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  1. macroevolution: The rise and fall of major taxonomic groups due to evolutionary radiations that bring new groups to prominence and mass extinctions in which groups are lost; the history of large-scale evolutionary changes over time. Compare microevolution.
  2. macromolecule: A large organic molecule formed by the bonding together of small organic molecules.
  3. macronutrient: A chemical element required by an organism in relatively large amounts. Compare micronutrient.
  4. macrophage: A type of white blood cell that engulfs or encapsulates invading pathogens.
  5. magnetic resonance imaging (MRI): An imaging technology that can detect which parts of the brain are active at any particular time.
  6. male: An individual of a sexually reproducing species that produces sperm. Compare female.
  7. malignant: Of or referring to a cancerous growth that begins as a single tumor and then spreads to other tissues in the body with life-threatening consequences. Compare benign.
  8. marrow: A tissue lying within vertebrate bone in which red and white blood cells are produced.
  9. mass extinction: A event during which large numbers of species become extinct throughout most of Earth.
  10. mast cell: A type of white blood cell that lives in connective tissue and releases histamine.
  11. mechanoreceptor: A sensory cell that initiates an action potential in response to a mechanical stimulus.
  12. meiosis: A specialized process of cell division in eukaryotes during which diploid cells divide to produce haploid cells. Meiosis has two division cycles and occurs exclusively in cells that produce gametes. Compare mitosis.
  13. meiosis I: The first cycle of cell division in meiosis. Meiosis I produces haploid daughter cells, each with half the chromosome number of the diploid parent cell.
  14. meiosis II: The second cycle of cell division in meiosis. Meiosis II is essentially mitosis, but in a haploid cell.
  15. memory cell: A type of B cell that remains in the body as a long-term record of the primary immune response to a particular pathogen. During a second exposure to that pathogen, the memory cells specific to that pathogen multiply rapidly to produce large numbers of lymphocytes that can attack that pathogen.
  16. menopause: The end of a human female's reproductive life.
  17. menstrual cycle: A series of hormonally controlled, cyclical changes that take place in the reproductive system of human females, which include the shedding of the lining of the uterus (menstruation) about every 28 days.
  18. meristem: A group of perpetually young, undifferentiated plant cells from which new cells arise.
  19. mesoderm: The cell layer in animal development that lies between the endoderm and ectoderm in the gastrula and gives rise to muscle tissue, connective tissue, and the kidney. Compare ectoderm, endoderm.
  20. messenger RNA (mRNA): A type of RNA that specifies the order of amino acids in a protein.
  21. metabolic heat: Heat generated as a product of respiration.
  22. metabolic pathway: A series of enzyme-controlled chemical reactions in a cell in which the product of one reaction becomes the substrate for the next.
  23. metabolism: All the chemical reactions that occur in a cell.
  24. metamorphosis: A dramatic developmental transformation from a reproductively immature to a reproductively mature form, involving great change in the form and function of the animal.
  25. metaphase: The stage of mitosis during which chromosomes become aligned at the equator of the cell.
  26. microevolution: Changes in allele or genotype frequencies in a population over time; the smallest scale at which evolution occurs. Compare macroevolution.
  27. microfilament: A protein fiber composed of actin monomers. Microfilaments are part of a cell's cytoskeleton and are important in cell movements.
  28. micronutrient: A chemical element required by an organism in tiny amounts. Compare macronutrient.
  29. microtubule: A protein fiber composed of tubulin monomers. Microtubules are part of the cell's cytoskeleton.
  30. midbrain: A portion of the brain stem that helps to maintain muscle tone and sends some sensory data to higher brain centers.
  31. middle ear: The portion of the vertebrate ear that contains three tiny bones that receive vibrations from the eardrum and transmit them to the inner ear.
  32. milk-letdown reflex: The reflexive release of milk by a human mother in response to the suckling behavior of an infant.
  33. mineral: A small inorganic molecule. Many minerals are needed by organisms in small amounts for proper nutrition.
  34. mismatch error: The insertion of an incorrect nitrogen base during DNA replication that is not detected and corrected.
  35. mitochondrion (pl. mitochondria): An organelle with a double membrane that is the site of oxidative phosphorylation. Mitochondria break down simple sugars to produce most of the ATP needed by eukaryotic cells.
  36. mitosis: The process of cell division in eukaryotes that produces two daughter nuclei, each with the same chromosome number as the parent nucleus. Compare meiosis.
  37. mitotic spindle: An arrangement of microtubules that guides the movement of chromosomes during mitosis.
  38. modular: Of or referring to the body of an organism, such as a plant, that grows by the repeated addition of a basic structural unit.
  39. molecule: An arrangement of two or more atoms linked by chemical bonds.
  40. monoglyceride: Along with fatty acids, one of the two components of lipids.
  41. monomer: A molecule that can be linked with other related molecules to form a larger polymer.
  42. monosaccharide: A simple sugar that can be linked to other sugars, forming a polysaccharide. Glucose is the most common monosaccharide in living organisms.
  43. morphogen: A chemical signal that influences the developmental fate of a cell.
  44. morphology: The form and structure of an organism.
  45. most recent common ancestor: The ancestral organism from which a group of descendants arose.
  46. motor neuron: A neuron that carries signals from the central nervous system to the body, thereby causing changes in muscles, organs, or endocrine glands. Compare sensory neuron, interneuron.
  47. motor protein: A protein that uses the energy of ATP to move organelles or proteins along microtubules.
  48. mRNA: See messenger RNA.
  49. multicellular: Made up of more than one cell.
  50. multiregional hypothesis: A hypothesis stating that anatomically modern humans evolved from Homo erectus populations scattered throughout the world. According to this idea, worldwide gene flow caused different human populations to evolve modern characteristics simultaneously and to remain a single species. Compare out-of-Africa hypothesis.
  51. muscle: A tissue unique to animals that can contract, or shorten, to produce movement.
  52. muscle fiber: The basic unit of muscle tissue, consisting of a collection of myofibrils.
  53. mutagen: A substance or energy source that alters DNA.
  54. mutation: A change in the sequence of an organism's DNA. New alleles arise only by mutation, so mutations are the original source of all genetic variation.
  55. mutualism: An interaction between two species in which both species benefit.
  56. mutualist: An organism that interacts with another organism to the mutual benefit of both.
  57. mycelium (pl. mycelia): The main body of a fungus, composed of hyphae.
  58. mycorrhiza (pl. mycorrhizae): A mutualism between a fungus and a plant, in which the fungus provides the plant with mineral nutrients while receiving organic nutrients from the plant.
  59. myelin: A fatty material that forms an insulating sheath around the axon of a neuron and greatly speeds the rate at which action potentials move along the axon.
  60. myofibril: A unit of a muscle fiber consisting of many sarcomeres attached end-to-end.
  61. myoglobin: An oxygen-binding pigment that helps store oxygen in muscle tissue.
  62. myosin filament: One of the two types of filaments that make up muscle, consisting of many molecules of the protein myosin. Compare actin filament.