Chapter 1 • Lesson 5



Chapter 1 • Lesson 5

The Cell Cycle Objective: 1.2.2

Key Terms

• cell division • parent cell • daughter cell • cell cycle • interphase • mitosis • prophase

• sister chromatids • centromere • metaphase • anaphase • telophase • cytokinesis

• asexual reproduction

Getting the Idea

Multicellular eukaryotic organisms must have a way of growing and replacing damaged cells. Existing cells contain the instructions for dividing to produce new cells. Growth and repair in eukaryotes takes place by a form of cell division, the process by which a cell divides to form two or more new cells.

Eukaryotic Cell Division and the Cell Cycle

Recall from Lesson 1 that DNA contains the instructions for cell activities. In eukaryotes, DNA is bundled into structures called chromosomes, which are located in the nucleus. When a cell reproduces, it must make an exact copy of its chromosomes to pass on to each new cell. The original cell is called the parent cell. Each new cell that forms from the parent cell is called a daughter cell.

The cell cycle is a continuous process in which individual cells grow, make copies of their chromosomes, and then divide to form daughter cells. The complete cell cycle consists of three phases: interphase, mitosis, and cytokinesis. These phases are shown in the diagram below. Depending on the type of cell involved, the length of the cell cycle can vary from less than an hour to several days.

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Interphase

As shown on the previous page, most of the cell cycle is spent in interphase, a period of growth and DMA replication (copying) that occurs between cell divisions. Interphase has three stages, in which different processes occur in different cell structures: growth 1, synthesis, and growth 2. During the growth 1, or G-i, stage, the cell increases in size and synthesizes proteins. The number of organelles doubles, and new cytoplasm forms. The synthesis, or S, stage is marked by duplication of the chromosomes. Between cell divisions, the chromosomes are invisible because they are uncoiled and spread out through the

nucleus. The cell enters the growth 2, or G2, stage after the S stage. During the G2 stage, the cell continues to grow as all the structures and proteins the cell needs for mitosis form.

Mitosis

The second phase of the cell cycle is mitosis, which makes up most of the M phase. In mitosis, the nucleus divides into two nuclei. Each nucleus contains a complete set of chromosomes. Mitosis is a continuous process. However, biologists have divided it into four phases—prophase, metaphase, anaphase, and telophase—to help describe the events that take place. The diagram below shows the phases of mitosis in a typical animal cell.

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During prophase, a substance called chromatin condenses to form visible chromosomes. Sister chromatids—paired strands of a duplicated chromosome—are attached at regions called centromeres. The nuclear membrane breaks down, and organelles called centrioles travel to opposite poles of the cell. A fanlike structure called a spindle forms around each centriole.

During metaphase, the sister chromatids line up at the cell's center along the spindle fibers. The centromere of each chromatid is attached to a spindle fiber.

Anaphase begins as the spindle fibers pull the sister chromatids apart at their centromeres. Each chromatid is an exact duplicate of its parent chromosome. The spindle fibers pull the sister chromatids of each pair toward opposite ends of the cell.

During telophase, the chromosomes reach opposite poles of the cell and begin to uncoil. The spindle breaks down. Nuclear membranes reform around the chromosomes at each pole. The cell now has two identical nuclei, each with a complete set of chromosomes. All that remains in cell division is the division of the cytoplasm and organelles. This process begins during telophase.

Cytokinesis follows telophase. Cytokinesis is the division of the cytoplasm into two separate cells. This process is different in plant and animal cells. In animal cells, a central groove called a cleavage furrow forms in the plasma membrane and pinches the two daughter cells apart. A new plasma membrane forms between the two nuclei. When cytokinesis is complete, two new daughter cells, each with its own nucleus and organelles, result. In plant cells, cytokinesis involves the formation of a cell plate between the nuclei. The cell plate forms part of the cell wall of each of the two new cells.

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Multicellular eukaryotes use mitosis to make new cells. These cells may enable the organism to grow larger. Organisms also use mitosis to replace cells that are damaged or no longer useful to the organism. For example, your body continuously makes new skin cells by mitosis to replace skin cells that are worn out. In unicellular eukaryotes and some multicellular organisms, mitosis is an important process in asexual reproduction.

Asexual reproduction is the production of offspring by a single parent. Because offspring produced in this way receive an exact copy of the genetic material of their parent, these offspring are genetically identical to that parent. Yeasts and freshwater animals called hydras reproduce in this way. Their offspring develop from buds on the parent's body. New potato plants can grow from "eyes," or buds, on potatoes, and many plants can reproduce asexually from runners or other parts.

Discussion Question

List the phases of the cell cycle, including all the stages of mitosis. For each stage, identify whether a cell has one set of chromosomes or twice the normal number of chromosomes.

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