1. The Cell Cycle and Cell Division7
The Cell Cycle and Cell Division

2. Chapter 7 The Cell Cycle and Cell Division
Key Concepts
7.1 Different Life Cycles Use Different Modes of Cell Reproduction
7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
7.3 Cell Reproduction Is Under Precise Control

3. Chapter 7 The Cell Cycle and Cell Division
Key Concepts
7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity
7.5 Programmed Cell Death Is a Necessary Process in Living Organisms

4. Organisms have two basic strategies for reproducing themselves:
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction
The lifespan of an organism is linked to cell reproduction—usually called cell division.
Organisms have two basic strategies for reproducing themselves:
Asexual reproduction
Sexual reproduction
Cell division is also important in growth and repair of tissues.
See Chapter 4

5. Figure 7.1 The Importance of Cell Division (Part 1)

6. Figure 7.1 The Importance of Cell Division (Part 2)

7. Figure 7.1 The Importance of Cell Division (Part 3)

8. Any genetic variations are due to mutations.
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction
In asexual reproduction the offspring are clones—genetically identical to the parent.
Any genetic variations are due to mutations.
A unicellular prokaryote may reproduce itself by binary fission.
Single-cell eukaryotes can reproduce by mitosis.
Other eukaryotes are also able to reproduce through asexual or sexual means.

9. Figure 7.2 Asexual Reproduction on a Large Scale

10. Gametes form by meiosis—a process of cell division.
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction
Sexual reproduction requires gametes—two parents each contribute one gamete to an offspring.
Gametes form by meiosis—a process of cell division.
Gametes—and offspring—differ genetically from each other and from the parents.

11. DNA in eukaryotic cells is organized into chromosomes.
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction
DNA in eukaryotic cells is organized into chromosomes.
A chromosome consists of a single molecule of DNA and proteins.
Somatic cells—body cells not specialized for reproduction
Each somatic cell contains two sets of chromosomes (homologs) that occur in homologous pairs.
VIDEO 7.1 Cell Visualization: From DNA to chromosomes
LINK The inheritance of characteristics such as seed shape is discussed in Chapter 8
See Chapter 4

12. Haploid cell—Number of chromosomes = n
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction
Gametes contain only one set of chromosomes—one homolog from each pair.
Haploid cell—Number of chromosomes = n
Fertilization—Two haploid gametes (female egg and male sperm) fuse to form a zygote.
Chromosome number in zygote = 2n and cells are diploid.

13. All kinds of sexual life cycles involve meiosis:
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction
All kinds of sexual life cycles involve meiosis:
Haplontic life cycle—in protists, fungi, and some algae—zygote is only diploid stage
After zygote forms it undergoes meiosis to form haploid spores, which germinate to form a new organism.
Organism is haploid, and produces gametes by mitosis—cells fuse to form diploid zygote.

14. Figure 7.3 All Sexual Life Cycles Involve Fertilization and Meiosis (Part 1)

15. Spores divide by mitosis to form the haploid generation (gametophyte).
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction
Alternation of generations—most plants, some protists; meiosis gives rise to haploid spores
Spores divide by mitosis to form the haploid generation (gametophyte).
Gametophyte forms gametes by mitosis.
Gametes then fuse to form diploid zygote (sporophyte), which in turn produces haploid spores by meiosis.

16. Figure 7.3 All Sexual Life Cycles Involve Fertilization and Meiosis (Part 2)

18. A mature organism is diploid and produces gametes by meiosis.
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction
Diplontic life cycle—animals and some plants; gametes are the only haploid stage
A mature organism is diploid and produces gametes by meiosis.
Gametes fuse to form diploid zygote; zygote divides by mitosis to form mature organism.

19. Figure 7.3 All Sexual Life Cycles Involve Fertilization and Meiosis (Part 3)

20. Four events must occur for cell division:
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
Four events must occur for cell division:
Reproductive signal—to initiate cell division
Replication of DNA
Segregation—distribution of the DNA into the two new cells
Cytokinesis—division of the cytoplasm and separation of the two new cells

21. Separates the DNA and cytoplasm into two cells through binary fission
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
In prokaryotes, cell division results in reproduction of the entire organism.
The cell:
Grows in size
Replicates its DNA
Separates the DNA and cytoplasm into two cells through binary fission

22. Two important regions in reproduction: ori - where replication starts
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
Most prokaryotes have one chromosome, a single molecule of DNA—usually circular.
Two important regions in reproduction:
ori - where replication starts
ter - where replication ends

23. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
As replication proceeds, the ori complexes move to opposite ends of the cell.
DNA sequences adjacent to the ori region actively bind proteins for the segregation, hydrolyzing ATP for energy.
An actin-like protein provides a filament along which ori and other proteins move.
LINK Review the description of the cytoskeleton and its components in Concept 4.4

24. Figure 7.4 Prokaryotic Cell Division

25. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
Cytokinesis begins after chromosome segregation by a pinching in of the plasma membrane—protein fibers form a ring.
As the membrane pinches in, new cell wall materials are synthesized resulting in separation of the two cells.
VIDEO 7.2 Cytokinesis in the euglenoid Phacus
VIDEO 7.3 Cytokinesis in a green alga, Micrasterias

26. Eukaryotic cells divide by mitosis followed by cytokinesis.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
Eukaryotic cells divide by mitosis followed by cytokinesis.
Replication of DNA occurs as long strands are threaded through replication complexes.
See Chapter 9

27. In eukaryotes, the chromosomes become highly condensed.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
In segregation of DNA after cell division, one copy of each chromosome ends up in each of the two new cells.
In eukaryotes, the chromosomes become highly condensed.
Mitosis segregates them into two new nuclei— the cytoskeleton is involved in the process.

28. Cytokinesis follows mitosis.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
Cytokinesis follows mitosis.
The process in plant cells (which have cell walls) is different than in animal cells (which do not have cell walls).

29. The cell cycle—the period between cell divisions
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
The cell cycle—the period between cell divisions
In eukaryotes it is divided into mitosis and cytokinesis—called the M phase—and a long interphase.
During interphase, the cell nucleus is visible and cell functions including replication occur
Interphase begins after cytokinesis and ends when mitosis starts.

30. Interphase has three subphases: G1, S, and G2.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
Interphase has three subphases: G1, S, and G2.
G1 (Gap 1)—variable, a cell may spend a long time in this phase carrying out its functions
S phase (Synthesis)—DNA is replicated
G2 (Gap 2)—the cell prepares for mitosis, synthesizes microtubules for segregating chromosomes
ANIMATED TUTORIAL 7.1 Mitosis

31. Figure 7.5 The Phases of the Eukaryotic Cell Cycle (Part 1)

32. Figure 7.5 The Phases of the Eukaryotic Cell Cycle (Part 2)

33. Figure 7.5 The Phases of the Eukaryotic Cell Cycle (Part 3)

34. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
In mitosis, one nucleus produces two daughter nuclei each containing the same number of chromosomes as the parent nucleus.
Mitosis is continuous, but can be can be divided into phases—prophase, prometaphase, metaphase, anaphase, and telophase.
VIDEO 7.4 Division of bacteria, Salmonella enteritidis

35. The chromatin (DNA) is not yet condensed.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
During interphase, only the nuclear envelope and the nucleolus are visible.
The chromatin (DNA) is not yet condensed.
Three structures appear in prophase:
The condensed chromosomes
Centrosome
Spindle
See Concept 4.3

36. Condensed chromosomes appear during prophase.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
Condensed chromosomes appear during prophase.
Sister chromatids—two DNA molecules on each chromosome after replication
Centromere—region where chromatids are joined
Kinetochores are protein structures on the centromeres, and are important for chromosome movement.
See Chapter 9

37. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
The karyotype of an organism reflects the number and sizes of its condensed chromosomes.
Karyotype analysis can be used to identify organisms, but DNA sequence is more commonly used.

38. Segregation is aided by other structures:
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
Segregation is aided by other structures:
The centrosome determines the orientation of the spindle apparatus.
Each centrosome can consist of two centrioles—hollow tubes formed by microtubules.
Centrosome is duplicated during S phase and each moves towards opposite sides of the nucleus.

39. Polar microtubules form a spindle and overlap in the center.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
Centrosomes serve as mitotic centers or poles; the spindle forms between the poles from two types of microtubules:
Polar microtubules form a spindle and overlap in the center.
Kinetochore microtubules—attach to kinetochores on the chromatids. Sister chromatids attach to opposite halves of the spindle.
VIDEO 7.5 Formation of mitotic spindles

40. Chromosome separation and movement is highly organized.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
Chromosome separation and movement is highly organized.
During prometaphase, the nuclear envelope breaks down.
Chromosomes consisting of two chromatids attach to the kinetochore mictotubules.
VIDEO 7.6 Cell Visualization: Mitosis and cell division

41. Figure 7.6 The Phases of Mitosis (1)

42. During metaphase, chromosomes line up at the mid-line of the cell.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
During metaphase, chromosomes line up at the mid-line of the cell.
During anaphase, the separation of sister chromatids is controlled by M phase cyclin- Cdk; cohesin is hydrolyzed by separase.
After separation, they move to opposite ends of the spindle and are referred to as daughter chromosomes.

43. Figure 7.6 The Phases of Mitosis (2)

44. Microtubules also shorten, drawing chromosomes toward poles.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
A protein at the kinetochores—cytoplasmic dynein—hydrolyzes ATP for energy to move chromosomes along the microtubules towards the poles.
Microtubules also shorten, drawing chromosomes toward poles.

45. Telophase occurs after chromosomes have separated: Spindle breaks down
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
Telophase occurs after chromosomes have separated:
Spindle breaks down
Chromosomes uncoil
Nuclear envelope and nucleoli appear
Two daughter nuclei are formed with identical genetic information

46. Division of the cytoplasm differs in plant and animals
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
Cytokinesis:
Division of the cytoplasm differs in plant and animals
In animal cells, plasma membrane pinches between the nuclei because of a contractile ring of microfilaments of actin and myosin.
VIDEO 7.7: Mitosis in a newt lung epithelial cell

47. Figure 7.7 Cytokinesis Differs in Animal and Plant Cells (Part 1)

48. These fuse to form a new plasma membrane.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
Plant cells:
Vesicles from the Golgi apparatus appear along the plane of cell division
These fuse to form a new plasma membrane.
Contents of vesicles form the cell plate—the beginning of the new cell wall.
VIDEO 7.8 Mitosis in a plant cell

49. Figure 7.7 Cytokinesis Differs in Animal and Plant Cells (Part 2)

50. Each daughter cell contains all of the components of a complete cell.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
After cytokinesis:
Each daughter cell contains all of the components of a complete cell.
Chromosomes are precisely distributed.
The orientation of cell division is important to development, but organelles are not always evenly distributed.

51. Concept 7.3 Cell Reproduction Is Under Precise Control
The reproductive rates of most prokaryotes respond to environmental conditions.
In eukaryotes, cell division is related to the needs of the entire organism.
Cells divide in response to extracellular signals, like growth factors.

52. Concept 7.3 Cell Reproduction Is Under Precise Control
The eukaryotic cell cycle has four stages: G1, S, G2, and M.
Progression is tightly regulated—the G1-S transition is called R, the restriction point.
Passing this point usually means the cell will proceed with the cell cycle and divide.

53. Figure 7.8 The Eukaryotic Cell Cycle

54. Concept 7.3 Cell Reproduction Is Under Precise Control
Transitions also depend on activation of cyclin- dependent kinases (Cdk’s).
A protein kinase is an enzyme that catalyzes phosphorylation from ATP to a protein.
Phosphorylation changes the shape and function of a protein by changing its charges.
See Chapter 5

55. Concept 7.3 Cell Reproduction Is Under Precise Control
Cdk is activated by binding to cyclin (by allosteric regulation); this alters its shape and exposes its active site.
The G1-S cyclin-Cdk complex acts as a protein kinase and triggers transition from G1 to S.
Other cyclin-Cdk’s act at different stages of the cell cycle, called cell cycle checkpoints.
See Chapter 3

56. Figure 7.10 Cyclins Are Transient in the Cell Cycle

57. Concept 7.3 Cell Reproduction Is Under Precise Control
Example of G1-S cyclin-Cdk regulation:
Progress past the restriction point in G1 depends on retinoblastoma protein (RB).
RB normally inhibits the cell cycle, but when phosphorylated by G1-S cyclin-Cdk, RB becomes inactive and no longer blocks the cell cycle.
See Chapters 3 and 5

58. Reduce the chromosome number from diploid to haploid
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity
Meiosis consists of two nuclear divisions but DNA is replicated only once. The function of meiosis is to:
Reduce the chromosome number from diploid to haploid
Ensure that each haploid has a complete set of chromosomes
Generate diversity among the products
ANIMATED TUTORIAL 7.2 Meiosis

59. Figure 7.11 Mitosis and Meiosis: A Comparison

60. Figure 7.12 Meiosis: Generating Haploid Cells (Part 1)

61. Figure 7.12 Meiosis: Generating Haploid Cells (Part 2)

62. Figure 7.12 Meiosis: Generating Haploid Cells (Part 3)

63. Figure 7.12 Meiosis: Generating Haploid Cells (Part 4)

64. Figure 7.12 Meiosis: Generating Haploid Cells (Part 5)

65. Meiotic division reduces the chromosome number. Two unique features:
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity
Meiotic division reduces the chromosome number. Two unique features:
In meiosis I, homologous pairs of chromosomes come together and line up along their entire lengths.
After metaphase I, the homologous chromosome pairs separate, but individual chromosomes made up of two sister chromatids remain together.

66. Meiosis I is preceded by an S phase during which DNA is replicated.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity
Meiosis I is preceded by an S phase during which DNA is replicated.
Each chromosome then consists of two sister chromatids, held together by cohesin proteins.
At the end of meiosis I, two nuclei form, each with half the original chromosomes—still composed of sister chromatids.

67. These four cells are not genetically identical.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity
Sister chromatids separate during meiosis II, which is not proceeded by DNA replication.
The products of meiosis I and II are four cells with a haploid number of chromosomes.
These four cells are not genetically identical.
Two processes may occur:
Crossing over and independent assortment
VIDEO 7.9 Meiosis in a cranefly spermatocyte

68. In prophase of meiosis I homologous chromosomes pair by synapsis.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity
In prophase of meiosis I homologous chromosomes pair by synapsis.
The four chromatids of each pair of chromosomes form a tetrad,or bivalent.
The homologs seem to repel each other but are held together at chiasmata.

69. Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity
Crossing over is an exchange of genetic material that occurs at the chiasma.
Crossing over results in recombinant chromatids and increases genetic variability of the products.

70. In-Text Art, Ch. 7, p. 138

71. Figure 7.13 Crossing Over Forms Genetically Diverse Chromosomes

72. Prophase I may last a long time.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity
Prophase I may last a long time.
Human males: Prophase I lasts about 1 week, and 1 month for entire meiotic cycle
Human females: Prophase I begins before birth, and ends up to decades later during the monthly ovarian cycle

73. The more chromosomes involved, the more combinations possible.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity
Independent assortment during anaphase I also allows for chance combinations and genetic diversity.
After homologous pairs of chromosomes line up at metaphase I, it is a matter of chance which member of a pair goes to which daughter cell.
The more chromosomes involved, the more combinations possible.
APPLY THE CONCEPT Meiosis halves the nuclear chromosome content and generates diversity

74. Either results in aneuploidy—chromosomes lacking or present in excess
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity
Meiotic errors:
Nondisjunction—homologous pairs fail to separate at anaphase I—sister chromatids fail to separate, or homologous chromosomes may not remain together
Either results in aneuploidy—chromosomes lacking or present in excess

75. Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity
Organisms with triploid (3n), tetraploid (4n), and even higher levels are called polyploid.
This can occur through an extra round of DNA duplication before meiosis, or the lack of spindle formation in meiosis II.
Polyploidy occurs naturally in some species, and can be desirable in plants.

76. Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity
If crossing over happens between non- homologous chromosomes, the result is a translocation.
A piece of chromosome may rejoin another chromosome, and its location can have profound effects on the expression of other genes.
Example: Leukemia
See Chapters 10 and 11

77. In-Text Art, Ch. 7, p. 140

78. Cell death occurs in two ways:
Concept 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms
Cell death occurs in two ways:
In necrosis, the cell is damaged or starved for oxygen or nutrients. The cell swells and bursts.
Cell contents are released to the extracellular environment and can cause inflammation.
See Concept 31.1
APPLY THE CONCEPT Programmed cell death is a necessary process in living organisms

79. Apoptosis is genetically programmed cell death. Two possible reasons:
7.5 ProConcept 7.ammed Cell Death Is a Necessary Process in Living Organisms
Apoptosis is genetically programmed cell death. Two possible reasons:
The cell is no longer needed, e.g., the connective tissue between the fingers of a fetus.
Old cells may be prone to genetic damage that can lead to cancer—blood cells and epithelial cells die after days or weeks.

80. Cell detaches from its neighbors
Concept 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms
Events of apoptosis:
Cell detaches from its neighbors
Cuts up its chromatin into nucleosome-sized pieces
Forms membranous lobes called “blebs” that break into fragments
Surrounding living cells ingest the remains of the dead cell
VIDEO 7.10 Apoptosis

81. Figure 7.14 Apoptosis: Programmed Cell Death (Part 1)

82. Cell death cycle is controlled by signals:
Concept 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms
Cell death cycle is controlled by signals:
Lack of a mitotic signal (growth factor)
Recognition of damaged DNA
External signals cause membrane proteins to change shape and activate enzymes called caspases—hydrolyze proteins of membranes.