Download 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.
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.
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.
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.