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7 HOW CELLS DIVIDE
EXTENDED LECTURE OUTLINE
Cell Division (p. 142)
7.1
7.2
7.3
7.4
Prokaryotes Have a Simple Cell Cycle (p. 142; Figs. 7.1)
A. Cell division takes place in two stages in prokaryotes: DNA is copied, then the cell splits in a
process called binary fission.
B. Each daughter cell contains one circular copy of the parent cell's DNA and is a functioning
cell.
Eukaryotes Have a Complex Cell Cycle (p. 143; Fig. 7.2)
A. In eukaryotes, cell division, along with replication of segments of DNA called chromosomes,
is more complex.
B. In duplicated form, eukaryotic chromosomes appear as an “X” still attached in the middle at
the centromere.
C. Somatic, or body cells undergo mitosis, while germ cells in reproductive organs undergo
meiosis. The life cycle of a cell is called the cell cycle.
1. The first, or G1 phase, is the primary growth phase and takes up most of the life span of
the cell.
2. The S, or synthesis phase follows, during which DNA is replicated.
3. Next is the G2 phase when the cell readies itself for cell division.
4. These three phases together are known as interphase.
5. Mitosis (M phase) follows next, during which the nucleus and chromosomes of the cell
are divided.
6. During cytokinesis (C phase) the cytoplasm is cleaved, resulting in two daughter cells.
Chromosomes (p. 143; Figs. 7.3, 7.4, 7.5, 7.6)
A. Discovery of Chromosomes
1. Chromosomes were first observed by Walter Fleming in 1882 while watching the rapidly
dividing cells of salamander larvae.
B. Chromosome Number
1. Eukaryotic somatic cells have two copies of each chromosome, known as homologous
chromosomes.
2. When cells have both copies of each type of chromosome, they are called diploid; when
cells have only one of each type, such as after meiosis, they are called haploid.
3. When chromosomes replicate, each replicated copy is known as a chromatid.
4. Humans have 46, or 23 pairs, of chromosomes.
C. Chromosome Structure
1. Eukaryotic cells have histones associated with their DNA to help hold the shape of the
large DNA molecule and coil it into a tightly compacted chromosome.
2. Chromosomes exist in somatic cells as homologues, or homologous chromosomes.
3. Cells that have both homologues are diploid.
4. Before cell division, each homologue replicates into two identical copies called sister
chromatids.
5. In the duplicated condition, there are 92 chromatids in a human somatic cell.
Cell Division (p. 146; Figs. 7.7, 7.8, 7.9)
A. Interphase
1. During interphase, chromosomes replicate and begin condensation.
B. Mitosis
1. The first stage of mitosis is prophase, during which condensation of chromosomes and
nuclear membrane breakdown occur, centrioles move toward the poles, and kinetochore
fibers extend outward from chromosomes.
2. Metaphase occurs second, when chromosomes are aligned at the center of the cell.
35
3.
7.5
Anaphase, the third phase of mitosis, occurs as when sister chromatids are separated and
pulled toward the poles.
4. The last phase is telophase, during which the mitotic spindle is disassembled, the nuclear
envelope reforms, and chromosomes decondense.
C. Cytokinesis
1. In animal cells, mitosis is followed immediately by cytokinesis when a cleavage furrow
forms, splitting the cytoplasm between two daughter cells.
2. Plant cells instead have a cell plate forming to separate the cytoplasm of daughter cells.
D. Cell Death
1. Human cells divide only up to 50 times, after which they are programmed to die.
2. Cancer cells differ in that they can divide rapidly and indefinitely.
Controlling the Cell Cycle (p. 148; Figs. 7.10, 7.11)
A. The complex cell of eukaryotes is controlled by feedback at three checkpoints.
1. Cell growth is assessed at the G1 checkpoint.
2. DNA replication is assessed at the G2 checkpoint.
3. Mitosis is assessed at the M checkpoint.
Cancer and the Cell Cycle (p. 150)
7.6
7.7
7.8
What Is Cancer? (p. 150; Figs. 7.12, 7.13)
A. Cancer is unrestrained cell proliferation caused by damage to genes regulating cell division.
B. A tumor, or cluster of undifferentiated cells, may result.
C. The cancerous growth may metastasize, forming new tumors at distant sites.
Cancer and Control of the Cell Cycle (p. 151; Fig. 7.14)
A. The “guardian angel gene”, also called p53, plays a key role in the G1 checkpoint of cell
division.
B. Mutations disabling key elements of the G1 checkpoint are associated with many cancers.
Curing Cancer (p. 152; Fig. 7.15)
A. Preventing the Start of Cancer
1. Mutations that increase the number of receptors on the cell surface (as is common with
breast cancer) amplify the division signal and so lead to cancer; cell therapies directed at
this stage use the human immune system in the form of monoclonal antibodies to attack
cancer cells.
2. The second step in the decision process is the passage of the signal into the cell’s surface
using a protein called Ras; mutated forms of this protein instruct the cell to divide when it
should not, and are responsible for 30% of all cancers.
3. Amplifying the signal once it reaches the cytoplasm employs a protein kinase; mistakes
in this step lead to 5% of all cancers and may be controlled in the future using “anti-sense
RNA”.
4. The fourth decision is to release the brake (tumor supressor protein Rb) used to block
transcription factor E2F; drugs that inhibit E2F are being developed.
5. Protein p53 checks to make certain everything is ready to divide, and a full 50% of all
cancers are due to a mutation leading to the production of abnormal p53; new therapies
are directed at using an adenovirus to induce the immune system to attack the cancer.
6. Inhibiting telomerase is also being explored as a means to inhibit cancer.
B. Preventing the Spread of Cancer
1. Inhibiting angiogenesis helps to reduce the size of tumors.
2. Preventing metastasis offers promise as an anticancer therapy.
Meiosis (p. 154)
7.9
Discovery of Meiosis (p. 154; Figs. 7.16, 7.17)
A. In 1882, Belgian cytologist Pierre-Joseph van Beneden discovered that the gametes of the
roundworm Ascaris contained two chromosomes while somatic cells contained four.
B. Fertilization
1. van Beneden proposed in 1887 that an egg and sperm could fuse to form a zygote.
2. The fusion of gametes is called fertilization or syngamy.
36
C.
7.10
7.11
7.12
7.13
Meiosis
1. A reduction division was required to reduce the number of chromosomes to half so
sexual reproduction could occur.
2. This reduction division is known as meiosis.
D. The Sexual Life Cycle
1. Adult cells are diploid and gametes are haploid.
2. Sexual reproduction joins haploid gametes to produce a new diploid individual.
The Sexual Life Cycle (p. 155; Figs. 7.18, 7.19)
A. Somatic Tissues
1. In the sexual life cycle, there is an alternation of diploid and haploid generations.
2. Normal diploid body cells are called somatic cells.
3. Somatic cells arise from the zygote and are all genetically identical.
B. Germ-Line Tissues
1. Cells that produce gametes are called germ-line tissues.
2. Germ-line cells will undergo meiosis to produce haploid gametes.
The Stages of Meiosis (p. 156; Figs. 7.20, 7.21, 7.22, 7.23)
A. Meiosis has two divisions: meiosis I serves to divide the two versions of each chromosome;
and meiosis II separates the two replicas of each chromosome.
B. Meiosis I
1. Meiosis is similar to mitosis, except that it involves two divisions, meiosis I and meiosis
II, and the resulting cells are haploid, rather than diploid like those produced by mitosis.
2. Also, a phenomenon called crossing over occurs during prophase I of meiosis I when
pieces of nonsister chromatids exchange places to promote new genetic combinations in
the offspring.
3. Prior to meiosis I, during interphase, the DNA replicates and the cell readies itself for
cell division.
4. Meiosis I consists of four stages: prophase I occurs as the chromosomes condense,
homologues pair, and crossing over occurs; metaphase I involves formation of the
spindle apparatus and alignment of chromosome pairs along the center of the cell; in
anaphase I, homologous chromosomes are separated and pulled toward the poles; and
during telophase I, the chromosomes gather at each pole and prepare for the second
division.
5. During meiosis I, the alignment of homologous pairs along the center of the cell is
random, with different combinations of parental chromosomes possible for each daughter
cell.
6. This process is known as independent assortment.
C. Meiosis II
1. Meiosis II, which also has four stages, follows after meiosis I, and the result is the
separation of the sister chromatids to form four haploid gametes.
D. The Important Role of Crossing Over
1. Because of crossing over, no two haploid cells are the same.
Comparing Meiosis and Mitosis (p. 160; Figs. 7.24, 7.25)
A. Meiosis has two unique features: synapsis and reduction division.
B. Synapsis
1. The process of pairing throughout the length of the homologous chromosomes and
exchanging genetic fragments is called synapsis.
C. Reduction Division
1. Since DNA only replicates once, before meiosis I, the two divisions result in halving the
chromosome number in the daughter cells (gametes).
Evolutionary Consequences of Sex (p. 162; Fig. 7.26, 7.27)
A. Sexual reproduction has the capacity to generate new genetic combinations.
B. Independent Assortment
1. The random alignment of homologues during meiosis I results in an astounding number
of possible kinds of gametes that can be produced.
C. Crossing Over
37
1. Crossing over allows for combinations of genes that may never have existed previously.
D. Random Fertilization
1. Since the new zygote is the product of two gametes, each with new variation within them,
fertilization adds even more genetic diversity.
E. Importance of Generating Diversity
1. The evolutionary process is revolutionary in that the pace of evolutionary change is
quickened by genetic recombination.
2. Sexual reproduction adds genetic versatility to the offspring, a trait favored in the
vertebrates.
LEARNING OBJECTIVES
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Explain how prokaryotic binary fission occurs.
List the phases of the eukaryotic complex cell cycle.
Describe a eukaryotic chromosome.
Know the significance of mitosis.
Be able to state the events that occur during the stages of mitosis.
Know the difference between cytokinesis in animal and plant cells.
Understand that cell division is controlled in normal cells, but loses control in cancer cells.
Describe the nature of the new molecular therapies used to treat cancer.
Diagram the sexual life cycle.
Understand the stages of meiosis.
Compare the outcome of meiosis with that of mitosis.
Discuss the unique features of meiosis, namely synapsis and reduction division.
Explain the evolutionary consequences of sex.
KEY TERMS
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binary fission (p. 140) Prokaryotes carry out this simple form of splitting into two cells.
chromosome (p. 143) The eukaryotic chromosome is a single DNA molecule with associated proteins.
mitosis (p. 143) The type of cell division used for growth and repair, mitosis occurs in somatic cells.
meiosis (p. 143) The type of cell division that occurs only in germ cells and leads to the production of
gametes.
complex cell cycle (p. 143) This consists of the G1 phase, S phase, G2 phase, mitosis, and cytokinesis.
histones (p. 143) Proteins with positive charges around which the DNA molecule coils.
homologous chromosomes (p. 144) Two identical copies of the same chromosome, each coming from
a different parent.
cytokinesis (p. 148) This occurs with a cleavage furrow in animal cells and a cell plate in plant cells.
cancer (p. 150) Simply put, cancer is a growth disorder of cells.
metastases (p. 150)
diploid (p. 154)
haploid (p. 154) Only one set of chromosomes (one of each homologous pair) is present in the cell.
crossing over (p. 156) A phenomenon in which DNA is exchanged between nonsister chromatids.
independent assortment (p. 156)
38
LECTURE SUGGESTIONS AND ENRICHMENT TIPS
1.
2.
3.
The Mechanics of Mitosis. Give each student in the class a length of string to represent a nuclear
membrane and two (or more) pairs of pipe cleaners. Two individual pipe cleaners represent two
chromosomes in normal condition; the second set of two can each be twisted together with one of the
first pair to represent chromosomes in duplicated condition. Using a suitable diagram on an overhead
projector or chalkboard, have students manipulate the “chromosomes” through the various stages of
mitosis. Something about actually going through the motions of manipulating cell parts in this way
helps students remember the stages of mitosis.
Cancer and Its Causes. Discuss with students what types of things cause cancer, including the
mutagens in cigarette smoke, inadequate nutrition or fats in the diet, mutagenic chemicals, and
ultraviolet radiation. Relate mutations in DNA to losing control over the cell cycle and having more
rapid than normal rates of cell division, producing numerous immature cells. End your discussion with
ideas on how students could cut their chances of developing cancer, based especially on behavior
modification.
How Cancer Chemotherapy Works. Discuss with your students how cancer chemotherapy works and
tie it in with the cell cycle and mitosis. Many different forms of chemotherapy are available, each
tailored to work on specific types of cancer in the body. Chemotherapy operates by specifically killing
cells when they are dividing. Since cancer cells divide much more rapidly than normal body cells,
chemotherapy drugs should kill the cancer out before they kill too many of the body's normal cells.
Cells of the body that are killed by chemotherapy are those that the body replaces frequently, such as
the lining of the small intestine or the lining of the mouth and throat. Thus, chemotherapy patients often
suffer intestinal disturbances plus dry or sore mouths and throats, among other side effects of the
treatment.
CHANGES TO THE NEW EDITION
Refer to the Johnson instructor web site at http://www.mhhe.com/biosci/genbio/tlw4 for a complete list of
changes to this edition.
CRITICAL THINKING QUESTIONS
1.
2.
3.
What are the possible consequences of an inaccurate process of mitosis during growth in a multicellular
organism?
Devise an evolutionary explanation for why eukaryotes, such as humans, have two copies of each
chromosome.
What are the sources of new genetic combinations for organisms, like many algae, that rarely, if ever,
engage in sexual reproduction?
FILMS/MEDIA SUGGESTIONS
(Telephone and fax numbers and/or web sites for the sources of the following materials are listed in the
Appendix.)
Mitosis and Meiosis. Exceptional microscopic images coupled with animated sequences help students
understand the processes of mitosis and meiosis. 1994. 23 minutes.
CLEARVUE/eav, WW6VH 1466 and also Carolina Biological Supply, CE-49-1004
Mitosis. This BSCS Classic Inquiry program challenges students to come to their own conclusions based
on the data presented. The interactive format helps students examine the nucleus and mitotic events. 10
minutes.
CLEARVUE/eav, WW5VH 1597
39
Meiosis: The Key to Genetic Diversity. Topics include: why meiosis is important in sexually reproducing
organisms, the stages of meiosis, and how genetic diversity helps a species. 26 minutes.
CLEARVUE/eav, WW5VH 1269 and also Carolina Biological Supply, CE-49-1003AV
Apoptosis: Cell Death and Cancer. This video explores the ground-breaking research of Scottish
investigators Wyllie and Currie into the process of apoptosis and the possible use of this process to cure
cancer.
Films for the Humanities & Sciences, BVL7412
Cancer and Metastasis. How a normal cell becomes cancerous and the steps leading to metastasis in
cancer are clearly presented in this program. 1995. 39 minutes.
Films for the Humanities & Sciences, BVL6904
40