5.1_reading_-_cell_cycle_and_cell_division by xuyuzhu


									                                                              • Step 1: DNA Replication. Just before the cell
5.1 Cell Division and the                                    divides, its DNA is copied in a process called DNA
                                                             replication. This results in two identical chromosomes
Cell Cycle                                                   instead of just one. This step is necessary
Lesson Objectives                                            so that when the cell divides, each daughter cell will
• Contrast cell division in prokaryotes and eukaryotes.      have its own chromosome.
• Identify the phases of the eukaryotic cell cycle.          • Step 2: Chromosome Segregation. The two
• Explain how the cell cycle is controlled.                  chromosomes segregate, or separate, and move to
• Define cancer, and relate it to the cell cycle.            opposite ends (known as poles) of the cell.
                                                             • Step 3: Cytokinesis. A new plasma membrane starts
Vocabulary                                                   growing into the center of the cell, and the cytoplasm
• binary fission       • cancer                              splits apart, forming two daughter cells. This process is
• cell cycle           • cell division                       called cytokinesis. The two daughter cells that result
• cytokinesis          • DNA replication                     are genetically identical to each other and to the parent
• interphase           • mitosis                             cell.
• tumor

Introduction                                                 Cell Division in Eukaryotes
                                                             Cell division is more complex in eukaryotes than
You consist of a great many cells, but like all other
                                                             prokaryotes. Prior to dividing, all the DNA in a
organisms, you started life as a single cell. How did
                                                             eukaryotic cell’s multiple chromosomes is replicated.
you develop from a single cell into an organism with
                                                             Its organelles are also duplicated. Then, when the cell
trillions of cells? The answer is cell division. After
                                                             divides, it occurs in two major steps:
cells grow to their maximum size, they divide into two
                                                             • The first step is mitosis, a multi-phase process in
new cells. These new cells are small at first, but
                                                             which the nucleus of the cell divides. During mitosis,
they grow quickly and eventually divide and produce
                                                             the nuclear membrane breaks down and later reforms.
more new cells. This process keeps repeating in a
                                                             The chromosomes are also sorted and separated to
continuous cycle.
                                                             ensure that each daughter cell receives a complete set
                                                             of chromosomes. Mitosis is described in greater detail
Cell Division                                                in Lesson 5.2.
Cell division is the process in which one cell, called       • The second major step is cytokinesis. As in
the parent cell, divides to form two new cells, referred     prokaryotic cells, during this step the cytoplasm divides
to as daughter cells. How this happens depends on            and two daughter cells form.
whether the cell is prokaryotic or eukaryotic.
Cell division is simpler in prokaryotes than eukaryotes
because prokaryotic cells themselves are simpler.
                                                             The Cell Cycle
                                                             Cell division is just one of several stages that a cell
Prokaryotic cells have a single circular chromosome,
                                                             goes through during its lifetime. The cell cycle
no nucleus, and few other organelles. Eukaryotic cells,
                                                             is a repeating series of events that include growth,
in contrast, have multiple chromosomes contained
                                                             DNA synthesis, and cell division. The cell cycle in
within a nucleus and many other organelles. All of
                                                             prokaryotes is quite simple: the cell grows, its DNA
these cell parts must be duplicated and then separated
                                                             replicates, and the cell divides. In eukaryotes, the cell
when the cell divides.
                                                             cycle is more complicated.

Cell Division in Prokaryotes                                 Eukaryotic Cell Cycle
Most prokaryotic cells divide by the process of binary
                                                             The diagram in Figure 5.3 represents the cell cycle of
fission. A bacterial cell dividing this way is depicted
                                                             a eukaryotic cell.
below in Figure 5.1
                                                                                           Figure 5.3: Eukaryotic Cell
                                                                                           Cycle. This diagram represents
                                                                                           the cell cycle in eukaryotes. The
                                                                                           G1, S, and G2 phases make up
                                                                                           interphase (I). The M phase
                                                                                           includes mitosis and cytokinesis.
                                                                                           After the M phase, two cells
Binary fission can be broken down into a series of                                         result.
three steps, although it is actually a continuous
process. Cell division is relatively simple in prokaryotic
As you can see, the eukaryotic cell cycle has several         Regulatory proteins control the cell cycle at key
phases. The mitosis phase (M) actually includes both         checkpoints, which are shown in Figure 5.4. There are
mitosis and cytokinesis. This is when the nucleus and        a number of main checkpoints.
then the cytoplasm divide. The other three phases (G1,       • The G1 checkpoint, just before entry into S phase,
S, and G2) are generally grouped together as                 makes the key decision of whether the cell should
interphase During interphase, the cell grows, performs       divide.
routine life processes, and prepares to divide. These        • The S checkpoint determines if the DNA has been
phases are discussed below. You can watch a                  replicated properly.
eukaryotic cell going through these phases of the cell       • The mitotic spindle checkpoint occurs at the point in
cycle at the following link:                                 metaphase where all the chromosomes should
http://www.cellsalive.com/cell_cycle.htm.                    have aligned at the mitotic plate.

Interphase                                                   Cancer and the Cell Cycle
Interphase of the eukaryotic cell cycle can be               Cancer is a disease that occurs when the cell cycle is
subdivided into the following three phases, which are        no longer regulated. This may happen because a
represented in Figure 5.3:                                   cell’s DNA becomes damaged. Damage can occur due
• Growth Phase 1 (G1): during this phase, the cell           to exposure to hazards such as radiation or toxic
grows rapidly, while performing routine metabolic            chemicals. Cancerous cells generally divide much
processes. It also makes proteins needed for DNA             faster than normal cells. They may form a mass of
replication and copies some of its organelles in             abnormal cells called a tumor. The rapidly dividing
preparation for cell division. A cell typically spends       cells take up nutrients and space that normal cells
most of its life in this phase.                              need. This can damage tissues and organs and
• Synthesis Phase (S): during this phase, the cell’s         eventually lead to death. Figure 5.5: These cells are
DNA is copied in the process of DNA replication.             cancer cells, growing out of control and forming a
• Growth Phase 2 (G2): during this phase, the cell           tumor.
makes final preparations to divide. For example,             Lesson Summary
it makes additional proteins and organelles.                 • Cell division is part of the life cycle of virtually all cells.
                                                             It is a more complicated process in eukaryotic
Control of the Cell Cycle                                    than prokaryotic cells because eukaryotic cells have
                                                             multiple chromosomes and a nucleus.
If the cell cycle occurred without regulation, cells might
                                                             • The cell cycle is a repeating series of events that cells
go from one phase to the next before they were
                                                             go through. It includes growth, DNA synthesis,
ready. What controls the cell cycle? How does the cell
                                                             and cell division. In eukaryotic cells, there are two
know when to grow, synthesize DNA, and divide?
                                                             growth phases, and cell division includes mitosis.
The cell cycle is controlled mainly by regulatory
                                                             • The cell cycle is controlled by regulatory proteins at
proteins. These proteins control the cycle by signaling
                                                             three key checkpoints in the cycle. The proteins
the cell to either start or delay the next phase of the
                                                             signal the cell to either start or delay the next phase of
cycle. They ensure that the cell completes the previous
                                                             the cycle.
phase before moving on.
                                                             • Cancer is a disease that occurs when the cell cycle is
                                                             no longer regulated. Cancer cells grow rapidly
                                                             and may form a mass of abnormal cells called a tumor.
Figure 5.4

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