mammalian families were present. The most advanced family of flowering plants, the Compositae, evolved during this epoch, as did songbirds and rodents. The chimpanzee and hominid lines diverged from a common ancestor late in the Miocene. See geologic time divisions, Pan. Mirabilis jalapa the four-o’clock, a variegated di- cotyledonous plant extensively studied in terms of plastid inheritance. See Appendix A, Plantae, Angio- spermae, Dicotyledoneae, Caryophyllales; Appendix C, 1909, Correns and Bauer.

miscarriage a spontaneous abortion (q.v.). mischarged tRNA a tRNA molecule to which an incorrect amino acid is attached. mismatch See mispairing. mismatch repair a system for correcting mis- matches between bases in the parent and daughter strands of DNA that make hydrogen bonding impos- sible. Recognition of the mismatch requires several proteins, including the one encoded by the MSH2 gene. After the segment is cut out, the resulting gap is filled by the actions of DNA polymerase I and DNA ligase (q.v.). See hereditary nonpolyposis colo- rectal cancer (HNPCC). mispairing the presence in one chain of a DNA double helix of a nucleotide not complementary to the nucleotide occupying the corresponding position in the other chain.

missense mutant a mutant in which a codon is mutated to one directing the incorporation of a dif- ferent amino acid. This substitution may result in an inactive or unstable product. Contrast with nonsense mutation. missing link an unknown or postulated intermedi- ate in an evolutionary sequence of fossil forms. Archeopteryx (q.v.) provided the missing link be- tween reptiles and birds. See living fossil. Mississippian See Carboniferous. mistranslation the insertion of an incorrect amino acid into a site on a growing polypeptide chain that is the result of environmental factors or mutations that effect either the tRNA, or the enzymes that at- tach specific amino acids to specific tRNAs, or the ribosome itself. mitochondrial cloud a mitochondria-surrounded Balbiani body (q.v.) found in the pre-vitellogenic Xenopus oocyte (q.v.) that functions in the assembly and translocation of specific RNAs to the vegetal pole, where the germ plasm (q.v.) is formed. The mitochondrial cloud originates near the nuclear membrane during early oogenesis (q.v.), increases in

Mitochondrial DNA (mtDNA)
size, and subsequently breaks down into smaller “is- lands” that become localized to the vegetal pole of the egg. See cytoplasmic localization, METRO, sponge body. mitochondrial DNA (mtDNA) the mitochondrial genome consists of a circular DNA duplex, and there are generally 5 to 10 copies per organelle. Hu- man mitochondrial DNA (q.v.) is a 16.6 kb cir- cle. The mtDNAs of plants are much larger (the mtDNA of Arabidopsis contains 367 kb), and fungi are intermediate (there are 75 kb in the mtDNA cir- cles of Saccharomyces). Introns have been found in mitochondrial genes from yeast but not from mam- mals.

The genetic code of mitochondria differs slightly from the “universal” genetic code. Since, of the two gametes, only the egg cell contributes sig- nificant numbers of mitochondria to the zygote, mtDNA is maternally inherited. Mitochondrial ge- nomes experience higher mutation rates than those in nuclear genomes and thus have more power to reveal evolutionary differences between closely re- lated species than nuclear genes have. In addition, mtDNAs do not undergo genetic recombination, so their sequences stay the same from one generation to the next, except when altered by mutations. Mi- tochondrial DNA is usually the only type of DNA to survive in long-dead specimens, due mainly to its abundance (500 to 1,000 copies per cell, rather than only 2 copies as in the nuclear DNA of diploid cells). Mitochondrial DNA sequences are available for about 500 species of eukaryotes (URL http:// See Appendix C, 1959, Che- vremont et al.; 1966, Nass; 1968, Thomas and Wil- kie; 1974, Dujon et al., Hutchison, et al.; 1979, Avise et al.; Barrell et al.; 1981, Anderson et al.; 1999, Andrews et al.; ATP synthase, human mito- chondrial DNA, mtDNA lineages, Neandertal, petites, Podospora anserina, promiscuous DNA, RNA editing, tRNA genes, universal code theory.

mitochondrial Eve See mtDNA lineages. mitochondrial DNA lineages evolutionary trees derived from data on mtDNAs. Human mitochon- dria are maternally inherited, and therefore mtDNA is contributed by the female parent to the next gen- eration. Furthermore, mitochondrial genes do not undergo recombination. For these reasons, it is much simpler to trace mutations in mtDNA than in genomic DNA.

In an analysis of mtDNAs published in1987, the restriction fragment polymorphisms were traced back to an mtDNA molecule carried by a hy- pothetical woman living in Africa about 200,000 years ago. In 1995, comparative studies of the entire mitochondrial genomes from three humans (one each from Africa, Japan, and Europe) and four apes (a chimpanzee, a bonobo, a gorilla, and an orang- utan) resulted in the estimate of 143,000 years as the time for the origin of the ancestral human mt- DNA, again in Africa. In the popular press, the woman with this ancestral mtDNA is sometimes called “African Eve” or “mitochondrial Eve.” Mito- chondrial Eve is our most recent common ancestor in the purely female line. Other women were contem- poraries of Eve, but their mitochondria have not been retained, usually because their link with today’s descendants is blocked because it passes through a male ancestor.

See Appendix C, 1979, Avise, Lans- man, and Shade; 1987, Cann, Stoneking, and Wil- son; 1995, Horae et al.; mitochondrial DNA (mt- DNA); contrast with Y chromosomal DNA lineages. mitochondrial proton transport the pumping of H+ ions (protons) from the matrix of the mitochon- drion through its inner membrane and into the inter- membrane space. The energy from NADH is used to transport these protons, and the result is a proton- motive force that drives the synthesis of ATP.

The inner mitochondrial membrane is impermeable to protons, but permeable to water, oxygen, and car- bon dioxide molecules. However, the protons trapped in the intermembrane space can pass through the proton channel in each ATP synthase (q.v.) embedded in the membrane. The reentry of the protons into the matrix supplies the energy to convert ADP to ATP.

See adenosine phosphate, chemiosmotic theory, cytochrome system, electron transport chain, nicotine-adenine dinucleotide (NAD). mitochondrial syndromes human disorders caused by mutations that impair the functioning of mito- chondrial genomes. Such diseases can affect both sexes, but are passed on only by affected mothers. See Leber hereditary optic neuropathy (LHON), Leigh syn- drome. mitochondrial translocases enzymes which form pores in mitochondrial membranes that allow pro- teins synthesized in the cytosol to enter the organ- elle.

Each mitochondrion imports about 1,000 dif- ferent proteins. Translocases located in the outer membrane are called TOMs, while those of the in- ner membrane are called TIMs. The proteins that make up TOMs and TIMs are encoded by nuclear genes and are synthesized in the cytosol. So these precursor proteins must also be imported into the mitochondrion, sorted, and directed to their appro- priate locations, where they are assembled into func- tioning enzymes. The outer mitochondrial mem- brane contains receptors that recognize those proteins of the cytosol destined to be internalized.


These preproteins contain amino acid sequences to which the receptors bind. The preproteins collected by the receptors are then passed by the TOMs into the fluid-filled space between the membranes. Here appropriate preproteins are passed by TIMs into the mitochondrial matrix for further processing, while some are retained in the inner membrane. mitochondrion a semiautonomous, self-reproduc- ing organelle that occurs in the cytoplasm of all cells of most, but not all, eukaryotes.

For example, micro- sporidia (q.v.) lack mitochondria. Each mitochon- drion is surrounded by a double limiting membrane. The inner membrane is highly invaginated, and its projections are called cristae. In most eukaryotes, the cristae are platelike, but certain protoctists (viz. cili- ates, sporozoa, diatoms, chrysophytes) have tubular cristae. Mitochondria are the sites of the reactions of oxydative phosphorylation (q.v.), which result in the formation of ATP.

Mitochondria contain distinctive ribosomes, tRNAs, aminoacyl-tRNA synthetases, and elongation and termination factors. Mitochon- dria depend upon genes within the nucleus of the cells in which they reside for many essential mRNAs. The proteins translated from these mRNAs in the cell cytosol are imported into the organelle. The mi- tochondrion only makes about 1% of its proteins in- ternally.

The rest are transported from the cytosol using enzymes called mitochondrial translocases (q.v.). Mitochondria are believed to have arisen from aerobic bacteria that established a symbiotic rela- tionship with primitive protoeukaryotes. See Appen- dix C, 1890, Altman; 1898, Benda; 1952, Palade; 1964, Luck and Reich; 1996, Burger et al.; 1998, Anderson et al.; ATP synthase, chloramphenicol, citric acid cycle, electron transport chain, endosymbiont theory, glycolysis, human mitochondrial DNA, kineto- plast, leader sequence peptide, mitosomes, petites, protein sorting, ribosome, Rickettsia prowazeki, serial symbiosis theory, sorting signals.

mitogen a compound that stimulates cells to un- dergo mitosis. See epidermal growth factor. mitomycin a family of antibiotics produced by Streptomyces caespitosus. Mitomycin C (shown above) prevents DNA replication by crosslinking the comple- mentary strands of the DNA double helix. mitosis mitosis or nuclear division is generally di- vided into four phases: prophase, metaphase, ana- phase, and telophase. During mitotic prophase, the centriole divides and the two daughter centrioles move apart. The chro- mosomes become visible within the nucleus because they coil up to produce a series of compact gyres. Each chromosome is longitudinally double except in

Mitomycin C

the region of the centromere, and each replicate strand of a chromosome is called a chromatid. The nucleolus (q.v.) and the nuclear envelope break down. During metaphase, the chromosomes move about within the spindle and eventually arrange themselves in the equatorial region of the spindle. The two chromatids are now ready to be separated and to move under the action of the traction fibers to the poles of the spindle. During anaphase, the block that prevented DNA replication in the centromere re- gion is removed and the centromere becomes func- tionally double. The chromatids are converted to in- dependent chromosomes that separate and move to opposite poles. During telophase, the spindle disap- pears and reconstruction of nuclear envelopes about the two groups of offspring chromosomes begins. As nuclear envelopes form around each group the chro- mosomes return to their extended state, and nucleoli reappear.

Next, cytokinesis occurs, and the cytoplasm is di- vided into two parts by a cleavage furrow in the case of animal cells or by a cell plate in the case of plant cells. The result of mitosis and cytokinesis is the production of two daughter cells with precisely the same nuclear content and approximately equal amounts of cytoplasm. In contrast to the situation in plants and animals, where the spindle appratus as- sembles and disassembles at each mitosis, in fungi the spindle with attached chromosomes and centro- mes persists throughout the cell cycle. See Appendix C, 1873, Schneider; 1879, 1882, Flemming; centro- mere, centrosome, checkpoint, chromosome conden- sation, cyclins, endomitosis, MPF, phragmoplast, sister chromatid cohesion, spindle pole body. Compare with meiosis. mitosis promoting factor See MPF. mitosomes organelles found in some protoctists that function in the assembly of iron-sulfur com- plexes. They are thought to represent mitochondria that have undergone regressive evolution (q.v.). See Giardia intestinales.


mitosporangium a sporangium containing spores produced by mitotic divisions. Compare with meio- sporangium. mitospore a spore produced by mitosis and there- fore having the same chromosome number as the mother cell. mitotic apparatus an organelle consisting of three components: (1) the asters, which form about each centrosome, (2) the gelatinous spindle, and (3) the traction fibers, which connect the centromeres of the various chromosomes to either centrosome. See Appendix C, 1877, Fol; 1952, Mazia and Dan; mito- sis, multipolar spindle.

mitotic center the agent that defines the poles to- ward which the chromosomes move during ana- phase. The centrioles function as mitotic centers in most animal cells. In plants with anastral mitosis, the nature of the mitotic center is unclear. mitotic chromosome See human mitotic chromo- somes. mitotic crossover somatic crossing over (q.v.). mitotic index the fraction of cells undergoing mi- tosis in a given sample. mitotic poison any chemical compound that kills dividing cells.

Some mitotic poisons are spindle poi- sons (q.v.), and others block topoisomerases (q.v.). Poisoned cells are arrested at the spindle or DNA damage checkpoints, and they subsequently undergo apoptosis (q.v.). mitotic recombination a process in which a dip- loid cell undergoing mitosis gives rise to daughter cells with allele combinations different from that in the parental cell. As in meiotic recombination, mi- totic recombination generally involves genetic ex- change between chromatids of homologous chromo- somes, but occurs less frequently than meiotic recombination. Mitotic recombination can give rise to tissues that are genetic mosaics, including tissues with twin spots (q.v.), and can be experimentally in- duced. See Appendix C, 1936, Stern; gene targeting, homologous recombination, somatic crossing over.

mitotic segregation in a eukaryotic cell contain- ing genetically different organelles, their random segregation during mitosis to generate some cells containing only mutant or nonmutant organelles. See cytohet, petites. mixis biparental sexual reproduction. mixoploidy the presence of more than one chro- mosome number in a cellular population. MLD medial lethal dose (q.v.). mM millimolar concentration. Mn manganese. MN blood group a human blood group system de- fined by red cell antigens specified by a gene on the long arm of chromosome 4 between bands 28.2 and 31.1.

The MN locus encodes a protein called glyco- phorin A (q.v.), and for this reason the gene symbol has been changed to GYPA. Each human red blood cell has about 500,000 of these proteins embedded in its plasma membrane. The M and N antigens are isoforms of glycophorin A that differ in the amino acids present at only two of the 131 sites. Mo molybdenum. mobile genetic element See transposable genetic elements. Mobius strip a topological figure, named after the German astronomer, A. F.

Mobius, made by putting a 180-degree twist in a long, rectangular strip, then pasting the ends together. The strip has only one surface and one edge. If a Mobius strip is cut longi- tudinally, it forms a double-sized ring. If a ring with a double twist is cut longitudinally, it forms inter- locked rings. A Mobius strip demonstrates the be- havior of a twisted ring chromosome when it at- tempts replication. modal class the class that contains more individu- als than any other in a statistical distribution. mode modal class. modification in nucleic acid metabolism, any changes made to DNA or RNA nucleotides after their original incorporation into a polynucleotide chain: e.g., methylation, deamination, formylation, etc.

See modification methylases. modification allele See DNA restriction enzyme. modification methylases bacterial enzymes that bind to the DNA of the cell at specific sites deter- mined by specific base sequences. Here they attach methyl groups to certain bases. This methylation pattern is unique to and protects the species from its own restriction endonucleases. Modification methyl- ases are coded for by modification alleles. See restric- tion and modification model. modified bases postsynthetically altered nucleo- tides of the four usual bases (A, T, G, and C) of DNA. See modification methylases.


modifier referring in the genetic literature to a gene that modifies the phenotypic expression of a nonallelic gene. modular organisms organisms that consist of pop- ulations of units or modules that are attached to- gether, but if separated are capable of independent growth and reproduction. An example would be plants like strawberries where segments of the branching root systems can generate new plants when severed from the parent. In such organisms, a mutation in a meristematic cell may be expressed during the subsequent growth of a module built from the mutant clone, and eventually the mutation may be contained in eggs or pollen. Thus, Weis- mann’s distinction between soma and germ cells does not apply to modular organisms, and evolution- ary changes in modular species may originate from both germinal and somatic mutations. See Appendix C, 1883, Weismann. modulating codon specific triplets that code for rare tRNAs. The translation of a mRNA molecule is slowed down when a modulating codon is encoun- tered.

moiety one of two roughly equal parts. molal descriptive of a solution that contains one mole (q.v.) of solute in 1000 grams of solvent, usu- ally water. Compare with molar. molar descriptive of a solution that contains one mole (q.v.) of a solute per liter of solvent. Compare with molal. mole the amount of a substance that has a weight in grams numerically equal to the molecular weight of the substance; also called gram molecular weight or gram molecule. See molar.

molecular biology a modern branch of biology concerned with explaining biological phenomena in molecular terms. Molecular biologists often use the biochemical and physical techniques to investigate genetic problems. molecular clock See DNA clock hypothesis, protein clock hypothesis. molecular cloning See gene cloning, recombinant DNA technology. molecular genetics that subdivision of genetics which studies the structure and functioning of genes at the molecular level. molecular hybridization base pairing between DNA strands derived from different sources or of a DNA strand with an RNA strand. molecular mass See relative molecular mass (Mr). molecular mimicry See eclipsed antigens. molecular motors molecules capable of generating torque or movement of other molecules.

The small- est known molecular motors are the ATPases re- sponsible for the rotation of bacterial flagellae. The only known enzyme component that must rotate to perform its catalytic function is the F1-ATPase sub- unit of the ATP synthase holoenzyme that spans the membranes of mitochondria. F1-ATPase is less than one-tenth the size of the motors that drive bacterial flagellae. See motor proteins. molecular sieve a crystalline alumino-silicate pel- let used to absorb water, carbon dioxide, hydrogen sulfide, and similar gases from gas mixtures and or- ganic solvents. Molecular sieves are also sometimes used as ion exchange media. molecular weight the sum of the atomic weights of all of the atoms in a given molecule. The term has largely been replaced by relative molecular mass (q.v.).

molecule that ultimate unit quantity of a com- pound that can exist by itself and retain all the chemical properties of the compound. Moloney murine leukemia virus (MoMLV) a vi- rus of mice producing lymphocytic leukemia that can be transmitted from an infected mother to her newborn progeny through her milk. The virus inte- grates into several locations in the mouse genome, and there is an integration site on human chromo- some 5p14. MoMLV is commonly used for gene transfer protocols. First, the viral genes that are re- quired for the production of replication-competent viruses are replaced with those genes that are desired to be transferred. Once the virus is administered, it binds to the host cells and is internalized, and the genes provided by the vector are then integrated randomly into the host chromosomes. Transfers are efficient and the integrations are stable.

See Appen- dix C, 1983, Mann, Mulligan, and Baltimore; gene therapy, proto-oncogene, retrovirus, virus. molting hormone See ecdysones. moltinism a polymorphism in terms of the num- ber of larval molts undergone by a given species. For example, in Bombyx mori there are strains that are known to molt three, four, or five times. molybdenum a biological trace element. Atomic number 24; atomic weight 95.94; most abundant isotopes 92Mo, 94Mo, 95Mo, 96Mo, 97Mo, 98Mo; radio-


isotope 99Mo, half life 67 hours, radiation emitted— beta particles. monad 1. a single organism, usually implying a free-living, unicellular, flagellate stage. 2. the hap- loid set of chromosomes found in the nucleus of an ootid or a spermatid. See meiosis. Monarch butterfly Danaus plexippus. Monarch larvae generally feed upon plants, such as milk- weeds, which contain molecules toxic to vertebrates. These molecules are sequestered by the insect, and they serve to render the animals unpalatable to avian predators. See automimicry. Monera a kingdom-level taxon, equivalent to the Prokaryotae, into which all of the bacteria are grouped. See Appendix A. monestrous mammal See estrous cycle. mongolism Down syndrome (q.v.). Mongoloid pertaining to a race of mankind, char- acterized by a faintly yellowish skin, an epicanthic fold, sparse body hair, and black straight head hair. monkey See catarrhine, Cercopithecus aethiops, Macaca mulatta, platyrrhine, woolly monkey.

monoallelic referring to a polyploid in which all alleles at a given locus are the same. In a hexaploid, for example, A1A1A1A1A1A1. monocentric designating a chromosome having a single centromere. monochromatic light light of a single wavelength. monochromatic radiation electromagnetic radia- tion of a single wavelength, or in which all of the photons have the same energy. monocistronic mRNA messenger ribonucleic acid coding for a single polypeptide chain; the typical mRNA of eukaryotic cells. Contrast with polycis- tronic mRNA. monoclonal antibodies immunoglobulins derived from a single clone of plasma cells.

Since all immu- noglobulins produced by a given plasma cell (or clone thereof) are chemically and structurally identi- cal, these antibodies constitute a pure population with highly specific antigen-binding properties. See HAT medium, hybridoma. Monocotyledoneae one of the two classes of flowering plants (see Appendix A, Kingdom 5, Plan- tae). The seeds of all monocots possess a single pri- mary leaf. See cotyledon, Dicotyledoneae. monocyte the largest leukocyte found in the blood; they are phagocytic, amoeboid cells. See agranulo- cytes, macrophages. Monodelphis domestica the gray, short-tailed opossum. the most widely used, laboratory-bred marsupial. This opossum has the most compact link- age map known among vertebrates. Crossing over in males is double that in females.

See Appendix A, Mammalia, Metatheria. monoecious having both staminate and pistillate flowers on the same plant. Compare with dioecious. See flower, hermaphrodite. monoenergetic radiation radiation of a given type (alpha, beta, neutron, gamma) in which all particles or photons have the same energy. monogamy an animal reproductive strategy in which a specific male and female form an exclusive mating pair during a reproductive cycle, a season, or a lifetime. Compare with polygamy. monogenic character a character determined by a single gene. monohybrid an individual that is heterozygous (e.g., Aa) for a pair of alleles at the locus under study. monohybrid cross a mating between two individ- uals, both of which are heterozygotes genetically identical at a given locus; e.g., Aa × Aa. monolayer a single layer of cells growing on a sur- face. monolepsis transmission to the offspring of char- acteristics of but one parent.

monomer a simple compound from which, by rep- etition of a single reaction, a polymer is made. For example, uridylic acid (U) can be polymerized to form polyuridylic acid (UUU . . . ). Compare with multimeric, oligomer, polymer. monomorphic locus a genetic locus at which the most common allele exceeds a frequency of 0.95 in the gene pool of the population. Compare with poly- morphic locus. monomorphic population a population showing only one trait (of potentially variable expression) due to fixation of one allelic form of the gene re- sponsible for that trait. mononuclear leukocyte See agranulocytes.

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