CHAPTER 12. “CELLULA E CELLULA”: UNDERSTANDING CELL REPRODCTION
Student Learning Outcomes
At the completion of this exercise, the student will be able to:
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6.
Discuss the functions of cell division.
Describe the cell cycle.
Distinguish among the three stages of interphase.
Describe the stages of mitosis.
Draw and label the stages of interphase and
Mitosis. Identify the stages of mitosis in an onion root
preparation and a whitefish blastula, using a microscope.
7. Discuss control of the cell cycle.
OVERVIEW
How can a one-celled human zygote grow to eventually become an adult consisting of
more than 100 trillion cells? How do those pesky weeds seem to pop up overnight in your
yard? How does a one-celled organism such as an amoeba reproduce? What is cancer?
These are a few of the many questions that can be explained with an understanding of the
process of cell division.
In 1855 the German physician Rudolf Virchow restated his observations
concerning the reproduction of cells as “Omnis cellula e cellula”- meaning that cells
come from preexisting cells. Living organisms, as well as the cells that comprise tissues,
are capable of reproduction and growth. Cell division is the mechanism by which new
cells are produced, whether for growth, repair, replacement, or forming a new organism.
In 1875, the German botanist Eduard Strasberger first described cell division in
plants, but the process of cell division was not described in detail until 1876. In that year,
the German zoologist Walther Flemming, while describing the development of
salamander eggs, coined the terms chromatin and mitosis (cell division) and also
established the framework for understanding the stages of cell division. Today, mitosis is
recognized as a major component of the process of cell division.
Where a cell arises, there a cell must have previously existed (omnis cellula e cellula),
just as an animal can spring only from an animal, and a plant only from a plant.
-Rudolf Virchow (1821-1902)
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Cell division is the biological process by which cellular and nuclear materials of somatic
cells (non-sex cells) are divided between two daughter cells formed from an original parent cell.
The resulting daughter cells are structurally and functionally similar to each other and to the
parent cell. In prokaryotic organisms (bacteria and cyanobacteria) the distribution of exact
replicas of genetic material is comparatively simple. In eukaryotic organisms such as animals,
however, the process of cell division is much more complex. The complexity is the result of the
presence of a larger cell, a nucleus, and more DNA (larger genome) on individual linear
chromosomes. Thus, any study of cell division in eukaryotes will include a discussion of the cell
cycle (Fig. 12.1) and its two components, interphase and mitosis.
THE CELL CYCLE
Cells that are dividing pass through a regular sequence of cell growth and division known as the
cell cycle. Cell type, hormones, and growth factors, as well as external conditions, influence the
time required to complete the cell cycle. The cell cycle is divided into two major stages:
interphase, when the cell is not actively dividing, and mitosis, when the cell is actively dividing.
At any given time, a cell exists in one of the stages of the cell cycle. The majority of cells in an
organism are in interphase.
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Figure 12.2 Sister chromosome (duplicated chromosome). A human chromosome, one of 23
pairs found in Homo sapiens. Chromosomes consist of paired chromatids which are joined in a
small region called the centromere.
Interphase
Interphase typically accounts for 90% of the time that elapses during each cell cycle. Classically
called the resting stage, interphase actually is a busy part of the cell cycle. During interphase and
before a cell begins the process of mitosis, it must undergo DNA replication, synthesize important
proteins, produce enough organelles to supply both daughter cells, and assemble the structures
used during cell division.
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Interphase is divided into three phases: gap 1 (G1), synthesis (S), and gap 2 (G2). In
recent years, a period known as G0 has been described. G0 or the quiescent phase occurs after G1
and represents the time that a cell is metabolically active but not proliferative. Actively dividing
cells and cancer cells either skip G0 or pass through the stage quickly. Some cells, such as nerve
cells, may never exit G0 once formed.
G1 Phase
The G1 phase, occurring after mitosis, serves as the cell’s primary growth phase. During this
time, the cell recovers from the previous division, increases in size, and synthesizes proteins,
lipids, and carbohydrates. Also the number of organelles and inclusions increases in number. In
cells that contain centrioles, the two centrioles begin to form during G1 (note that cells of
flowering plants, fungi, and nematodes do not contain centrioles). The G1 phase occupies the
major portion of the lifespan of a typical cell. Slow-growing cells, such as some liver cells, can
remain in G1 for more than a year. Fast growing cells, such as epithelial cells and those of bone
marrow, remain in G1 for 16 to 24 hours.
S Phase
In the S phase of interphase, which follows the G1 phase, each chromosome replicates to produce
two daughters copies, termed sister chromatids (Fig 12.2). The two copies remain attached at a
point of constriction called the centromere. During this time, these structures are not visible
under the light microscope. Among the numerous proteins manufactured during this phase are
histones and other proteins that coordinate the various events taking place within the nucleus and
cytoplasm. Duplication of the centrioles is completed, and they organize to migrate to the
opposite poles of the cell. In addition, the microtubules that will become part of the spindle
apparatus are synthesized.
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G2 Phase: The G2 phase of interphase, which occurs after the S phase, involves further
replication of membranes, microtubules, mitochondria, and other organelles. In addition, the
newly replicated sister chromatids, which are diffusely distributed throughout the nucleus, begin
to coil and become more compact. The start of chromosome condensation at the completion of G2
signals the beginning of mitosis. By the end of G2, the volume of the cell has nearly doubled.
Mitosis
Cell division includes both the division of the nucleus, called karyokinesis, and the division of
the cytoplasm, cytokinesis. The overall action is to distribute the replicated chromosomes of the
parent cell to the two newly formed identical daughter cells. Keep in mind that cell division is a
dynamic and continuous process, but for ease in explanation and study, it has been divided into
several stages – prophase, metaphase, anaphase, and telophase.
Prophase: is the longest and the first active stage of mitosis, characteristically accounting for
70% of the time that a cell spends in the mitotic process. During this stage, the chromosomes
progressively become more visible as they shorten and thicken. Upon close examination in late
prophase, the sister chromatids and their centromeres can be seen easily. Prophase also is
characterized by migration of the centriole pairs toward opposite poles and disintegration of the
nuclear envelope and nucleolus.
Short tubules known as asters appear and begin to radiate from the centrioles. The aster
is believed to function to stiffen the point of microtubular attachment during the retraction of the
spindle. Polar microtubules, between the centrioles, begin to form the spindle fibers that
eventually will expand from one pole to another meeting at the equatorial plane.
The term prometaphase has become increasingly popular to describe events of late
prophase. It usually is distinguished by the attachment of sister chromatids to the spindle fibers by
means of a proteinaceous hook, or kinetochore.
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Metaphase: is the brief second stage of mitosis, when the centromere joining each pair of sister
chromatids is attached to the spindle. Eventually the chromatid pairs appear to align along an
imaginary line called the equatorial plane, or metaphase plane.
Anaphase: is the third and most intriguing stage of mitosis. Although brief, this stage is
characterized by the separation of the sister chromatids from their centromere, and the classic
movement of the chromatids as they are pulled back along the spindle to the opposite poles.
Errors during anaphase could result in an unequal distribution of genetic material.
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Telophase: the final stage of mitosis, telophase, resembles a dumbbell with a set of
chromosomes at each end. Cytokinesis, the division of cytoplasm upon completion of nuclear
division, begins during this stage. Cytokinesis appears as a pinching-off of the cells along a
cleavage furrow. During telophase, the spindle is disassembled, the nuclear envelope and
nucleolus re-form, and the cellular inclusions and organelles organize. Eventually the cleavage
furrow is completed and two daughter cells are formed. Upon completion of telophase, the
daughter cells enter the G1 phase and the cell cycle starts anew. In plant cells, instead of
undergoing cytokinesis, a cell plate is formed, dividing the mother cell into two daughter cells
(Fig. 12.3 and Fig 12.4).
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CELL REGULATION
The cell cycle is strictly regulated by a number of mechanisms that attempt to reduce the number
of errors and ensure a smooth transition between the stages. Hormones such as the growth
hormone control cell proliferation. Too much or too little growth hormone can influence an
organism greatly. Epidermal growth factors (EGFs) are important in repairing wounds, and
fibroblast growth factors are important in repairing and forming vessels.
Checkpoints in the cell cycle ensure that cell division is occurring properly. Kinasis are
enzymes, and cyclins are proteins that influence the activity of cell checkpoints. The first check
point, which occurs at the end of G1, monitors the size of the cell and whether the DNA has been
damaged. If it detects a problem, it can initiate repair, or apoptosis (cell suicide). The second
checkpoint, which occurs at the end of G2, essentially checks if DNA damage has occurred, that
replication has occurred properly, and f the cell is ready to proceed to mitosis. The final
checkpoint checks the spindle assembly and controls the onset of anaphase.
In humans, the p53 gene is essential in the first checkpoint in the apoptosis of damaged
cells. If the p53 does not function properly, damaged and mutated cells will be allowed to
proliferate. Many forms of cancer in humans are related to a malfunctioning p53 gene, and cancer
can be thought of as cell division that has lost control.
Figure 12.3 The stages of mitosis on an onion root tip.
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Figure 12.4 The stages of animal mitosis followed by cytokinesis above, below, and
pages 9-10. Whitefish blastula (All 1000X).
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The single chromosomes (former chromatids– see anaphase) continue to lengthen as the nuclear
membrane reforms. Cell division is complete and the newly formed cells grow and mature.
1. Two daughter nuclei.
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Did You Know?
The Greek God of Medicine, Asclepius, was best known for prescribing rest and proper diet for
his patients. Great temples of healing were built in his honor. In these temples, holy snakes and dogs were
allowed to slither and walk among the sick and injured. Supposedly, the snakes would keep away evil
spirits and the dogs would promote the healing by licking the wounds.
Today it is known that dogs contain a great amount of epidermal growth factors (EGFs) in their
saliva, which promote cell division and speed healing. The next time your dog detects a scratch on your
arm, notice the concern! EGFs are an integral part of modern medicine, used in cornea transplants and
burn treatment.
STUDENT ACTIVITY – ANIMAL CELL MITOSIS
Observing Mitosis in Animal Cells
In this exercise, you will observe the cell cycle in prepared slides of a whitefish blastula. The blastula
occurs in the early stage in the embryonic development of an animal, seen as a ball of cells, each cell in
one of the stages of interphases or mitosis.
Materials
1. Prepared slide of whitefish blastula
2. Compound light microscope
3. Colored pencils
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Procedure 12.1 Observing Animal Mitosis
1. Obtain a prepared slide of a whitefish blastula.
2. Obtain a compound light microscope, and follow the safety and handling procedures for a light
microscope.
3. Observe the whitefish blastula first on low power. In this field you will be able to see many slices
of depth of the blastula. Focusing on low power, you should see individual cells.
4. Switch to high power and sketch and describe each of the stages of interphase and mitosis in the
space below.
5. Draw each stage of cell division and describe what is happening within the cell in detail.
Interphase
Prophase
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Metaphase
Anaphase
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Telophase
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STUDENT ACTIVIY – PLANT CELL MITOSIS
Observing Mitosis in Plant Cells
In this exercise, you will observe the cell cycle in prepared slides of green onion (scallions) root tip
mitosis. In plants, the root tips often are used to study the cell cycle. In plants, cell growth occurs in the
meristematic (apical meristem) regions, located at the tips of stems and roots.
Materials
1. Prepared slides of onion root tip mitosis
2. Compound light microscope
3. Colored pencils
Procedure 12.2. Observing Plant Mitosis
1. Obtain a prepared slide of onion root tip mitosis.
2. Obtain a compound light microscope, and follow safety and handling procedures for a light
microscope.
3. Observe the onion root tip mitosis first on low power. In this field, you will be able to see many
stages of interphase and mitosis. Focusing on low power, you should see individual cells.
4. Switch to high power and sketch and describe each of the stages of interphase and mitosis, in the
space provided on the following page.
CHECK YOUR UNERSTANDING
(1) When studying cell cycle, why is it necessary to use an embryonic cell to observe the stages?
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(2) Describe cytokinesis and the cell plate. Which stage of mitosis is associated with each?
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Interphase
Prophase
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Metaphase
Anaphase
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Telophase
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Last Name, First Name [lab partner N0. 1]
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Last Name, First Name [lab partner N0. 2]
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Last Name, First Name [lab
partner N0. 3]
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Section
Last Name, First Name [lab
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group #
partner N0. 4]
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Date
Review Questions Lab 12: Understanding Cell Reproduction
1. What is the significance of the S phase of interphase?
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2. Draw, label and describe the formation of sister chromatids.
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3. Why did early scientists call interphase the "resting stage"?
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4. What are some differences between animal and plant mitosis?
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5. What is the relationship between cell division and cancer?
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6. How does the interphase set up the prophase?
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7. What is the difference between karyokinesis and cytokinesis?
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8. Draw and label the cell cycle.
9. What are the three checkpoints of the cell cycle, and what is the basic function of each?
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(2) ________________________________________________________________________
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10. What is the function of cell division?
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