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 and6. 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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CHAPTER 12. “CELLULA E CELLULA”: UNDERSTANDING CELL REPRODCTION

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).

CELL REGULATION

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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 blastula2. Compound light microscope3. 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 MITOSISObserving 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 mitosis2. Compound light microscope3. 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?

                                                                                                                                                                                                                                                                                                _____________

                                                                                                                                                                                                                                                                                                _____________

(2) Describe cytokinesis and the cell plate. Which stage of mitosis is associated with each?

                                                                                                                                                                                                                                                                                              _____________  

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Interphase Prophase

__________________________________ ______________________________________

______________________________________ _________________________________________

Metaphase Anaphase

__________________________________ ______________________________________

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Telophase

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Review Questions Lab 12: Understanding Cell Reproduction1. 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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___________________________________________ ____________________________________________Last Name, First Name [lab partner N0. 1] Last Name, First Name [lab partner N0. 2]

_______________________________ _______________________________Last Name, First Name [lab partner N0. 3] Last Name, First Name [lab partner N0. 4]

___________________________ _______________ ____________________Section group # Date

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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