morphogen   Diffusible signal molecule that can impose a pattern on a field of cells by causing cells in different places to adopt different fates. (Figure 21–8) morphogenesis   Developmental process in which cells undergo movements and deformations in order to assemble into tissues and organs with specific shapes and sizes.
motor protein   Protein that uses energy derived from nucleoside triphosphate hydrolysis to propel itself along a linear track (protein filament or other polymeric molecule).
mRNA degradation control Regulation by a cell of gene expression by selectively preserving or destroying certain mRNA molecules in the cytoplasm.
mTOR   The mammalian version of the large protein kinase called TOR, involved in cell signaling; mTOR exists in two functionally distinct multiprotein complexes.
multidrug resistance An observed phenomenon in which cells exposed to one anticancer drug evolve a resistance not only to that drug, but also to other drugs to which they have never been exposed.
multidrug resistance (MDR) protein   Type of ABC transporter protein that can pump hydrophobic drugs (such as some anticancer drugs) out of the cytoplasm of eukaryotic cells.
multipass transmembrane protein   Membrane protein in which the polypeptide chain crosses the lipid bilayer more than once. (Figure 10–17) multivesicular bodies Intermediates in the endosome maturation process; early endosomes that are on their way to becoming late endosomes.
mutation Heritable change in the nucleotide sequence of a chromosome. (Panel 8–2, pp. 486–487) mutation rate The rate at which changes (mutations) occur in DNA sequences.
mutualism   Ecological relationship between microbes and their host in which both the microbe and host benefit.
Myc   Transcription regulatory protein that is activated when a cell is stimulated to grow and divide by extracellular signals. It activates the transcription of many genes, including those that stimulate cell growth. (Figure 17–61) 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. (Figure 11–33) myeloid cell Any white blood cell other than a lymphocyte. (Figure 22–31) myoblast   Mononucleated, undifferentiated muscle precursor cell. A skeletal muscle cell is formed by the fusion of multiple myoblasts. (Figure 22–19) myofibril Long, highly organized bundle of actin, myosin, and other proteins in the cytoplasm of muscle cells that contracts by a sliding filament mechanism.
myosin   Type of motor protein that uses the energy of ATP hydrolysis to move along actin filaments.
Na+-K+ pump (Na+-K+ ATPase) Transmembrane carrier protein found in the plasma membrane of most animal cells that pumps Na+ out of and K+ into the cell, using energy derived from ATP hydrolysis. (Figure 11–15)
NAD+/NADH (nicotinamide adenine dinucleotide/reduced nicotinamide adenine dinucleotide) Electron carrier system that participates in oxidation–reduction reactions, such as the oxidation of food molecules. NAD+ accepts the equivalent of a hydride ion (H–, a proton plus two electrons) to become the activated carrier NADH. The NADH formed donates its high-energy electrons to the ATP-generating process of oxidative phosphorylation. (Figure 2–36)
NADH dehydrogenase complex   First of the three electron-driven proton pumps in the mitochondrial respiratory chain, also known as Complex I. It accepts electrons from NADH and passes them to a quinone. (Figure 14–18)