Download The Cell Cycle

WEB TUTORIAL 9.1
The Cell Cycle
Text Sections
Section 9.3 DNA Is Packaged with Chromosomes, p. 142
Introduction
The cell cycle is a series of steps in which the cell prepares to reproduce itself by
mitosis. In this tutorial, we’ll explore the stages of the cell cycle and the events that
occur at each stage.
Learning Objectives
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Discuss the main events that occur during the two phases of the cell cycle.
Know the phases of interphase.
Understand what sister chromatids are.
Narration
Overview of the Cell Cycle
Actively dividing cells pass through a series of steps known as the cell cycle. There
are two main phases in this cycle. First there is interphase: that portion of the cell
cycle in which the cell simultaneously carries out its work and, in preparation for
division, duplicates its chromosomes. Second, there is the mitotic phase (or M
phase): that portion of the cell cycle that includes both mitosis and cytokinesis.
The cycle begins when cells, recently created by the division of parent cells, grow,
duplicate organelles, and prepare the machinery needed to replicate DNA. This is
called the G1 phase, and it signifies the first gap between the birth of the new cell
and DNA replication.
The next stage of the cell cycle is the S phase, or synthesis phase. During this stage,
each DNA molecule in the nucleus is duplicated.
When DNA replication is complete, the cell enters another gap stage, called the G2
phase. Cell growth and the duplication of organelles continue as the machinery for
cell division is prepared.
Together, the two gap phases and the S phase make up interphase, an interval during which cells do not divide. Cells spend most of their time in interphase.
The cell then proceeds through a series of events—collectively called M phase (for
mitosis)—that result in the formation of two daughter cells.
Number of Chromosomes vs. DNA Molecules
In preparation for cell division, chromosomes must be duplicated and then distributed so that each daughter cell receives an identical copy of the complete genome.
During the G1 phase, cells possess the standard diploid number of chromosomes—
in this example, four. Each chromosome contains one DNA molecule.
During the S phase, each DNA molecule is copied, or replicated, and the two "sister chromatids" remained joined. Thus, the number of DNA molecules doubles to
eight during the S phase, but the number of chromosomes is still four.
During the G2 phase, both the number of chromosomes and the number of DNA
molecules remain constant.
In M phase, mitosis, the nuclear membrane breaks down and chromosomes
become separated into two equivalent sets. The chromosomes condense, line up in
a row, and then the sister chromatids separate and move to opposite poles of the
cell.
After the chromosomes separate, the nuclear membrane re-forms around each set
of chromosomes, and the cell undergoes cytokinesis, the division of the cytoplasm.
Finally, when cell division is complete, each daughter cell contains a complete set
of chromosomes. That is, each daughter cell has four chromosomes and four DNA
molecules, as did the parent cell.
You should now be able to…
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Explain what the "gap" phases of interphase are.
Describe the events that occur during each phase of interphase.
Compare the number of chromosomes and the number of DNA molecules
present in a cell during each phase of interphase.
Discuss the length of time that a typical cell spends in interphase and mitosis.