why cells divide
Imagine the first stages of life. In humans and other animals, a sperm fertilizes an egg, forming the first cell. But humans are made up of trillions of cells, so where do the new cells come from? Remember that according to the cell theory, all cells come from existing cells. Once a sperm and egg cell unite and the first cell, called a zygote, forms, an entire baby will develop. And each cell in that baby will be genetically identical, meaning that each cell will have exactly the same DNA. How does a new life go from one cell to so many? The cell divides in half, creating two cells. Then those two cells divide, for a total of four cells. The new cells continue to divide and divide. One cell becomes two, then four, then eight, and so on ( Figure 1.1). This continual process of a cell dividing and creating two new cells is known as cell division. Cell division is part of a cycle of cellular growth and division known as the cell cyclecells must grow before they divide. The cell cycle describes the "life" of a eukayrotic cell. In addition to cell division, the cell cycle includes the division of the nucleus and the cytoplasm. Most cell division produces genetically identical cells, meaning they have the same DNA. The process of mitosis, which specifically is the division of the nucleus, ensures that each cell has the same DNA. During mitosis, the chromosomes equally separate, thus making sure each nucleus in each resulting cell after cell division is genetically identical. A special form of cell division, called meiosis, produces cells with half as much DNA as the parent cell. These cells are used for reproduction. In prokaryotic organisms, cell division is how those organisms reproduce. Besides the development of a baby, there are many other reasons that cell division is necessary for life: Cells divide repeatedly to produce an em- bryo. Previously the one-celled zygote (the first cell of a new organism) divided to make two cells (a). Each of the two cells divides to yield four cells (b), then the four cells divide to make eight cells (c), and so on. Through cell division, an entire embryo forms from one initial cell. 1. To grow and develop, you must form new cells. Imagine how often your cells must divide during a growth spurt. Growing just an inch requires countless cell divisions. Your body must produce new bone cells, new skin cells, new cells in your blood vessels and so on. 2. Cell division is also necessary to repair damaged cells. Imagine you cut your finger. After the scab forms, it will eventually disappear and new skin cells will grow to repair the wound. Where do these cells come from? Some of your existing skin cells divide and produce new cells. 3. Your cells can also simply wear out. Over time you must replace old and worn-out cells. Cell division is essential to this process.
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the cell cycle describes the lives of prokaryotic and eukaryotic cells.
a. true --> b. false
mitosis ensures that the two cells resulting from cell division will be genetically identical.
--> a. true b. false
which statement is not true concerning cell division?
a) all organisms undergo cell division. b) cell division is part of the cell cycle. --> c) mitosis produces cells with half as much dna as the parent cell. d) in single-celled organisms, cell division is reproduction.
the cell cycle consists of
--> a) the phases that a cell goes through from one cell division to the next. b) the process in which a parent cell divides to form daughter cells. c) the division of the nucleus. d) the production of reproductive cells.
mitosis followed by division results in
a) two genetically different cells. --> b) two genetically identical cells. c) two cells with half as much dna as the parent cell. d) four cells with half as much dna as the parent cell.
which is the best definition of mitosis?
a) the division of the cell. b) the division of the chromosomes. --> c) the division of the nucleus. d) the growth of the cell followed by cell division.
how does a zygote form?
a) first the nucleus divides, followed by cell division. --> b) a sperm and egg unite. c) a zygote forms after meiosis. d) none of the above
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