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apoptosis: The process by which a cell dies at a natural, "pre-programmed" time.

cell: The structural and functional unit of an organism that join together to form tissues. Examples: muscle cells, skin cells, and nerve cells.

chromosome: Thread-like structure in the nucleus that carries genetic information. A normal human body has 23 pairs of chromosomes (46 total chromosomes). Twenty two pairs are called somatic, or "body" chromosomes. The remaining 2 chromosomes are called "sex" chromosomes and determine wether you are a male or a female.

enzyme: A protein that initiates biochemical reactions in the body. It is neither created, nor destroyed during the reaction. Examples: telomerase (helps prevent chromosomes from shortening), lactase (helps dissolve milk sugar), and lipase (helps break down fat).

germ cell: A cell that becomes sex cells (sperm and egg).

malignant: A cell type that can invade and move to distal parts of the body and end the life of a patient. Example: Cancer.

mitosis: Division of a somatic (body) cell to form two daughter cells.

organ: A group of tissues that come together to perform a common function. Examples: the heart is a collection of cardiac tissue and the brain is a collection of nervous tissue.

organelle: The smallest unit of a cell that perform specific functions. Examples: the nucleus contains genetic information, the ribosome packages materials, and the mitochondria make energy.

organ system: A collection of organs that work together to perform a specific function. Example: Digestion is performed by the mouth, esophagus, stomach, small intestine, and large intestine.

RNA: Ribonucleic acid. It is a strand of base pairs that helps code for new DNA strands when a cell divides.

senescence: The process of cellular aging.

somatic cell: A cell other than a germ cell.

tissue: A collection of cells that come together to perform a specific function. Examples: Muscles are formed by muscle cells, nerves are formed by nervous cells, and glands are formed by secretory cells.