M phase Period of the eucaryotic cell cycle during which the nucleus and
cytoplasm divide. macromolecule Molecule such as a protein, nucleic acid, or polysaccharide
with a molecular mass greater than a few thousand daltons. macrophage Phagocytic cell derived from blood monocytes, typically resident
in most tissues. It has both scavenger and antigen-presenting functions in
immune responses. major histocompatibility complex (MHC) Complex of highly polymorphic
genes in vertebrates. They code for a large family of cell-surface glycoproteins
(MHC molecules) that bind peptide fragments of foreign proteins and present
them to T cells to induce an immune response. (24 50.) malaria Potentially fatal human disease caused by the protozoan parasite Plasmodium, which is transmitted by the bite of an infected mosquito. malignant Describes tumors and tumor cells that are invasive and/or able to
undergo metasis. A malignant tumor is a cancer. mannose 6-phosphate (M6P) Unique marker attached to the oligosaccharides
on some glycoproteins destined for lysosomes. map unit see genetic map distance MAP see microtubule-associated protein MAP-kinase (mitogen-activated protein kinase) Protein kinase that
performs a crucial step in relaying signals from the plasma membrane to the
nucleus. Turned on by a wide range of proliferation- or differentiationinducing
signals. mating-type locus (MAT locus) In budding yeast, the locus that determines
the mating type (a or a) of the haploid yeast cell. matrix space (1) Central subcompartment of a mitochondrion, bounded by the
inner mitochondrial membrane. (2) The corresponding compartment in a
chloroplast, which is more commonly known as the stroma. M-Cdk see M-phase Cdk Mcm proteins Proteins in the eucaryotic cell that bind to origin recognition
complexes in DNA in early G1 and are involved in forming the pre-replicative
complex. M-cyclin Type of cyclin found in all eucaryotic cells that promotes the events
of mitosis. MDR protein see multidrug resistance protein mega- Prefix denoting 106. (From Greek megas, huge, powerful.) megakaryocyte Large myeloid cell with a multilobed nucleus that remains in
the bone marrow when mature. It buds off platelets from long cytoplasmic
processes. meiosis Special type of cell division by which eggs and sperm cells are
produced. It comprises two successive nuclear divisions with only one round
of DNA replication, which produces four haploid daughter cells from an initial
diploid cell. melanocyte Cell that produces the dark pigment melanin. Responsible for the
pigmentation of skin and hair. membrane The lipid bilayer plus associated proteins that encloses all cells
and, in eucaryotic cells, many organelles as well. membrane-bound ribosome Ribosome attached to the cytosolic face of the
endoplasmic reticulum. The site of synthesis of proteins that enter the
endoplasmic reticulum. membrane channel Transmembrane protein complex that allows inorganic
ions or other small molecules to diffuse passively across the lipid bilayer. membrane potential Voltage difference across a membrane due to a slight
excess of positive ions on one side and of negative ions on the other. A typical
membrane potential for an animal cell plasma membrane is 60 mV (inside
negative relative to the surrounding fluid). membrane protein Protein that is normally closely associated with a cell
membrane. membrane transport Movement of molecules across a membrane mediated
by a membrane transport protein. membrane transport protein Membrane protein that mediates the passage of
ions or molecules across a membrane. Examples are ion channels and carrier
proteins. meristem An organized group of dividing cells whose derivatives give rise to
the tissues and organs of a flowering plant. Key examples are the apical
meristems at the tips of shoots and roots. mesenchyme Immature, unspecialized form of connective tissue in animals,
consisting of cells embedded in a thin extracellular matrix. mesoderm Embryonic tissue that is the precursor to muscle, connective tissue,
skeleton and many of the internal organs. messenger RNA (mRNA) RNA molecule that specifies the amino acid
sequence of a protein. Produced by RNA splicing (in eucaryotes) from a larger
RNA molecule made by RNA polymerase as a complementary copy of DNA.
It is translated into protein in a process catalyzed by ribosomes. metabolism The sum total of the chemical processes that take place in living
cells. metaphase Stage of mitosis at which chromosomes are firmly attached to the
mitotic spindle at its equator but have not yet segregated toward opposite poles. metaphase plate Imaginary plane at right angles to the mitotic spindle and
midway between the spindle poles; the plane in which chromosomes are
positioned at metaphase. metaplasia A change in the pattern of cell differentiation in a tissue. metastasis Spread of cancer cells from their site of origin to other sites in the
body. methyl (CH3) Hydrophobic chemical group derived from methane (CH4). MHC molecule One of a large family of ubiquitous cell-surface glycoproteins
encoded by genes of the major histocompatibility complex (MHC). They bind
peptide fragments of foreign antigens and present them to T cells to induce an
immune response. (See also class I MHC molecule, class II MHC molecule.) MHC see major histocompatibility complex micro- Prefix denoting 10 6. microelectrode, micropipette Piece of fine glass tubing pulled to an even
finer tip. Used to penetrate a cell to study its physiology or to inject electric
current or molecules. microfilament see actin filament micrograph Photograph of an image seen through a microscope. May be
either a light micrograph or an electron micrograph depending on the type of
microscope employed. microinjection Injection of molecules into a cell using a micropipette. micron (mm or micrometer) Unit of measurement often applied to cells and
organelles. Equal to 10 6 meter or 10 4 centimeter. micropipette see microelectrode microsome Small vesicle that is derived from fragmented endoplasmic
reticulum produced when cells are homogenized. microtubule Long hollow cylindrical structure composed of the protein
tubulin. It is one of the three major classes of filaments of the cytoskeleton. microtubule-associated protein (MAP) Any protein that binds to
microtubules and modifies their properties. Many different kinds have been
found, including structural proteins, such as MAP-2, and motor proteins, such
as dynein. microtubule-organizing center (MTOC) Region in a cell, such as a
centrosome or a basal body, from which microtubules grow. microvillus (microvilli) Thin cylindrical membrane-covered projection on the
surface of an animal cell containing a core bundle of actin filaments. Present in
especially large numbers on the absorptive surface of intestinal epithelial cells. midbody Structure formed at the end of cleavage that can persist for some
time as a tether between the two daughter cells in animals. milli- Prefix denoting 10 3. minus end The end of a microtubule or actin filament at which the addition of
monomers occurs least readily; the "slow-growing" end of the microtubule or
actin filament. The minus end of an actin filament is also known as the pointed
end. mismatch repair DNA repair process that corrects mismatched nucleotides
inserted during DNA replication. A short stretch of newly synthesized DNA
including the mismatched nucleotide is removed and replaced with the correct
sequence with reference to the template strand. mitochondrial precursor protein Mitochondrial protein encoded by a nuclear
gene, synthesized in the cytosol, and subsequently transported into
mitochondria. mitochondrion (mitochondria) Membrane-bounded organelle, about the size
of a bacterium, that carries out oxidative phosphorylation and produces most
of the ATP in eucaryotic cells. mitogen An extracellular substance, such as a growth factor, that stimulates
cell proliferation. mitogen-activated protein kinase see MAP-kinase mitosis Division of the nucleus of a eucaryotic cell, involving condensation of
the DNA into visible chromosomes, and separation of the duplicated
chromosomes to form two identical sets. (From Greek mitos, a thread,
referring to the threadlike appearance of the condensed chromosomes.) mitotic chromosome Highly condensed duplicated chromosome with the two
new chromosomes still held together at the centromere as sister chromatids. mitotic spindle Array of microtubules and associated molecules that forms
between the opposite poles of a eucaryotic cell during mitosis and serves to
move the duplicated chromosomes apart. model organism A species, such as Drosophila melanogaster or Escherichia coli, that has been studied intensively over a long period and thus serves as a
"model" of the biology of a particular type of organism. module In proteins or nucleic acids, a unit of structure or function that is found
in a variety of different contexts in different molecules. molar Describes a solution with a concentration of 1 mole of a substance
dissolved in 1 liter of solution (abbreviated as 1 M). mole X grams of a substance, where X is its relative molecular mass
(molecular weight). A mole consists of 6 × 1023 molecules of the substance. molecular chaperone see chaperone molecular weight Numerically, the same as the relative molecular mass of a
molecule expressed in daltons. For example, a protein of relative molecular
mass 20,000 has a molecular weight of 20,000. molecule Group of atoms joined together by covalent bonds. monoclonal antibody Antibody secreted by a hybridoma clone. Because each
such clone is derived from a single B cell, all of the antibody molecules
produced are identical. monocyte Type of white blood cell that leaves the bloodstream and matures
into a macrophage in tissues. monomer Small molecular building block that can serve as a subunit, being
linked to others of the same type to form a larger molecule (a polymer). monosaccharide Simple sugar with the general formula (CH2O) n, where n = 3 to 8. morphogen Signal molecule that can impose a pattern on a field of cells by
causing cells in different places to adopt different fates. mosaic In developmental biology, an organism made of a mixture of cells with
different genotypes. motif Element of structure or pattern that recurs in many contexts.
Specifically, a small structural domain that can be recognized in a variety of
proteins. motor protein Protein that uses energy derived from nucleoside triphosphate
hydrolysis to propel itself along a protein filament or another polymeric
molecule. M-phase Cdk (M-Cdk) Complex formed in vertebrate cells by an M-cyclin
and the corresponding cyclin-dependent kinase (Cdk). mRNA see messenger RNA MTOC see microtubule-organizing center multidrug resistance protein (MDR protein) Type of ABC transporter
protein that can pump hydrophobic drugs (such as some anti-cancer drugs) out
of the cytoplasm of eucaryotic cells. multipass transmembrane protein Membrane protein in which the
polypeptide chain crosses the lipid bilayer more than once. mutant Organism in which a mutation has occurred that makes it different
from wild-type or from the 'normal' extent of variation in the population. mutation rate The rate at which observable changes occur in a DNA sequence. mutation Heritable change in the nucleotide sequence of a chromosome. myelin sheath Insulating layer of specialized cell membrane wrapped around
vertebrate axons. Produced by oligodendrocytes in the central nervous system
and by Schwann cells in the peripheral nervous system. myeloid cell Any white blood cell other than lymphocytes. myoblast Mononucleated, undifferentiated muscle precursor cell. A skeletal
muscle cell is formed by the fusion of multiple myoblasts. myoepithelial cell Type of unstriated muscle cell found in epithelia, e.g. in the
iris of the eye and in glandular tissue. myofibril Long, highly organized bundle of actin, myosin, and other proteins
in the cytoplasm of muscle cells that contracts by a sliding filament
mechanism.
