Section 9.2

Your 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. When the cell prepares to divide, chromatin fibers condense, becoming visible as the compact structures called chromosomes.

The cell duplicates all of its chromosomes before cell division. These 2 identical joined copies are called sister chromatids. The region where the two chromatids are joined tightly together is called the centromere.

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 reproduces. (Figure 9-5)

Interphase Consists of 3 smaller phases: G 1 S G 2 Mitotic Phase Consists of 2 smaller phases : Mitosis Cytokinesis

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. 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.

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

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. Mitosis is a very accurate way of distributing identical copies of a large amount of genetic material to two daughter cells. Mitosis is unique to eukaryotes.

1. Describe how the appearance of chromosomes changes as a cell is about to divide. 2. Interphase used to be described as a "resting phase." Why is this description inaccurate? 3. Summarize the events that occur during mitosis and cytokinesis.