1. 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. Multicellular organisms depend on cell division for:
Fig. 12-2
Multicellular organisms depend on cell division for:
Development from a fertilized cell
Growth
Repair
100 µm
200 µm
20 µm
Figure 12.2 The functions of cell division
(a) Reproduction
(b) Growth and
development
(c) Tissue renewal

3. 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)
Somatic cells (nonreproductive cells) have two sets of chromosomes
Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

4. Cellular Organization of the Genetic Material
All the DNA in a cell constitutes the cell’s genome
DNA molecules in a cell are packaged into chromosomes
Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division
Sister chromatids have identical copies of a chromsome held together by a centromere
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

5. The cell cycle consists of
Concept 12.2: The mitotic phase alternates with interphase in the cell cycle
In 1882, the German anatomist Walther Flemming developed dyes to observe chromosomes during mitosis and cytokinesis
The cell cycle consists of
Mitotic (M) phase (mitosis and cytokinesis)
Interphase (cell growth and copying of chromosomes in preparation for cell division)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

6. Interphase (about 90% of the cell cycle) can be divided into subphases:
G1 phase (“first gap”)
S phase (“synthesis”)
G2 phase (“second gap”)
The cell grows during all three phases, but chromosomes are duplicated only during the S phase
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

7. 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

8. 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.
BioFlix: Mitosis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

9. Fig. 12-6a
Figure 12.6 The mitotic division of an animal cell

10. Mitosis the process During prophase chromosomes form
Chromatin shortens and condenses (wraps around nucleosome cores)
Assembly of spindle microtubules begins in the centrosome, the microtubule organizing center.
The centrosome replicates, forming two centrosomes that migrate to opposite ends of the cell, as spindle microtubules grow out from them
An aster (a radial array of short microtubules) extends from each centrosome
For the Cell Biology Video Spindle Formation During Mitosis, go to Animation and Video Files.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

11. Fig. 12-6c
Figure 12.6 The mitotic division of an animal cell

12. During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes
At metaphase, the chromosomes are all lined up at the metaphase plate, the midway point between the spindle’s two poles
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

13. Fig. 12-7
Aster
Centrosome
Sister
chromatids
Microtubules
Chromosomes
Metaphase
plate
Kineto-
chores
Centrosome
1 µm
Figure 12.7 The mitotic spindle at metaphase
Overlapping
nonkinetochore
microtubules
Kinetochore
microtubules
0.5 µm

14. The microtubules shorten by depolymerizing at their kinetochore ends
In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell
The microtubules shorten by depolymerizing at their kinetochore ends
For the Cell Biology Video Microtubules in Anaphase, go to Animation and Video Files.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

15. Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell
In telophase, genetically identical daughter nuclei form at opposite ends of the cell
For the Cell Biology Video Microtubules in Cell Division, go to Animation and Video Files.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

16. Cytokinesis: A Closer Look
In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow
In plant cells, a cell plate forms during cytokinesis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

17. 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

18. The Evolution of Mitosis
Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission
Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

19. The frequency of cell division varies with the type of cell
Concept 12.3: The eukaryotic cell cycle is regulated by a molecular control system
The frequency of cell division varies with the type of cell
These cell cycle differences result from regulation at the molecular level
The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

20. The Cell Cycle Control System
The cell cycle control system is regulated by both internal and external controls
Three major checkpoints are found in the G1, G2 and M phases.
Go-ahead signal  completes the cell cycle and divides
No go-ahead signal  switches to a nondividing state, the G0 phase
Most human cells are in this phase.
Liver cells can be “called back” to the cell cycle by external cues (growth factors), but highly specialized nerve and muscle cells never divide.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

21. 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

22. The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases
MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase
Protein kinases that catalyze the transfer of a phosphate group from ATP to a target protein
Composed of
Cyclin-dependent kinases (Cdks)-activate the change from interphase to mitosis
Cyclins-production constant, but concentration varies
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

23. (a) Fluctuation of MPF activity and cyclin concentration during
Fig a M G1 S G2 M G1 S G2 M G1
MPF activity
Cyclin
concentration
Figure Molecular control of the cell cycle at the G2 checkpoint
Time
(a) Fluctuation of MPF activity and cyclin concentration during
the cell cycle

24. (b) Molecular mechanisms that help regulate the cell cycle
Fig b G1 S
Cdk
Cyclin accumulation M
Degraded
cyclin G2 G2
checkpoint
Cdk
Figure Molecular control of the cell cycle at the G2 checkpoint
Cyclin is
degraded
Cyclin
MPF
(b) Molecular mechanisms that help regulate the cell cycle

25. Internal and External factors of Contol
Growth factors, proteins released by certain cells that stimulate other cells to divide.
For example, platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture
An example of an internal signal is that kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

26. Scalpels Petri plate Without PDGF cells fail to divide With PDGF
Fig
Scalpels
Petri
plate
Without PDGF
cells fail to divide
Figure The effect of a growth factor on cell division
With PDGF
cells prolifer-
ate
Cultured fibroblasts
10 µm

27. An example of external signals is density-dependent inhibition, in which crowded cells stop dividing
Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum in order to divide
Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

28. 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

29. Loss of Cell Cycle Controls in Cancer Cells
Cancer cells do not respond normally to the body’s control mechanisms
Cancer cells may not need growth factors to grow and divide:
They may make their own growth factor
They may convey a growth factor’s signal without the presence of the growth factor
They may have an abnormal cell cycle control system
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

30. Cancer
A normal cell is converted to a cancerous cell by a process called transformation
most are usually destroyed by the immune system.
Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue (begin a primary tumor)
If abnormal cells remain at the original site, the lump is called a benign tumor (like a mole) cells eventually stop dividing.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

31. Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors
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.

32. Cancer
Cancer cells contain several mutated genes
These almost always include
*mutations in genes that are involved in mitosis
Oncogenes: their mutated or overexpressed products stimulate mitosis even though normal growth factors are absent

33. Cancer
Examples
1. SIS- the gene for platelet derived growth factor (PDGF)
2. erbB-the gene encoding the receptor for epidermal growth factor (EGF)
3. Tumor supressor genes-normally inhibit mitosis
Ex: p53 gene causes apoptosis (cell suicide) in cells with DNA damage.