1. Chapter 12
The Cell Cycle

2. Multicellular organisms depend on cell division for:
In unicellular organisms, division of one cell reproduces the entire organism
Multicellular organisms depend on cell division for:
Development from a fertilized cell
Growth
Repair
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3. Cell division results in genetically identical daughter cells
Most cell division (mitosis) results in daughter cells with identical genetic information, DNA
A special type of division (meiosis) produces nonidentical daughter cells (gametes, or sperm and egg cells)
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4. Cellular Organization of the Genetic Material
All the DNA in a cell constitutes the cell’s genome
A genome can consist of
a single DNA molecule (prokaryotic cells)
a number of DNA molecules (eukaryotic cells)
DNA molecules in a cell are packaged into chromosomes
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5. Somatic cells (nonreproductive cells) have two sets of chromosomes
Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus
Somatic cells (nonreproductive cells) have two sets of chromosomes
Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells
Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein (histones).
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6. Distribution of Chromosomes During Eukaryotic Cell Division
Each duplicated chromosome has two sister chromatids, which separate during cell division
The centromere is where the two chromatids are most closely attached
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

7. Fig. 12-4 0.5 µm Chromosomes DNA molecules Chromo- some arm Chromosome
duplication
(including DNA
synthesis)
Centromere
Sister
chromatids
Figure 12.4 Chromosome duplication and distribution during cell division
Separation of
sister chromatids
Centromere
Sister chromatids

8. Eukaryotic cell division consists of:
Mitosis, the division of the nucleus
Cytokinesis, the division of the cytoplasm
Gametes are produced by a variation of cell division called meiosis
Meiosis yields nonidentical daughter cells that have only one set of chromosomes, half as many as the parent cell
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9. Phases of the Cell Cycle
The cell cycle consists of
Interphase (cell growth and copying of chromosomes in preparation for cell division)
G1 phase- cell grows
S phase- chromosomes duplicate
G2 phase- cell grows/prepare for division
Mitotic (M) phase
mitosis
cytokinesis
Video: MITOSIS
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10. G1 S Cytokinesis Mitosis G2 MITOTIC (M) PHASE Prophase Telophase and
Fig. 12-UN1
INTERPHASE G1 S
Cytokinesis
Mitosis G2
MITOTIC (M) PHASE
Prophase
Telophase and
Cytokinesis
Prometaphase
Anaphase
Metaphase

11. Mitosis is conventionally divided into five phases:
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Cytokinesis: (division of cytoplasm)
For the Cell Biology Video Myosin and Cytokinesis, go to Animation and Video Files.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

12. PROPHASE Chromatids condense and become visible Nucleoli dissapears
Centrosomes (pair of centrioles) move to the poles
Spindle begins to form

13. PROMETAPHASE Nuclear envelope breaks
Microtubules form from centrosome and move toward the middle of cell
Sister chromatids have a kinetochore (specialized protein located on centromere)
Spindle attach to kinetochore

14. METAPHASE Longest stage of mitosis Spindle is completely formed
Spindle attaches to kinetochore of chromatids
Spindle moves chromatids to middle of cell (metaphase plate)

15. ANAPHASE Shortest stage of Mitosis
Spindle pulls sister chromatids apart (become chromosomes)
Chromosomes are pulled toward the poles
Spindle shortens and cell elongates
Chromosomes reach poles

16. TELOPHASE Nuclear envelopes reappear Spindle dissappears
2 identical nuclei and 1 cytoplasm
Video: MITOSIS

17. Cytokinesis: A Closer Look
Division of cytoplasm
In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow
In plant cells, a cell plate forms during cytokinesis
Vesicles
forming
cell plate
Wall of
parent cell
New cell wall
Cleavage furrow
Cell plate
Figure 12.9 Cytokinesis in animal and plant cells
Contractile ring of
microfilaments
Daughter cells
Daughter cells
(a) Cleavage of an animal cell (SEM)
(b) Cell plate formation in a plant cell (TEM)

18. The Cell Cycle Control System
The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock
The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received
Video: CELL CYCLE
Video: CONTROL SYSTEM CELL CYCLE
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19. G1 checkpoint Control system S G1 G2 M M checkpoint G2 checkpoint
Figure Mechanical analogy for the cell cycle control system
M checkpoint
G2 checkpoint

20. Cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide
Cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase G0
G1 checkpoint G1 G1
Cell receives a go-ahead
signal
Cell does not receive a
go-ahead signal
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings