1. Cell Cycle & Cell Reproduction
CHAPTER 9
Cell Cycle & Cell Reproduction

2. I. The Cell Cycle
A. Orderly set of stages & sub stages between one division & the next
B. Two major stages of the cell cycle:
1. Interphase Stage: ~ 90% of cycle
a. Very active phase
b. Some cells (nerve, muscle) don’t complete cell cycle (in G stage)
2. Mitotic (M) Stage: ~ 10% of cycle
a. Cell nucleus divides
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3. The Cell Cycle

4. 2. Sub-phases of interphase
a. G1 - Doubles # of organelles, gathers raw materials for DNA synthesis
b. S - DNA is replicated (synthesis)
 Chromosomes start single- stranded (1 chromatid)
 End up with 2 identical chromatids
c. G2 - cell synthesizes proteins necessary for next cell division
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5. a. Mitosis (karyokinesis) • Nuclear division
2. Mitotic (M) Stage
Includes:
a. Mitosis (karyokinesis)
• Nuclear division
• Daughter chromosomes are distributed to 2 daughter nuclei
b. Cytokinesis  Cytoplasm division
 Results in 2 genetically identical daughter cells
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6. II. Control of the Cell Cycle
A. Controlled by internal and external signals.
1. A signal is a molecule that stimulates or inhibits a metabolic event.
2. These signals ensure that the stages follow one another in the normal sequence
3. Growth factors are external signals received at the plasma membrane
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7. B. Cell Cycle Checkpoints
1. Cyclins are a family of internal signaling proteins that increase & decrease as the cell cycle continues.
a. These have to be present in order for the cell to proceed from G stage to S stage and to proceed from G2 stage to M stage.
• If DNA is damaged at the G1 and G2 checkpoints the cell cycle stops.
b. M checkpoint will stop cycle if chromosomes are not aligned properly.
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8. The Cell Cycle

9. C. Apoptosis & Cell Division
1. Programmed cell death
2. Mitosis & apoptosis are opposing forces:
a. Mitosis increases cell number
b. Apoptosis decreases number
3. Fertilized egg divides to produce many cells; eventually some must die for organism to take shape
a. Tadpole losing tail
b. Hands losing webbing
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10. Apoptosis

11. III. Eukaryotic chromosomes
A. More than 50% proteins. When not undergoing cell division the:
1. DNA is in long thin threads
2. DNA is wound around proteins called histones.
3. 8 histones make up a nucleosome.
a. These look like beads on a necklace.
4. Nucleosomes coil up tight.
5. This creates a tangled mass of thin threads called chromatin.
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12. Levels of Chromosome Structure

13. B. Before mitosis begins:
1. Chromatin becomes highly coiled and condensed
2. Individual chromosomes become visible.
3. Each species has a characteristic number of chromosomes
a. humans = 46
b. goldfish = 94
c. corn = 20
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14. C. Chromosome #s in cells
1. Diploid cells
a. Cells that have two sets of homologous chromosomes
• # of chromosomes is called the diploid number • This is referred to as 2n
n = # of chromosomes in one complete set
• 2n = 46 for humans
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15. a. Cells with single set of chromosomes b. Found in gametes
2. Haploid cells
a. Cells with single set of chromosomes
b. Found in gametes
• Sex cells: eggs & sperm
• Referred to as n
• n = 23 for humans
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17. D . Chromosome appearance
1. During S stage of interphase the DNA replicates, duplicating the chromosomes in the parent cell.
2. Each new chromosome consists of identical DNA molecules called sister chromatids
3. They are attached to each other at a region called a centromere
4. Kinetochores develop on either side of centromere during division.
***Count centromeres to count chromosomes***
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18. Duplicated Chromosome

19. a. Centromeres holding sister chromatids together break
5. During mitosis:
a. Centromeres holding sister
chromatids together break
b. Sister chromatids separate
c. Each chromatid becomes a daughter chromosome
d. Sister of each type distributed to opposite daughter nuclei
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20. IV. Mitosis in Animal Cells
A. Just outside nucleus is the centrosome
1. This is the microtubule organizing center
2. Organizes mitotic spindle
a. Contains many fibers
b. Each composed of a bundle of microtubules
3. These replicated in S-phase so there are now two centrosomes
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21. a. Oriented at right angles to each other within centrosome
4. In animals cells the centrosome contains two barrel-shaped centrioles
a. Oriented at right angles to each other within centrosome
b. Each with 9 triplets of microtubules arranged in a cylinder.
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22. V. Phases of Mitosis A. Prophase
1. Chromatin condenses into visible 2-stranded chromosomes
2. Nucleolus disappears
3. Nuclear envelope breaks down
4. Spindle begins to form
5. Centrosomes move away from each other.
6. Star-like microtubules (asters) appear
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23. Prophase

24. a. Centromeres of chromosomes develop two kinetochores
B. Metaphase
1. Prometaphase
a. Centromeres of chromosomes develop two kinetochores
• These physically hook sister chromatids up with specialized kinetochore spindle fibers
• These connect sisters to opposite poles of mother cell
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26. a. Chromosomes pulled by kinetochore fibers
2. Metaphase proper:
a. Chromosomes pulled by kinetochore fibers
b. Forced to line up across the equatorial plane of cell
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27. C. Anaphase 1. Centromere dissolves releasing sister chromatids
2. Sister chromatids separate
a. Now called daughter chromosomes
b. They consist of one chromatid now
3. Pulled to opposite poles along spindle fibers
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28. Anaphase

29. D. Telophase 1. Spindle disappears
2. Two clusters of daughter chromosomes
3. Nuclear envelope forms around two new sets of chromosomes
3. Chromosomes uncoil to chromatin
4. Nucleolus reappears in each
***Cytokinesis usually occurs during this phase. May start in late anaphase***
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30. Telophase

31. VI. Cytokinesis A. Animal Cells 1. Cleavage furrow appears
2. Microfilaments of actin help to pinch the cell in half
3. Like pulling on a draw string
4. Pinches mother cell in two dividing cytoplasm up equally
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33. 1. Rigid walls do not permit furrowing 2. Cell plate forms
B. Plant Cells
1. Rigid walls do not permit furrowing
2. Cell plate forms
a. Small vesicles appear
b. Eventually fuse into one thin vesicle extending across cell
3. Membranes of cell plate become plasma member of daughter cells
4. Content of vesicles becomes part of cell wall
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34. Cytokinesis in Plants

35. VII. Functions of Mitosis
A. Permits growth & repair
1. In plants, meristematic tissue retains ability to divide through life of plant
2. In many animals, mitosis is necessary to turn a fertilized egg into an embryo and fetus.
3. Allow a repairs of damaged body parts
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36. (See Science Focus, p. 158, for a description of these processes)
VIII. Stem Cells
A. Many mammalian organs contain stem cells, which retain the ability to divide. These are called adult stem cells.
B. Red bone marrow stem cells could possibly be used to produce various types of tissues in the lab
1. These could be used for:
a. therapeutic cloning
b. reproductive cloning
(See Science Focus, p. 158, for a description of these processes)
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37. IX. Cancer A. Disease of cell cycle
1. Cancer cells result from mutation of genes that regulate the cell cycle
B. Neoplasms (tumors) are masses of cells that are dividing excessively
1. Benign tumor = not cancer. Do not invade neighboring tissues.
2. Malignant tumor = cancer. The mass of cells is capable of spreading to other tissues (Metastasis)
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38. C. Carcinogenesis 1. Development of cancer
a. Tends to be gradual; can take decades to develop.
2. Characteristics of cancer cells:
a. They lack differentiation; not specialized. Keep on dividing; seem immortal.
b. Have abnormal nuclei; chromosomes have mutated but they fail to undergo apoptosis
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39. c. Cancer cells form tumors because they lack:
• contact inhibition - they don’t stop dividing when they come into contact with a neighbor
• need to anchor themselves to surfaces or neighbors
d. Cancer cells undergo metastasis & angiogenesis.
• Cancer cells produce enzymes that allow them to invade tissues
• Angiogenesis - form new blood vessels to bring nutrients & O2 to tumor
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41. 3. Origin of Cancer
a. Mutations in following genes are associated with development of cancer
• Proto-oncogenes promote the cell cycle in various ways
• Tumor suppressor genes inhibit cell cycle in various ways
b. If either mutates, the cell may lose control and proto-oncogenes become oncogenes, cancer-causing genes
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42. • These usually get shorter each time a cell divides
c. DNA segments, called telomeres, occur at ends of chromosomes and protect them from damage.
• These usually get shorter each time a cell divides
• In cancer cells, an enzyme called telomerase keeps telomeres from shortening so that cancer cells keep on dividing.
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43. Causes of Cancer

44. X. Prokaryotic Cell Division
A. Produces two new individuals, a form of asexual reproduction
B. Prokaryotic chromosome:
1. Appears as a dense region called the nucleoid
2. Chromosome is a circular loop about 1,000 times length of cell
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45. C. Binary fission
1. Process that produces two daughter cells identical to original parent cell
2. Steps:
a. Cell enlarges
b. DNA replicates & chromosomes attach to plasma membrane
c. Cell elongates & pulls the chromosomes apart
d. Cell wall & plasma membrane grow inward until they divide cell
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46. Binary Fission