Cell: The basic structural and functional unit in people and all living things. Each cell is a small container of chemicals and water wrapped in a membrane.

Each cell in the human body -- there are 100 trillion cells in each of us -- contains the entire human genome, all the genetic information necessary to build a human being. This information is encoded within the cell nucleus in 6 billion base pairs, subunits of DNA, packaged in 23 pairs of chromosomes, one chromosome in each pair coming from each parent. Each of the 46 human chromosomes contains the DNA for thousands of individual genes, the units of heredity.

There are notable exceptions including the egg and sperm cells (each of which have only 23 chromosomes containing half the usual amount of DNA) and mature red blood cells (which no longer have a nucleus and so lack chromosomes and DNA).

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