The cell is the basic unit of life. Every organism is either made up of large interconnected groups of cells or is itself a free-floating and independent cell. The variety of cell types is staggering. The human body alone contains more than two hundred different types of cells that vary in size, shape, and function.

Genetic information contained in the DNA in our cells influences the structure and function of those cells. It does so by dictating which proteins each cell should create and when it should create them. The process of DNA replication ensures that all of this detailed information is passed not only from parent cell to daughter cell whenever a cell divides, but also from parent to offspring.

Growth, development, and cell replacement occur through cell division. Given that all cells obtain nutrients and eliminate wastes through their membranes, there is a limit to cell size. In order for an organism to grow and to create specialized tissues such as skin, muscle, and nerve, the organism must create fully functional cells with a full complement of DNA. This is achieved though a process called mitosis, in which cells create a duplicate copy of their DNA prior to cell division.

Sexual reproduction presents a special challenge. A human cell contains two sets of 23 chromosomes. During fertilization, a sperm cell and an egg cell combine their genetic material. In order to preserve the chromosome numbers, they must produce gametes (sperm cells and egg cells) that contain just one, rather than two, sets of chromosomes. This is accomplished though another type of cell division called meiosis. Meiosis ensures that a fertilized egg, or zygote, will contain an equal amount of genetic material from the mother and from the father.

Even when the development of an organism is complete, cell division must be ongoing. Cells are regularly damaged and must be replaced if tissues and organs are to be maintained. In humans, they are replaced at a rate of approximately 50 billion each day. However, it is critical that cell division not outpace cell death. This requires that the body regulate cell division and cell death with precision. An imbalance typically results in disease, either from cell loss or cancerous cell proliferation.

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