9.2 The cell cycle multiplies cells

Objectives Describe the structure of a chromosome. Name the stages of the cell cycle and explain what happens during each stage. Key Terms –chromatinchromatin –chromosomechromosome –sister chromatidsister chromatid –centromerecentromere –cell cyclecell cycle –interphaseinterphase –mitotic phasemitotic phase –mitosismitosis –cytokinesiscytokinesis

At this moment, millions of cells in your body are dividing, each forming two new cells. However, the vast majority of your cells (about 200 trillion) aren't dividing but are going about other cell activities— building proteins, breaking down food, consuming energy, and so on. How does cell division fit into the life of a cell?

Chromosomes and Cell Division Almost all the genes of a eukaryotic cell are located in the cell nucleus. Most of the time, this genetic material exists as a mass of very long fibers that are too thin to be seen under a light microscope. These fibers consist of chromatin, a combination of DNA and protein molecules. As a cell prepares to divide, its chromatin fibers condense, becoming visible as the compact structures called chromosomes.chromatinchromosomes

The number of chromosomes in a eukaryotic cell depends on the species. For example, human body cells generally each have 46 chromosomes. Each chromosome may contain many hundreds of genes.

Before cell division begins, a cell duplicates all of its chromosomes. Each chromosome now consists of two identical joined copies called sister chromatids. (Biologists use the term "sister" to identify joined chromatids, but it does not imply female gender.) The region where the two chromatids are joined tightly together is called the centromere.sister chromatidscentromere

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Each chromosome in a reproducing cell undergoes the duplication and separation process shown here. Duplication occurs during interphase, in preparation for cell division.

The Cell Cycle How often a cell divides depends on the type of cell. Some cells divide once a day. Some divide more often; others, less often. Some highly specialized cells, such as mature muscle cells, do not divide at all. Eukaryotic cells that do divide undergo an orderly sequence of events known as the cell cycle. The cell cycle extends from the "birth" of a cell as a result of cell reproduction to the time the cell itself reproducescell cycle

Interphase The cell may spend as much as 90 percent of the cell cycle in interphase. Interphase is the stage during which a cell carries out its metabolic processes and performs its functions. For example, a cell in your stomach lining might be making and releasing enzyme molecules that help digest your food. During interphase, a cell increases its supply of proteins, increases the number of many of its organelles (such as mitochondria and ribosomes), and grows in size. Interphase

Interphase also includes cellular functions leading up to cell division. One key event is the duplication of the DNA in the cell's chromosomes. This period is called the S phase (S stands for DNA synthesis). The interphase periods before and after the S phase are called the G1 and G2 phases (G stands for gap). During the G2 phase, each duplicated chromosome remains loosely packed as chromatin fibers. The cell is now ready to begin mitosis.

Mitotic Phase The stage of the cell cycle when the cell is actually dividing is called the mitotic phase (M phase). The mitotic phase includes two processes, mitosis and cytokinesismitotic phase During mitosis, the nucleus and the duplicated chromosomes divide and are evenly distributed, forming two "daughter" nuclei.mitosis Cytokinesis is the process by which the cytoplasm is divided in two. Cytokinesis usually begins before mitosis is completed..Cytokinesis

The combination of mitosis and cytokinesis produces two genetically identical daughter cells since the chromosomes were duplicated precisely in the S phase. Each daughter cell has a single nucleus, some surrounding cytoplasm, and a plasma membrane

This diagram of the cell cycle, showing only one chromosome, highlights the mitotic phase (M). The duplicated chromosomes are separated during mitosis and distributed into daughter cells that form through cytokinesis. (The term "daughter" refers to offspring cells and does not imply female gender.)

Mitosis is a very accurate way of distributing identical copies of a large amount of genetic material to two daughter cells. Experiments with yeast cells, for example, indicate that an error in chromosome distribution occurs only once in about 100,000 cell divisions. Mitosis is unique to eukaryotes