1. Mitosis, Meiosis, and Life Cycles
Chapter 12

2. LEARNING OBJECTIVE 1
Distinguish between a haploid cell and a diploid cell
Define homologous chromosomes

3. KEY TERMS DIPLOID (2N) HAPLOID (N)
The condition of having two sets of chromosomes per nucleus
In somatic cells of diploid organisms, chromosomes are present in pairs
HAPLOID (N)
The condition of having one set of chromosomes per nucleus

4. KEY TERMS HOMOLOGOUS CHROMOSOMES
Members of a chromosome pair that are similar in size, shape, and genetic constitution

5. LEARNING OBJECTIVE 2 Identify the phases in the cell cycle
Describe the main events of each phase

6. KEY TERMS
CELL CYCLE
Cyclic series of events in the life of a dividing eukaryotic cell

7. KEY TERMS
INTERPHASE
Stage of the cell cycle between successive mitotic divisions

8. Interphase During interphase Interphase is divided into
cell grows and prepares for next division
DNA replicates
Interphase is divided into
first gap phase (G1)
synthesis phase (S)
second gap phase (G2)

9. KEY TERMS
MITOSIS
Division of the cell nucleus resulting in two daughter nuclei, each with the same number of chromosomes as the parent nucleus

10. KEY TERMS
CYTOKINESIS
Stage of cell division in which the cytoplasm divides to form two daughter cells

11. Cell Cycle

12. (Mitosis and cytokinesis)
INTERPHASE G1
(First gap phase) S
(Synthesis phase) G2
(Second gap
phase)
Figure 12.1: The cell cycle.
The cell cycle includes interphase (G1, S, and G2) and M phase (mitosis and cytokinesis). The time required for each stage varies with cell type and species. Most cells spend about 90 percent of their cell cycle in interphase.
M PHASE
(Mitosis and cytokinesis)
Fig. 12-1, p. 242

13. LEARNING OBJECTIVE 3
Describe the events that occur in each stage of mitosis

14. Stages of Mitosis
Prophase
Metaphase
Anaphase
Telophase

15. Prophase Chromatin condenses into chromosomes
Nucleolus disappears
Nuclear envelope breaks down
Mitotic spindle begins to form
At the end of prophase, each duplicated chromosome is composed of two sister chromatids

16. A Duplicated Chromosome

17. Centromere region Microtubules Kinetochore Sister chromatids
Figure 12.3: A duplicated chromosome.
Each duplicated chromosome is composed of two identical sister chromatids that are tightly associated at their centromere regions. Associated with each centromere is a kinetochore, which serves as a microtubule attachment site.
Kinetochore
Sister
chromatids
Fig. 12-3, p. 245

18. KEY TERMS
SPINDLE
Structure consisting mainly of microtubules that provides the framework for chromosome movement during cell division

19. Metaphase
Duplicated chromosomes line up along midplane of cell

20. Anaphase
Sister chromatids separate and move to opposite poles of the cell
Each chromatid is now a separate chromosome

21. Telophase Nuclear envelope forms around each set of chromosomes
Nucleoli reappear
Chromosomes lengthen and become chromatin
Spindle disappears

22. KEY TERMS CELL PLATE Cytokinesis generally takes place in telophase
Structure that forms during cytokinesis in plants, separating two daughter cells produced by mitosis
Cytokinesis generally takes place in telophase

23. Cytokinesis

24. Cell plate forming Nucleus Vesicles gather on cell's midplane
Small vesicles
fuse, forming
larger vesicles
Eventually one
large vesicle
exists
New cell walls
(from vesicle
contents)
Figure 12.4: Cytokinesis.
Stages in the formation of plasma membranes and cell walls from the fusing vesicles of the cell plate. The electron micrograph shows cytokinesis in a maple (Acer) leaf cell. Note the cell plate, which consists of gathering vesicles.
Plasma
membrane
Cell
wall
Cell plate
forming
New plasma
membranes
(from vesicle
membranes)
Fig. 12-4, p. 247

25. Interphase and Mitosis

26. Condensing chromosome (consists of 2 sister chromatids)
(a) Interphase
(b) Prophase
(c) Metaphase
(d) Anaphase
(e) Telophase
Figure 12.2: Interphase and the stages of mitosis.
The micrographs are onion (Allium cepa) root tip cells prepared with stains. The diploid number for the cells shown in the diagrams is four.
Condensing chromosome
(consists of 2 sister chromatids)
Sister chromatids (now called chromosomes) move to opposite ends of cell
Cell plate
New
nuclei
Spindle microtubules
Fig. 12-2, p. 244

27. Condensing chromosome (consists of 2 sister chromatids) (c) Metaphase
(a) Interphase
(b) Prophase
Condensing chromosome
(consists of 2 sister chromatids)
(c) Metaphase
Spindle microtubules
(d) Anaphase
Sister chromatids (now called chromosomes) move to opposite ends of cell
(e) Telophase
New
nuclei
Cell plate
Figure 12.2: Interphase and the stages of mitosis.
The micrographs are onion (Allium cepa) root tip cells prepared with stains. The diploid number for the cells shown in the diagrams is four.
Stepped Art
Fig. 12-2, p. 244

28. LEARNING OBJECTIVE 4
Explain why meiosis is needed at some point in the life cycle of every sexually reproducing organism
Describe the events that occur during meiosis I and meiosis II

29. KEY TERMS
MEIOSIS
Process in which a 2n cell undergoes successive nuclear divisions, potentially producing four n nuclei
Leads to formation of spores in plants

30. Meiosis
Meiosis must occur at some time in the life of a sexually reproducing organism if gametes are to be haploid
Meiosis consists of two cell divisions
Meiosis I
Meiosis II

31. Meiosis I
Members of each homologous pair of chromosomes separate and are distributed into separate nuclei in two daughter cells
Chromosomes were duplicated prior to meiosis I, so each consists of two chromatids

32. Meiosis II
Chromatids separate into individual chromosomes and are distributed into different haploid daughter cells
Four haploid cells form

33. KEY TERMS
SYNAPSIS
Physical association of homologous chromosomes during prophase I of meiosis

34. Crossing Over Crossing over
Exchange of segments of homologous chromosomes
Synapsis and crossing over occur during prophase I of meiosis

35. Crossing Over

36. Sister chromatids Crossing over Kinetochores Fig. 12-6, p. 249
Figure 12.6: A pair of homologous chromosomes during late prophase I of meiosis.
Note the four chromatids that make up the paired homologous chromosomes. Crossing over is visible at two sites.
Fig. 12-6, p. 249

37. Meiosis

38. Meiosis

39. (a) Prophase I (b) Metaphase I (c) Anaphase I (d) Telophase I
Cell plate
Figure 12.5: The stages of meiosis.
The micrographs are Easter lily (Lilium longiflorum) flower cells prepared with stains and flattened on slides. The diploid number for the cells shown in the diagrams is four.
New nuclei
Each chromosome
consists of two chromatids
Homologous
chromosomes
Spindle
microtubules
Fig (a-d), p. 248

40. (e) Prophase II (f) Metaphase II (g) Anaphase II (h) Telophase II
Figure 12.5: The stages of meiosis.
The micrographs are Easter lily (Lilium longiflorum) flower cells prepared with stains and flattened on slides. The diploid number for the cells shown in the diagrams is four.
New nuclei
Chromosomes
(each with two
chromatids)
Spindle
microtubules
New cell walls
and plasma
membranes
Fig (e-h), p. 249

41. LEARNING OBJECTIVE 5
Compare and contrast mitosis and meiosis

42. Mitosis
Involves a single nuclear division in which the two daughter cells formed are genetically identical to each other and to the original cell
Synapsis of homologous chromosomes does not occur during mitosis

43. Meiosis
Involves two successive nuclear divisions and forms four haploid cells, each with a different combination of genes
Synapsis of homologous chromosomes occurs during prophase I of meiosis

44. Mitosis and Meiosis

45. Mitosis Meiosis Synapsis (pairing of homologous chromosomes)
Parent cell
(2n = 4)
Parent cell
(2n = 4)
Prophase I
Prophase
Nuclear
envelope
Nuclear
envelope
1st
meiotic
division
Mitosis
Prophase II
2nd
meiotic
division
Figure 12.7: Mitosis compared with meiosis.
The diploid number for each cell is four.
Two daughter cells (each 2n = 4)
identical to parent cell
Four daughter cells (each n = 2)
genetically different from parent cell
Mitosis
Meiosis
Fig. 12-7, p. 250

46. LEARNING OBJECTIVE 6
Define alternation of generations

47. KEY TERMS Alternation of generations SPOROPHYTE GAMETOPHYTE
Plants alternate haploid and diploid generations
SPOROPHYTE
2n, spore-producing stage in plant life cycle
GAMETOPHYTE
n, gamete-producing stage in plant life cycle

48. The Sporophyte
The 2n, spore-producing stage in the life cycle of a plant
A diploid sporophyte plant forms haploid spores by meiosis
A spore divides mitotically to form a haploid gametophyte plant

49. The Gametophyte
The n, gamete-producing stage in the life cycle of a plant
The gametophyte produces haploid gametes by mitosis
Two gametes fuse to form a diploid zygote, which divides mitotically to produce a diploid sporophyte

50. Plant Life Cycle

51. Gametophyte (n) (multicellular haploid organism) Mitosis Mitosis
Spores (n)
Gametes (n)
Meiosis
Fertilization
Zygote (2n)
Figure 12.8: Plant life cycle.
The life cycle of plants involves an alternation of generations between diploid and haploid stages.
Mitosis
Sporophyte (2n)
(multicellular
diploid organism)
Fig. 12-8, p. 251

52. Animation: The Cell Cycle
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53. Animation: Comparing Mitosis and Meiosis
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