Science / Biology Glossary

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 a . . . View Full Definition

Messenger Rna (MRNA): Blueprint for protein synthesis that is transcribed from one strand of the DNA (gene) and which is translated at the ribosome into a polypeptide sequence.

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 . . . View Full Definition

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 stage of eukaryotic cell division (mitosis or meiosis) in which the chromosomes line up at the equator of the cell.

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.

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.

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