1. Overview: The Key Roles of Cell Division
The ability of organisms to reproduce best distinguishes living things from nonliving matter (reproduction is an emergent property of life)
The continuity of life is based on the reproduction of cells, or cell division
Cell Theory: all cells come from pre-existing cells

2. Division in unicellular vs multicellular organisms
In unicellular organisms, standard cell division of one cell reproduces the entire organism
Multicellular organisms depend on standard cell division only for:
Development from a fertilized cell
Growth
Repair
Not for reproducing the entire organism

3. Cell division is carefully controlled in all normal cells
In eukaryotes division is a part of the cell cycle-a programmed series of events that governs the life of a cell

4. Concept 12.1: Cell division results in genetically identical daughter cells
Standard cell division results in daughter cells with “identical” genetic information and DNA
-clones
-vegetative reproduction
-asexual reproduction
A special type of division produces nonidentical daughter cells for reproduction of the organism (gametes, or sperm and egg cells)
- sexual reproduction

5. Genetic material must be duplicated and separated to produce clones
All the DNA in a cell constitutes the cell’s genome (nuclear genome, organelle 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 eukaryotic cell are packaged into organelles called chromosomes

6. The number of chromosomes is characteristic for each species
Gametes (reproductive cells: sperm and eggs) contain n chromosomes
Somatic cells (nonreproductive cells) have two sets of chromosomes or 2n (2n = 46 for humans)
Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein

7. Genetic material must be duplicated to produce clones
In preparation for cell division, DNA is copied or “replicated”.
A somatic cell temporarily has 2 x 2n chromosomes

8. Genetic material must be separated to produce clones
DNA molecules are many times longer than their cells.
The chromatin that contains this DNA must be tightly packed for travel to daughter cells.
Chromatin “packing” is called condensation

9. Packing Ratio
10, ,000 X

10. Structural features of chromosomes
Packed and duplicated chromosomes each have two sister chromatids
The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached
The centromere defines two “arms” (p and q) for each chromatid
The end of the chromatid is the telomere

11. The modern cell cycle consists of
Concept 12.2: The active division phase alternates with non-division in the cell cycle
In the 1800’s the active division phase was named “mitosis” and the non-dividing phase named “interphase”
The two terms have been adapted for modern usage to describe the cell cycle
The modern cell cycle consists of
Mitotic (M) phase (mitosis and cytokinesis)- nuclear and cytoplasmic division
Interphase (cell growth and copying of chromosomes in preparation for cell division)

12. Interphase (about 90% of the time of the cell cycle) can be subdivided further:
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

13. S (DNA synthesis) G1 Cytokinesis G2 Mitosis
Eukaryotic Cell Cycle
INTERPHASE S
(DNA synthesis) G1
Cytokinesis G2
Mitosis
Figure 12.5 The cell cycle
MITOTIC
(M) PHASE

14. Mitosis is conventionally subdivided into four parts:
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis overlaps telophase
For the Cell Biology Video Myosin and Cytokinesis, go to Animation and Video Files.

15. Chromosome, consisting of two sister chromatids
Prophase-the cell prepares to divide: chromosome condensation begins,
Spindle starts to assemble
G2 of Interphase
Prophase
Prometaphase
Centrosomes
(with centriole
pairs)
Chromatin
(duplicated)
Early mitotic
spindle
Aster
Centromere
Fragments
of nuclear
envelope
Nonkinetochore
microtubules
Figure 12.6 The mitotic division of an animal cell
Nucleolus
Nuclear
envelope
Plasma
membrane
Chromosome, consisting
of two sister chromatids
Kinetochore
Kinetochore
microtubule
Prometaphase-preparation continues: final condensation, spindle finishes, nuclear membrane fragments

16. Telophase and Cytokinesis
Metaphase-chromosomes line up at the middle of the cell
Anaphase-sister chromatids separate to form new “daughter” chromosomes,
pulled to opposite spindle poles
Metaphase
Anaphase
Telophase and Cytokinesis
Metaphase
plate
Cleavage
furrow
Nucleolus
forming
Figure 12.6 The mitotic division of an animal cell
Daughter
chromosomes
Nuclear
envelope
forming
Spindle
Centrosome at
one spindle pole
Telophase- daughter cells reorganize, reverse of prophase

17. G2 of Interphase Prophase Prometaphase
Fig. 12-6a
Figure 12.6 The mitotic division of an animal cell
G2 of Interphase
Prophase
Prometaphase

18. Telophase and Cytokinesis
Fig. 12-6c
Figure 12.6 The mitotic division of an animal cell
Metaphase
Anaphase
Telophase and Cytokinesis

19. The Mitotic Spindle
The mitotic spindle is an apparatus of microtubules that controls chromosome movement during mitosis
During prophase, 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
For the Cell Biology Video Spindle Formation During Mitosis, go to Animation and Video Files.

20. During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes
Different microtubules overlap with microtubules coming from the other centrosome (spindle pole).

21. Cytokinesis
In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow
In plant cells, a cell plate forms during cytokinesis

22. Figure 12.9 Cytokinesis in animal and plant cells
Vesicles
forming
cell plate
Wall of
parent cell
1 µm
100 µm
Cleavage furrow
Cell plate
New cell wall
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)

23. Binary Fission
Prokaryotes (bacteria and archaea) reproduce by a simpler 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

24. 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

25. 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

26. Evidence for Cytoplasmic Signals
The cell cycle appears to be driven by specific chemical signals present in the cytoplasm
Some evidence for this hypothesis comes from experiments in which cultured mammalian cells at different phases of the cell cycle were fused to form a single cell with two nuclei

27. EXPERIMENT RESULTS Experiment 1 Experiment 2 S G1 M G1 S S M M
When a cell in the
S phase was fused
with a cell in G1, the G1
nucleus immediately
entered the S
phase—DNA was
synthesized.
When a cell in the
M phase was fused with
a cell in G1, the G1
nucleus immediately
began mitosis—a
spindle formed and
chromatin condensed,
even though the
chromosome had not
been duplicated.
Figure Do molecular signals in the cytoplasm regulate the cell cycle?

28. The sequential events of the cell cycle are directed by a cytoplasmic factors
The cycle has specific checkpoints where the it stops until a go-ahead signal is received
Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks)
The cell makes Cdks continuously but cyclins only when needed for division
The activity of cyclins and Cdks fluctuates during the cell cycle-they serve as go-ahead signal at checkpoints

29. 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

30. G1 S Cyclin accumulation Cdk M Degraded G2 cyclin G2 Cdk checkpoint
Figure Molecular control of the cell cycle at the G2 checkpoint
Cyclin is
degraded
Cyclin
MPF

31. An example of an internal signal is that kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase
Some external signals are growth factors, proteins released by certain cells that stimulate other cells to divide

32. 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

33. A normal cell is converted to a cancerous cell by a process called transformation
Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue
If abnormal cells remain at the original site, the lump is called a benign tumor
Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors

34. Multistep Model for Cancer
Tumor suppressors hold back cancer, oncogenes are genes that are capable
of initiating cancer
Mutant oncogenes or tumor suppressor genes can be inherited

35. Environmental factors can trigger cancer
Carcinogens are chemicals that lead to cancer
Proto-oncogenes mutate to become fully active oncogenes
Above shows several ways that proto-oncogenes can mutate

36. Viruses can trigger cancer
Virus genes can remain in cells and act as oncogenes (v-oncogenes)
Virus genes can remain in cell and activate cellular oncogenes (c-oncogenes)
Cancer viruses, tumor viruses or oncoviruses

37. Example
Mutations can lead to altered function at any step

38. Environmental factors, genetic factors, viruses can trigger cancer
They act through a multi-step model to de-regulate cell division
They work through genes called oncogenes