| macroevolution |
| The combination of events associated with the origin, diversification, extinction, and interactions of organisms which produced the species that currently inhabit the Earth. Large scale evolutionary change such as the evolution of new species (or even higher taxa) and extinction of species. |
| |
| macromolecules |
| Large molecules made up of many small organic molecules that are often referred to as monomers; e.g., carbohydrates, lipids, proteins, and nucleic acids. Macromolecules are polymers of monomers. |
| |
| macronucleus |
| In ciliates, the large nucleus that carries up to several hundred copies of the genome and controls metabolism and asexual reproduction. |
| |
| macronutrients |
| 1. Elements needed by plants in relatively large (primary) or smaller (secondary) quantities. 2. Foods needed by animals daily or on a fairly regular basis. |
| |
| macrophages |
| A type of white blood cell derived from monocytes that engulf invading antigenic molecules, viruses, and microorganisms and then display fragments of the antigen to activate helper T cells; ultimately stimulating the production of antibodies against the antigen. |
| |
| malformation |
| A structural defect inherited in an organ or part of an organ that results from abnormal fetal development.
|
| |
| Malpighian tubules |
| The excretory organs of insects; a set of long tubules that open into the gut. |
| |
| mammal-like reptiles |
| Group of Permian-Triassic reptiles having some possible mammalian features, notably a more prominent dentary (tooth-bearing) bone and reduction of the incus and malleus (which are part of the reptilian jaw along with the dentary). The mammal-like reptiles are thought to have been the reptile group from which the mammals later evolved. |
| |
| mantle |
| In mollusks, a membranous or muscular structure that surrounds the visceral mass and secretes a shell if one is present. |
| |
| mapping |
| The process of deducing schematic representations of DNA. Three types of DNA maps can be constructed: physical maps, genetic maps, and cytogenetic maps, with the key distinguishing feature among these three types being the landmarks on which they are based.
|
| |
| marine biome |
| The aquatic biome consisting of waters containing 3.5% salt on average; includes the oceans and covers more than 70% of the Earth's surface; divided into benthic and pelagic zones. |
| |
| marker |
| Also known as a genetic marker, a segment of DNA with an identifiable physical location on a chromosome whose inheritance can be followed. A marker can be a gene, or it can be some section of DNA with no known function. Because DNA segments that lie near each other on a chromosome tend to be inherited together, markers are often used as indirect ways of tracking the inheritance pattern of genes that have not yet been identified, but whose approximate locations are known.
|
| |
| marsupials |
| Pouched mammals. The young develop internally, but are born while in an embryonic state and remain in a pouch on the mother's abdomen until development is complete; this group includes kangaroos, koalas, and opossums. One of the three reproductive "strategies" of living mammals g-laying and placental being the other two), marsupials finish development in a pouch or under hairy coverings attached to the mother. |
| |
| mass extinction |
| A time during which extinction rates are generally accelerated so that more than 50% of all species then living become extinct; results in a marked decrease in the diversity of organisms. Mass extinctions are thought to have occurred numerous times in Earth history, often from a variety of reasons: impacts, tectonism, changes in primary productivity of the seas, etc. |
| |
| mast cells |
| Cells that synthesize and release histamine, as during an allergic response; found most often in connective tissue surrounding blood vessels. |
| |
| matter |
| Anything that has mass and occupies space. |
| |
| matter cycling |
| The þow of matter through various organisms and the physical environment of an ecosystem. |
| |
| maximum sustainable yield (MSY) |
| The maximum number of a food or game population that can be harvested without harming the population's ability to grow back. |
| |
| medulla |
| 1. A term referring to the central portion of certain organs; e.g., the medulla oblongata of the brain and the adrenal medulla, which synthesizes epinephrine and norepinephrine. 2. In more common usage, the area in the brain that regulates breathing, heartbeat, blood pressure and similar activities. |
| |
| medulla oblongata |
| The region of the brain that, with the pons, makes up the hindbrain; controls heart rate, constriction and dilation of blood vessels, respiration, and digestion. |
| |
| medusa |
| The motile bell-shaped form of body plan in cnidarians; e.g., jellyfish. |
| |
| megakarocytes |
| Cells found in the bone marrow that produce platelets. |
| |
| megaspore mother cell |
| Cells that undergo meiosis to produce megaspores. |
| |
| megaspores |
| Four haploid cells produced by meiosis in the ovule of a þower. Usually, three of these cells degenerate, with the remaining cell becoming the female gametophyte phase of the plant's life cycle. Large (palynologists consider the megaspores to generally be above 200 micrometers in diameter) spores that develop into the megagametophyte, which in turn produces eggs. |
| |
| meiosis |
| Cell division in which the chromosomes replicate, followed by two nuclear divisions. Each of the resulting gametes (in animals, spores in plants) receives a haploid set of chromosomes. Reduction/division by which ploidy, the number of sets of homologous chromosomes, is reduced in the formation of haploid cells that become gametes (or gametophytes in plants). |
| |
| Meissner's corpuscles |
| Sensory receptors concentrated in the epidermis of the fingers and lips that make these areas very sensitive to touch. |
| |
| melanin |
| A pigment that gives the skin color and protects the underlying layers against damage by ultraviolet light; produced by melanocytes in the inner layer of the epidermis. |
| |
| melanocytes |
| The cells in the inner layer of the epidermis that produce melanin. |
| |
| melanoma |
| Cancer of the cells in the skin that produce melanin, a brown pigment. Melanoma often begins in a mole.
|
| |
| membrane-attack complex (MAC) |
| A large cylindrical multiprotein complex formed by the complement system; kills invading microorganisms by embedding in their plasma membrane, creating a pore through which þuid þows, ultimately causing the cell to burst. |
| |
| Mendel, Johann (Gregor) |
| Austrian biologist, born in 1822 and died in 1884, who laid the foundations for the science of genetics. Mendel was a monk whose controlled experiments with breeding peas in the monastery garden led him to conclude that the heritable units we now call genes were not blends of parental traits but separate physical entities passed individually in specific proportions from one generation to the next. Mendel's discoveries were ignored for several decades, but other biologists finally recognized their significance early in the 20th century.
|
| |
| Mendelian inheritance |
| Manner in which genes and traits are passed from parents to children. Examples of Mendelian inheritance include autosomal dominant, autosomal recessive, and sex-linked genes.
|
| |
| menstrual cycle |
| The recurring secretion of hormones and associated uterine tissue changes; typically 28 days in length. |
| |
| menstruation |
| The process in which the uterine endometrium breaks down and sheds cells, resulting in bleeding; occurs approximately once a month. The first day marks the beginning of the menstrual and ovarian cycles. |
| |
| meristematic tissue |
| Embryonic tissue located at the tips of stems and roots and occasionally along their entire length; can divide to produce new cells; one of the four main tissue systems in plants. |
| |
| mesentary |
| Epithelial cells supporting the digestive organs. |
| |
| mesoderm |
| The middle layer of cells in embryonic development; gives rise to muscles, bones, and structures associated with reproduction. The middle embryonic tissue layer. Cells and structures arising from the mesoderm include the bone, blood, muscle, skin, and reproductive organs. |
| |
| mesoglea |
| A gel-like matrix that occurs between the outer and inner epithelial layers in cnidarians. |
| |
| mesophyll |
| Layer of leaf tissue between the epidermis layers; literally meaning "middle of the leaf". |
| |
| mesophytic leaves |
| The leaves of plants that grow under moderately humid conditions with abundant soil and water. |
| |
| Mesozoic Era |
| The period of geologic time beginning 245 million years ago and ending 65 million years ago; the age of the dinosaurs and cycads, the Mesozoic falls between the Paleozoic and Cenozoic Eras and includes the Triassic, Jurassic, and Cretaceous Periods. |
| |
| messenger RNA (mRNA) |
| Template for protein synthesis. Each set of three bases, called codons, specifies a certain protein in the sequence of amino acids that comprise the protein. The sequence of a strand of mRNA is based on the sequence of a complementary strand of DNA.
|
| |
| metabolic pathway |
| A series of individual chemical reactions in a living system that combine to perform one or more important functions. The product of one reaction in a pathway serves as the substrate for the following reaction. Examples include glycolysis and Kreb's cycle. |
| |
| metabolism |
| The sum of all chemical reactions (energy exchanges) in cells. |
| |
| metamorphosis |
| The process of changing from one form to another; e.g., in insects, from the larval stage to the pupal stage to the reproductive adult stage. |
| |
| metaphase |
| The phase of mitosis, or cell division, when the chromosomes align along the center of the cell. Because metaphase chromosomes are highly condensed, scientists use these chromosomes for gene mapping and identifying chromosomal aberrations.
|
| |
| metastasis |
| The process in which cancer cells break away from the original tumor mass and establish new tumor sites elsewhere in the body. |
| |
| methanogens |
| A group of archaebacteria that produce methane as a by product of their metabolism. |
| |
| methionine |
| The amino acid coded for by the initiation codon; all polypeptides begin with methionine, although post-translational reactions may remove it. |
| |
| micelles |
| Structures formed when bile salts surround digested fats in order to enable the water-insoluble fats to be absorbed by the epithelial cells lining the small intestine. |
| |
| microarray technology |
| A new way of studying how large numbers of genes interact with each other and how a cell's regulatory networks control vast batteries of genes simultaneously. The method uses a robot to precisely apply tiny droplets containing functional DNA to glass slides. Researchers then attach fluorescent labels to DNA from the cell they are studying. The labeled probes are allowed to bind to complementary DNA strands on the slides. The slides are put into a scanning microscope that can measure the brightness of each fluorescent dot; brightness reveals how much of a specific DNA fragment is present, an indicator of how active it is.
|
| |
| microevolution |
| A small-scale evolutionary event such as the formation of a species from a preexisting one or the divergence of reproductively isolated populations into new species. |
| |
| microfilaments |
| Rods composed of actin that are found in the cytoskeleton and are involved in cell division and movement. |
| |
| microgametophyte |
| Stage of the plant life cycle that develops from or within a microspore. The microgametophyte produces sperm in specialized structures known as antheridia. |
| |
| micronucleus |
| In the protistan group known as the ciliates, the small nucleus containing a single copy of the genome that is used for sexual reproduction. |
| |
| micronutrients |
| Elements that are required by plants in very small quantities, but are toxic in large quantities: iron, manganese, molybdenum, copper, boron, zinc, and chloride. |
| |
| microsporangia |
| Structures of the sporophyte in which microspores are produced by meiosis. In flowering plants the microsporangia are known as anther sacs. |
| |
| microspore mother cell |
| Cells in the microsporangium that undergo meiosis to produce microspores. In flowering plants the microspore is known as the pollen grain, and contains a three-celled male. |
| |
| microspores |
| Four haploid cells produced by the meiotic division in the pollen sacs of þowers or microsporangia of gymnosperms. Microspores undergo mitotic division and become encased in a thick protective wall to form pollen grains. Small, size usually less than 200 micrometers, spores produced by meiosis. Microspores either germinate into the male gametophyte or have the male gametophyte develop inside the microspore wall. |
| |
| microtubules |
| Filaments about 25 nanometers in diameter found in cilia, þagella, and the cytoskeleton. |
| |
| microvilli |
| Hair-like projections on the surface of the epithelial cells of the villi in the small intestine; increase the surface area of the intestine to improve absorption of digested nutrients. |
| |
| midbrain |
| A network of neurons that connects with the forebrain and relays sensory signals to other integrating centers. |
| |
| middle lamella |
| A layer composed of pectin that cements two adjoining plant cells together. |
| |
| migration |
| Movement of organisms either permanently (as in the migration of humans to the Americas) or temporarily (migratory birds such as Canadian geese). |
| |
| mineralocorticoids |
| A group of steroid hormones produced by the adrenal cortex that are important in maintaining electrolyte balance. |
| |
| minerals |
| Trace elements required for normal metabolism, as components of cells and tissues, and in nerve conduction and muscle contraction. |
| |
| minimum viable population (MVP) |
| The smallest population size that can avoid extinction due to breeding problems or random environmental þuctuations. |
| |
| mitochondria |
| Self-replicating membrane-bound cytoplasmic organelles in most eukaryotic cells that complete the breakdown of glucose, producing NADH and ATP (singular term: mitochondrion). The powerhouse of the cell. Organelles within eukaryotes that generate (by chemiosmosis) most of the ATP the cell needs to function and stay alive. |
| |
| mitochondrial DNA |
| The genetic material of the mitochondria, the organelles that generate energy for the cell.
|
| |
| mitosis |
| The division of the cell's nucleus and nuclear material of a cell; consists of four stages: prophase, metaphase, anaphase, and telophase. Cell xeroxing. Mitosis occurs only in eukaryotes. The DNA of the cell is replicated during interphase of the cell cycle and then segregated during the four phases of mitosis. |
| |
| mitotic spindle |
| A network of microtubules formed during prophase. Some microtubules attach to the centromeres of the chromosomes and help draw the chromosomes apart during anaphase. |
| |
| molecular biology |
| Field of biology that studies the molecular level of organization. |
| |
| molecules |
| Units of two or more atoms held together by chemical bonds. The combination of atoms by chemical bonds with the component atoms in definite porportions, such as water (two H to one O). |
| |
| Monera |
| Prokaryotic kingdom that includes (in the most widely accepted classification system) archaebacteria, eubacteria, and cyanobacteria. Members of this kingdom were among the first forms of life over 3.5 billion years ago. |
| |
| monocots |
| One of the two major types of þowering plants; characterized by having a single cotyledon, þoral organs arranged in threesd or multiples of three, and parallel-veined leaves; include grasses, cattails, lilies, and palm trees. One of the two major groups in the Angiosperms, monocots are characterized by having a single seed leaf (cotyledon), flower parts in 3's or multiples of 3, monoaperturate pollen (although some dicots also have this feature), parallel veins in their leaves, and scattered vascular bundles in their stems. |
| |
| monoculture |
| The growth of only one species in a given area; such as a cornfield or other agricultural field. |
| |
| monocytes |
| White blood cells that clean up dead viruses, bacteria, and fungi and dispose of dead cells and debris at the end of the inþammatory response. |
| |
| monohybrid cross |
| In genetics, a cross that involves only one characteristic. |
| |
| monomer |
| An organic chemical unit linked to other units (usually by a covalent bond formed by the removal of water) to produce a larger molecule (macromolecule) known as a polymer. |
| |
| monophyletic group |
| A group of organisms descended from a common ancestor. For example: your immediate family may be considered such a group, being descended from a common ancestral group (grandparents, etc.). |
| |
| monosaccharides |
| Simple carbohydrates, usually with a five- or six-carbon skeleton; e.g., glucose and fructose. A carbohydrate composed of a single sugar unit, such as glucose, ribose, deoxyribose, etc. |
| |
| monosomy |
| Possessing only one copy of a particular chromosome instead of the normal two copies.
|
| |
| monotremes |
| Egg-laying mammals; e.g., the spiny anteater and the duck-billed platypus. |
| |
| morph |
| A distinct phenotypic variant within a population. |
| |
| morphological convergence |
| The evolution of basically dissimilar structures to serve a common function. For example: the wings of birds and insects. |
| |
| morula |
| The solid-ball stage of the pre-emplantation embryo. |
| |
| mosaic evolution |
| A pattern of evolution where all features of an organism do not evolve at the same rate. Some characteristics are retained from the ancestral condition while others are more recently evolved. |
| |
| motor (efferent) pathways |
| The portion of the peripheral nervous system that carries signals from the central nervous system to the muscles and glands. |
| |
| motor neurons |
| Neurons that receive signals from interneurons and transfer the signals to effector cells that produce a response. Nerve cells connected to a muscle or gland. Sometimes also known as effector neurons. |
| |
| motor output |
| A response to the stimuli received by the nervous system. A signal is transmitted to organs that can convert the signals into action, such as movement or a change in heart rate. |
| |
| motor units |
| Consist of a motor neuron with a group of muscle fibers; form the units into which skeletal muscles are organized; enable muscles to contract on a graded basis. |
| |
| mouse model |
| A laboratory mouse useful for medical research because it has specific characteristics that resemble a human disease or disorder. Strains of mice having natural mutations similar to human ones may serve as models of such conditions. Scientists can also create mouse models by transferring new genes into mice or by inactivating certain existing genes in them.
|
| |
| mouth |
| The oral cavity; the entrance to the digestive system where food is broken into pieces by the teeth and saliva begins the digestion process. |
| |
| mucus |
| A thick, lubricating fluid produced by the mucous membranes that line the respiratory, digestive, urinary, and reproductive tracts; serves as a barrier against infection and, in the digestive tract, moistens food, making it easier to swallow. |
| |
| multicellular |
| Organisms composed of multiple cells and exhibiting some division of labor and specialization of cell structure and function. |
| |
| multinucleate |
| Cells having more than one nucleus per cell. |
| |
| multiple endocrine neoplasia, type 1 (MEN1) |
| A rare inherited disorder that affects the endocrine glands, which release hormones into the bloodstream. The disorder, also known as Wermer's syndrome, can cause multiple tumors in the parathyroid and pituitary glands and in the pancreas. These tumors are almost always benign, but they can cause the glands to become overactive and secrete abnormal levels of hormones. Those abnormal secretions, in turn, can cause a variety of medical problems, ranging from kidney stones and fatigue to fertility problems and life-threatening ulcers.
|
| |
| muscle fibers |
| Long, multinucleated cells found in skeletal muscles; made up of myofibrils. One of the four major groups of vertebrate cell/tissue types. Muscle cells contract/relax, allowing movement of and/or within the animal. |
| |
| muscular system |
| One of eleven major body organ systems in animals; allows movement and locomotion, powers the circulatory, digestive, and respiratory systems, and plays a role in regulating temperature. |
| |
| mutation |
| A permanent structural alteration in DNA. In most cases, such DNA changes either have no effect or cause harm, but occasionally a mutation can improve an organism's chance of surviving and passing the beneficial change on to its descendants.
|
| |
| mutation rate |
| The average occurrence of mutations in a species per a given unit of time. |
| |
| mutualism |
| A form of symbiosis in which both species benefit. A type of symbiosis where both organisms benefit. The classic example is lichens, which is a symbiosis between an alga and a fungus. The alga provides food and the fungus provides water and nutrients. |
| |
| mycelium |
| The mass of interwoven filaments of hyphae in a fungus. |
| |
| mycorrhiza |
| Occurs when a fungus (basidiomycete or zygomycete) weaves around or into a plant's roots and forms a symbiotic relationship. Fungal hyphae absorb minerals from the soil and pass them on to the plant roots while the fungus obtains carbohydrates from the plant (pl.: mycorrhizae). |
| |
| myelin sheath |
| Layers of specialized glial cells, called Schwann cells, that coat the axons of many neurons. |
| |
| myofibrils |
| Striated contractile microfilaments in skeletal muscle cells. |
| |
| myosin |
| Thick protein filaments in the center sections of sarcomeres. |
| |
Top ]
Discuss this topic ]