CHAPTER 12. “CELLULA E CELLULA”: UNDERSTANDING CELL
REPRODCTION
Student Learning Outcomes
At the completion of this exercise, the student will be able
to:
1. Discuss the functions of cell division.
2. Describe the cell cycle.
3. Distinguish among the three stages of interphase.
4. Describe the stages of mitosis.
5. Draw and label the stages of interphase and
6. 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)
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.
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.
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.
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.
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.
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).
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).
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.
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
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] Last Name, First Name [lab partner N0.
2]_______________________________
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N0. 3] Last Name, First Name [lab partner N0.
4]___________________________ _______________
____________________Section group #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?
(1)
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(2)
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(3)
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10. What is the function of cell division?
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