1. Overview: The Key Roles of Cell Division
The ability of organisms to reproduce best distinguishes living things from nonliving matter
The continuity of life is based on the reproduction of cells, or cell division
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

2. Fig. 12-1
Figure 12.1 How do a cell’s chromosomes change during cell division?

3. 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
Cell division is an integral part of the cell cycle, the life of a cell from formation to its own division
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

4. Concept 12.1: Cell division results in genetically identical daughter cells
Most cell division results in daughter cells with identical genetic information, DNA
A special type of division produces nonidentical daughter cells (gametes, or sperm and egg cells)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

5. 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 (common in prokaryotic cells) or a number of DNA molecules (common in eukaryotic cells)
DNA molecules in a cell are packaged into chromosomes
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

6. Fig. 12-3
Figure 12.3 Eukaryotic chromosomes
20 µm

7. Every eukaryotic species has a characteristic number of chromosomes in their cell nuclei
Somatic cells (body cells) have 46 chromosomes; also called the diploid chromosome number (2n)
Gametes (sex cells) are haploid and have half the number of chromosomes as a diploid cell
Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

8. Distribution of Chromosomes During Eukaryotic Cell Division
In preparation for cell division, DNA is replicated and the chromosomes condense
When chromosomes are replicated, each duplicated chromosome consists of two sister chromatids attached by a centromere, the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

9. 0.5 µm Chromosomes DNA molecules Chromo- some arm Chromosome
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

10. 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

11. Phases of the Cell Cycle
The cell cycle consists of
Mitotic (M) phase (mitosis and cytokinesis)
Interphase (cell growth and copying of chromosomes in preparation for cell division)
Nucleolus is visible
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

12. Interphase (90% of the cell cycle) consists of G1 , S and G2 phases
G1 : cell grows while carrying out appropriate functions
S: cell carries out normal functions and duplicates chromosomes (copies DNA)
G2 : gap after the chromosomes have been duplicated and just before mitosis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

13. S (DNA synthesis) G1 Cytokinesis G2 Mitosis
Fig. 12-5
INTERPHASE S
(DNA synthesis) G1
Cytokinesis G2
Mitosis
Figure 12.5 The cell cycle
MITOTIC
(M) PHASE

14. Mitosis is conventionally divided into five phases:
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Cytokinesis is well underway by late telophase
For the Cell Biology Video Myosin and Cytokinesis, go to Animation and Video Files.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

15. Prophase
Chromatin become more tightly coiled into discrete chromosomes
Nuclear membrane begins to disintegrate
The mitotic spindle (an apparatus of microtubules extending from two centrosomes) begins to form in the cytoplasm
The spindle will include the centrosomes, the spindle microtubules, and the asters (radial array of short microtubules)

16. Chromosome, consisting of two sister chromatids
Fig. 12-6
G2 of Interphase
Prophase
Prometaphase
Metaphase
Anaphase
Telophase and Cytokinesis
Centrosomes
(with centriole
pairs)
Chromatin
(duplicated)
Early mitotic
spindle
Aster
Centromere
Fragments
of nuclear
envelope
Nonkinetochore
microtubules
Metaphase
plate
Cleavage
furrow
Nucleolus
forming
Figure 12.6 The mitotic division of an animal cell
Daughter
chromosomes
Nucleolus
Nuclear
envelope
Plasma
membrane
Chromosome, consisting
of two sister chromatids
Kinetochore
Kinetochore
microtubule
Spindle
Centrosome at
one spindle pole
Nuclear
envelope
forming

17. Prometaphase
Nuclear envelope begins to fragment, allowing microtubules to attach to chromosomes
Two chromatids of each chromosome are held together by protein kinetochores in the centromere region
Microtubules will attach chromatids to kinetochores

18. Chromosome, consisting of two sister chromatids
Fig. 12-6
G2 of Interphase
Prophase
Prometaphase
Metaphase
Anaphase
Telophase and Cytokinesis
Centrosomes
(with centriole
pairs)
Chromatin
(duplicated)
Early mitotic
spindle
Aster
Centromere
Fragments
of nuclear
envelope
Nonkinetochore
microtubules
Metaphase
plate
Cleavage
furrow
Nucleolus
forming
Figure 12.6 The mitotic division of an animal cell
Daughter
chromosomes
Nucleolus
Nuclear
envelope
Plasma
membrane
Chromosome, consisting
of two sister chromatids
Kinetochore
Kinetochore
microtubule
Spindle
Centrosome at
one spindle pole
Nuclear
envelope
forming

19. Metaphase (meta = middle)
Chromosomes line up in a single file on the equator or metaphase plate
Centrioles migrate to opposite poles in the cell, riding along the developing spindle

20. Chromosome, consisting of two sister chromatids
Fig. 12-6
G2 of Interphase
Prophase
Prometaphase
Metaphase
Anaphase
Telophase and Cytokinesis
Centrosomes
(with centriole
pairs)
Chromatin
(duplicated)
Early mitotic
spindle
Aster
Centromere
Fragments
of nuclear
envelope
Nonkinetochore
microtubules
Metaphase
plate
Cleavage
furrow
Nucleolus
forming
Figure 12.6 The mitotic division of an animal cell
Daughter
chromosomes
Nucleolus
Nuclear
envelope
Plasma
membrane
Chromosome, consisting
of two sister chromatids
Kinetochore
Kinetochore
microtubule
Spindle
Centrosome at
one spindle pole
Nuclear
envelope
forming

21. Anaphase
Spindle fibers pull apart sister chromosomes, separating centromeres of each chromosome
Cell elongates
By the end, opposite ends of the cell both contain complete and equal sets of chromosomes

22. Chromosome, consisting of two sister chromatids
Fig. 12-6
G2 of Interphase
Prophase
Prometaphase
Metaphase
Anaphase
Telophase and Cytokinesis
Centrosomes
(with centriole
pairs)
Chromatin
(duplicated)
Early mitotic
spindle
Aster
Centromere
Fragments
of nuclear
envelope
Nonkinetochore
microtubules
Metaphase
plate
Cleavage
furrow
Nucleolus
forming
Figure 12.6 The mitotic division of an animal cell
Daughter
chromosomes
Nucleolus
Nuclear
envelope
Plasma
membrane
Chromosome, consisting
of two sister chromatids
Kinetochore
Kinetochore
microtubule
Spindle
Centrosome at
one spindle pole
Nuclear
envelope
forming

23. Telophase
Nuclear envelope reforms around the sets of chromosomes located at opposite ends of the cell
Chromosomes unravel and become less condensed and return to their normal, pre-cell division condition as threadlike strands

24. Chromosome, consisting of two sister chromatids
Fig. 12-6
G2 of Interphase
Prophase
Prometaphase
Metaphase
Anaphase
Telophase and Cytokinesis
Centrosomes
(with centriole
pairs)
Chromatin
(duplicated)
Early mitotic
spindle
Aster
Centromere
Fragments
of nuclear
envelope
Nonkinetochore
microtubules
Metaphase
plate
Cleavage
furrow
Nucleolus
forming
Figure 12.6 The mitotic division of an animal cell
Daughter
chromosomes
Nucleolus
Nuclear
envelope
Plasma
membrane
Chromosome, consisting
of two sister chromatids
Kinetochore
Kinetochore
microtubule
Spindle
Centrosome at
one spindle pole
Nuclear
envelope
forming

25. Cytokinesis begins and the cytoplasm of the cell is divided
Animal cells: a cleavage furrow forms that eventually divides the cytoplasm
Plant cells: a cell plate forms that divides the cytoplasm

27. Binary Fission
Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission
In binary fission, the chromosome replicates (beginning at the origin of replication), and the two daughter chromosomes actively move apart
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

28. Cell wall Origin of replication Plasma membrane E. coli cell Bacterial
Fig
Cell wall
Origin of
replication
Plasma
membrane
E. coli cell
Bacterial
chromosome
Two copies
of origin
Figure Bacterial cell division by binary fission

29. Cell wall Origin of replication Plasma membrane E. coli cell Bacterial
Fig
Cell wall
Origin of
replication
Plasma
membrane
E. coli cell
Bacterial
chromosome
Two copies
of origin
Origin
Origin
Figure Bacterial cell division by binary fission

30. Cell wall Origin of replication Plasma membrane E. coli cell Bacterial
Fig
Cell wall
Origin of
replication
Plasma
membrane
E. coli cell
Bacterial
chromosome
Two copies
of origin
Origin
Origin
Figure Bacterial cell division by binary fission

31. Cell wall Origin of replication Plasma membrane E. coli cell Bacterial
Fig
Cell wall
Origin of
replication
Plasma
membrane
E. coli cell
Bacterial
chromosome
Two copies
of origin
Origin
Origin
Figure Bacterial cell division by binary fission

32. Steps of the cell cycle are controlled by a cell cycle control system
Concept 12.3: The eukaryotic cell cycle is regulated by a molecular control system
Steps of the cell cycle are controlled by a cell cycle control system
Moves the cell through its stages by a series of checkpoints, during which signals tell the cell to continue dividing or stop dividing
Checkpoints include G1 phase checkpoint, G2 phase checkpoint and M phase checkpoint
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

33. G1 checkpoint Control system S G1 G2 M M checkpoint G2 checkpoint
Fig
G1 checkpoint
Control
system S G1 G2 M
Figure Mechanical analogy for the cell cycle control system
M checkpoint
G2 checkpoint

34. For many cells, the G1 checkpoint seems to be the most important one
If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide
If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

35. G0 G1 G1 G1 checkpoint (b) Cell does not receive a go-ahead signal
Fig G0
G1 checkpoint
Figure The G1 checkpoint G1 G1
Cell receives a go-ahead
signal
(b) Cell does not receive a
go-ahead signal

36. The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases
Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks)
Specific kinases give the go-ahead signals at the G1 and G2 checkpoints
MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

37. Stopping cell division
During anaphase, MPF switches itself off by starting a process that leads to the destruction of cyclin molecules
Without cyclin, Cdk molecules become inactive, bringing mitosis to a close

38. Normal cell division has two key characteristics:
Density-dependent inhibition, in which crowded cells stop dividing
Anchorage dependence, in which they must be attached to a substratum, like the extracellular matrix of a tissue, to divide
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

39. Density-dependent inhibition
Fig
Anchorage dependence
Density-dependent inhibition
Density-dependent inhibition
Figure Density-dependent inhibition and anchorage dependence of cell division
25 µm
25 µm
(a) Normal mammalian cells
(b) Cancer cells

40. A tumor is a mass of abnormal cells within otherwise normal tissue
Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence
The process that changes a normal cell to a cancer cell is transformation
A tumor is a mass of abnormal cells within otherwise normal tissue
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

41. If the abnormal cells remain at the original site, the lump is benign
A malignant tumor becomes invasive enough to impair the functions of one or more organs
An individual with a malignant tumor has cancer
Malignant tumors may have cells that separate from the original tumor and enter blood or lymph vessels and spread (metastasis)

42. Lymph vessel Tumor Blood vessel Cancer cell Glandular tissue
Fig
Lymph
vessel
Tumor
Blood
vessel
Cancer
cell
Glandular
tissue
Metastatic
tumor 1
A tumor grows
from a single
cancer cell. 2
Cancer cells
invade neigh-
boring tissue. 3
Cancer cells spread
to other parts of
the body. 4
Cancer cells may
survive and
establish a new
tumor in another
part of the body.
Figure The growth and metastasis of a malignant breast tumor