Macromolecule: Any large molecule (usually made of many monomeric building blocks) with a molecular weight of more than a few thousand daltons.

Macrophage: A particular type of leukocyte, or white blood cell, specialized in phagocytosis-the process by which it can engulf foreign material. It is involved in both primary and specific immune responses.

Major Histocompatability Complex (MHC): A number of genes which encode the class I and II MCH molecules which bind peptides and present them to T-cell receptors. Different MHCs can result in tissue graft rejection.

Meiosis: In eukaryotes a specialized process of cell division that results in haploid cells from a diploid cell. Cells double their DNA once, but there are two subsequent nuclear divisions, resulting in half the DNA present in the original cell. Meiosis generates gametes important to fertilization.

Mesoderm: The middle layer of tissue produced during embryogenesis.

Metastasize: To undergo metastasis. Metastasis is the movement of tumor or cancer cells from one place to another where another tumor develops as a result.

Metastatic: Relating to metastasis. Metastasis is the movement of tumor or cancer cells from one place to another, where another tumor develops as a result.

Micelles: Concentrically arranged phospholipid or detergent molecules that spontaneously organize in a hydrophilic environment. The hydrophilic heads face the hydrophilic environment, whereas the hydrophobic tails are clustered together and face inward away from the hydrophilic environment.

Microscopy: The process of using a microscope to examine the internal structure of cells and tissues.

Microtubules: Long, cylindrical structures that make up one component of the cytoskeleton. They are composed of tubulin dimers arranged in 13 protofilaments. Microtubules have a distinct polarity, with (+) and (-) ends, and can polymerize or depolymerize according to the local concentration of tubulin dimers and other factors.

Missense: Referring to a genetic mutation that alters a codon, causing it to specify an incorrect amino acid during protein synthesis.

Mitochondria: Double-membrane organelles found in eukaryotic cells. They are self-replicating and can carry out protein synthesis. They are responsible for the energy demands of the cell through the production of ATP by oxidative phosphorylation.

Mitosis: The physical separation of a dividing cell into two new daughter cells. Mitosis occurs after DNA synthesis and in mammalian cells lasts for only an hour. It is characterized by the dissolution of the nucleus, karyokinesis, and cytokinesis. Mitosis is subdivided into prophase, metaphase, anaphase, and telophase.

Mixed Micelles: A concentrically arranged group of hydrophobic molecules of mixed variety. A typical mixed micelle is one that is generated by detergent solubilization of cells, resulting in a mixed micelle containing phospholipid and detergent molecules with their polar heads facing outward, their hydrophobic tails facing inward.

Modular: In reference to a protein, describing one with multiple functions carried out by distinct regions. Functional domains of a modular protein, when attached to unrelated proteins, sometimes retain the original function.

Morphogenesis: The change in form, usually of an embryo during development.

Morphogens: Molecules secreted by a site in an embryo that can elicit morphogenesis or differentiation, often in a radial pattern originating from that site. It is thought that morphogens also provide positional cues to cells during development. Few morphogens have been chemically defined.

Motif: In proteins, a sequence of amino acids that exhibits a particular three-dimensional structure that is usually associated with a specific function.

MPF (Mitosis-Promoting Factor; Maturation-Promoting Factor): A protein macromolecular complex that contains cyclin and a protein kinase that triggers mitosis. It was discovered simultaneously in both fertilized eggs and dividing somatic eukaryotic cells.

mRNA: Messenger RNA, the molecule that carries the genetic instructions specifying the amino acid sequence of a protein during its synthesis.

MTOC (Microtubule-Organizing Center): A part of the cell that organizes and stabilizes microtubules. MTOCs act as nucleating centers for microtubules. Basal bodies are examples of MTOCs.

Muscle Cells: Cells that are contractile in nature. Muscle cells are considered to be smooth, skeletal, or cardiac and can be recognized by the arrangement of their actin and myosin filaments.

Mutant: In genetics, an organism that carries a permanent, transmissible change in its genome that can result in an altered phenotype.

Mutation: A permanent, transmissible change in the nucleotide sequence of a gene that can lead to a change or a loss of function.