9 The Cell Cycle.

9 The Cell Cycle

Overview: The Key Roles of Cell Division
The ability of organisms to produce more of their own kind best distinguishes living things from nonliving matter The continuity of life is based on the reproduction of cells, or cell division © 2014 Pearson Education, Inc. 2

Figure 9.1 Figure 9.1 How do a cell’s chromosomes change during cell division? 3

In unicellular organisms, division of one cell reproduces the entire organism
Cell division enables multicellular eukaryotes to develop from a single cell and, once fully grown, to renew, repair, or replace cells as needed Cell division is an integral part of the cell cycle, the life of a cell from formation to its own division © 2014 Pearson Education, Inc. 4

(a) Reproduction (b) Growth and development (c) Tissue renewal 100 m
Figure 9.2 100 m 200 m (a) Reproduction (b) Growth and development Figure 9.2 The functions of cell division 20 m (c) Tissue renewal 5

(a) Reproduction 100 m Figure 9.2a
Figure 9.2a The functions of cell division (part 1: reproduction) (a) Reproduction 6

(b) Growth and development
Figure 9.2b 200 m Figure 9.2b The functions of cell division (part 2: growth and development) (b) Growth and development 7

(c) Tissue renewal 20 m Figure 9.2c
Figure 9.2c The functions of cell division (part 3: tissue renewal) (c) Tissue renewal 8

Concept 9.1: Most cell division results in genetically identical daughter cells
Most cell division results in the distribution of identical genetic material—DNA—to two daughter cells DNA is passed from one generation of cells to the next with remarkable fidelity © 2014 Pearson Education, Inc. 9

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 © 2014 Pearson Education, Inc. 10

Eukaryotic chromosomes
Figure 9.3 Figure 9.3 Eukaryotic chromosomes 20 m Eukaryotic chromosomes 11

Somatic cells (nonreproductive cells) have two sets of chromosomes
Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein 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 one set of chromosomes © 2014 Pearson Education, Inc. 12

Distribution of Chromosomes During Eukaryotic Cell Division
In preparation for cell division, DNA is replicated and the chromosomes condense Each duplicated chromosome has two sister chromatids, joined identical copies of the original chromosome The centromere is where the two chromatids are most closely attached © 2014 Pearson Education, Inc. 13

Sister chromatids Centromere 0.5 m Figure 9.4
Figure 9.4 A highly condensed, duplicated human chromosome (SEM) 0.5 m 14

Once separate, the chromatids are called chromosomes
During cell division, the two sister chromatids of each duplicated chromosome separate and move into two nuclei Once separate, the chromatids are called chromosomes © 2014 Pearson Education, Inc. 15

Chromosomal DNA molecules
Figure 9.5-1 Chromosomal DNA molecules Chromosomes 1 Centromere Chromosome arm Figure Chromosome duplication and distribution during cell division (step 1) 16

Figure 9.5-2 Chromosomal DNA molecules Chromosomes 1 Centromere Chromosome arm Chromosome duplication 2 Sister chromatids Figure Chromosome duplication and distribution during cell division (step 2) 17

Figure 9.5-3 Chromosomal DNA molecules Chromosomes 1 Centromere Chromosome arm Chromosome duplication 2 Sister chromatids Figure Chromosome duplication and distribution during cell division (step 3) Separation of sister chromatids 3 18

Eukaryotic cell division consists of
Mitosis, the division of the genetic material in 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 © 2014 Pearson Education, Inc. 19

Concept 9.2: 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 © 2014 Pearson Education, Inc. 20

Phases of the Cell Cycle
The cell cycle consists of Mitotic (M) phase, including mitosis and cytokinesis Interphase, including cell growth and copying of chromosomes in preparation for cell division © 2014 Pearson Education, Inc. 21

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 © 2014 Pearson Education, Inc. 22

S G1 (DNA synthesis) G2 Cytokinesis Mitosis Figure 9.6
Figure 9.6 The cell cycle 23

Mitosis is conventionally divided into five phases
Prophase Prometaphase Metaphase Anaphase Telophase Cytokinesis overlaps the latter stages of mitosis © 2014 Pearson Education, Inc. 24

Video: Microtubules Mitosis
Video: Animal Mitosis Video: Microtubules Mitosis Animation: Mitosis Video: Microtubules Anaphase Video: Nuclear Envelope © 2014 Pearson Education, Inc. 25

Two sister chromatids of
Figure 9.7a 10 m G2 of Interphase Prophase Prometaphase Chromosomes (duplicated, uncondensed) Early mitotic spindle Centromere Fragments of nuclear envelope Centrosomes (with centriole pairs) Nonkinetochore microtubules Aster Figure 9.7a Exploring mitosis in an animal cell (part 1: G2 of interphase through prometaphase) Plasma membrane Nucleolus Kinetochore Kinetochore microtubule Nuclear envelope Two sister chromatids of one chromosome 26

Telophase and Cytokinesis
Figure 9.7b 10 m Metaphase Anaphase Telophase and Cytokinesis Metaphase plate Cleavage furrow Nucleolus forming Figure 9.7b Exploring mitosis in an animal cell (part 2: metaphase through cytokinesis) Nuclear envelope forming Spindle Centrosome at one spindle pole Daughter chromosomes 27

Two sister chromatids of
Figure 9.7c G2 of Interphase Prophase Centrosomes (with centriole pairs) Chromosomes (duplicated, uncondensed) Early mitotic spindle Centromere Aster Figure 9.7c Exploring mitosis in an animal cell (part 3: G2 of interphase and prophase) Plasma membrane Nucleolus Nuclear envelope Two sister chromatids of one chromosome 28

Prometaphase Metaphase Fragments of nuclear envelope Nonkinetochore
Figure 9.7d Prometaphase Metaphase Fragments of nuclear envelope Nonkinetochore microtubules Metaphase plate Figure 9.7d Exploring mitosis in an animal cell (part 4: prometaphase and metaphase) Kinetochore Kinetochore microtubule Spindle Centrosome at one spindle pole 29

Figure 9.7e Anaphase Telophase and Cytokinesis Cleavage furrow Nucleolus forming Figure 9.7e Exploring mitosis in an animal cell (part 5: anaphase, telophase and cytokinesis) Nuclear envelope forming Daughter chromosomes 30

G2 of Interphase 10 m Figure 9.7f
Figure 9.7f Exploring mitosis in an animal cell (part 6: G2 of interphase micrograph) G2 of Interphase 31

Figure 9.7g 10 m Figure 9.7g Exploring mitosis in an animal cell (part 7: prophase micrograph) Prophase 32

Prometaphase 10 m Figure 9.7h
Figure 9.7h Exploring mitosis in an animal cell (part 8: prometaphase micrograph) Prometaphase 33

Figure 9.7i 10 m Figure 9.7i Exploring mitosis in an animal cell (part 9: metaphase micrograph) Metaphase 34

Figure 9.7j 10 m Figure 9.7j Exploring mitosis in an animal cell (part 10: anaphase micrograph) Anaphase 35

Figure 9.7k 10 m Figure 9.7k Exploring mitosis in an animal cell (part 11: telophase and cytokinesis micrograph) Telophase and Cytokinesis 36

The Mitotic Spindle: A Closer Look
The mitotic spindle is a structure made of microtubules and associated proteins It controls chromosome movement during mitosis In animal cells, assembly of spindle microtubules begins in the centrosome, the microtubule organizing center © 2014 Pearson Education, Inc. 37

The centrosome replicates during interphase, forming two centrosomes that migrate to opposite ends of the cell during prophase and prometaphase An aster (radial array of short microtubules) extends from each centrosome The spindle includes the centrosomes, the spindle microtubules, and the asters © 2014 Pearson Education, Inc. 38

During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes Kinetochores are protein complexes that assemble on sections of DNA at centromeres At metaphase, the centromeres of all the chromosomes are at the metaphase plate, an imaginary structure at the midway point between the spindle’s two poles Video: Mitosis Spindle © 2014 Pearson Education, Inc. 39

Aster Centrosome Sister chromatids Metaphase plate (imaginary) Kineto-
Figure 9.8 Aster Centrosome Sister chromatids Metaphase plate (imaginary) Kineto- chores Microtubules Chromosomes Overlapping nonkinetochore microtubules Figure 9.8 The mitotic spindle at metaphase Kinetochore microtubules 1 m 0.5 m Centrosome 40

Microtubules Chromosomes 1 m Centrosome Figure 9.8a
Figure 9.8a The mitotic spindle at metaphase (part 1: TEM) 1 m Centrosome 41

Kinetochores Kinetochore microtubules 0.5 m Figure 9.8b
Figure 9.8b The mitotic spindle at metaphase (part 2: kinetochore TEM) 0.5 m 42

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 Chromosomes are also “reeled in” by motor proteins at spindle poles, and microtubules depolymerize after they pass by the motor proteins © 2014 Pearson Education, Inc. 43

Experiment Results Conclusion Kinetochore Spindle pole Chromosome
Figure 9.9 Experiment Results Kinetochore Spindle pole Conclusion Chromosome movement Mark Kinetochore Figure 9.9 Inquiry: At which end do kinetochore microtubules shorten during anaphase? Motor protein Tubulin subunits Microtubule Chromosome 44

Experiment Kinetochore Spindle pole Mark Figure 9.9a
Figure 9.9a Inquiry: At which end do kinetochore microtubules shorten during anaphase? (part 1: experiment) 45

Results Conclusion Chromosome movement Kinetochore Motor Tubulin
Figure 9.9b Results Conclusion Chromosome movement Figure 9.9b Inquiry: At which end do kinetochore microtubules shorten during anaphase? (part 2: results and conclusion) Kinetochore Motor protein Tubulin subunits Microtubule Chromosome 46

Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell At the end of anaphase, duplicate groups of chromosomes have arrived at opposite ends of the elongated parent cell Cytokinesis begins during anaphase or telophase and the spindle eventually disassembles © 2014 Pearson Education, Inc. 47

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 Animation: Cytokinesis Video: Cytokinesis and Myosin © 2014 Pearson Education, Inc. 48

(a) Cleavage of an animal cell (SEM)
Figure 9.10 (a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (TEM) 100 m Cleavage furrow Vesicles forming cell plate Wall of parent cell 1 m Cell plate New cell wall Figure 9.10 Cytokinesis in animal and plant cells Contractile ring of microfilaments Daughter cells Daughter cells 49

(a) Cleavage of an animal cell (SEM)
Figure 9.10a (a) Cleavage of an animal cell (SEM) 100 m Cleavage furrow Figure 9.10a Cytokinesis in animal and plant cells (part 1: cleavage of animal cell) Contractile ring of microfilaments Daughter cells 50

Cleavage furrow 100 m Figure 9.10aa
Figure 9.10aa Cytokinesis in animal and plant cells (part 1a: cleavage of animal cell, SEM) Cleavage furrow 100 m 51

(b) Cell plate formation in a plant cell (TEM)
Figure 9.10b (b) Cell plate formation in a plant cell (TEM) Vesicles forming cell plate Wall of parent cell 1 m Cell plate New cell wall Figure 9.10b Cytokinesis in animal and plant cells (part 2: cell plate formation in plant cell) Daughter cells 52

Vesicles Wall of parent 1 m forming cell cell plate Figure 9.10ba
Figure 9.10ba Cytokinesis in animal and plant cells (part 2a: cell plate formation in plant cell, TEM) Vesicles forming cell plate Wall of parent cell 1 m 53

Chromosomes condensing Nucleus Nucleolus Chromosomes 10 m Prophase
Figure 9.11 Chromosomes condensing Nucleus Nucleolus Chromosomes 10 m 1 Prophase 2 Prometaphase Cell plate Figure 9.11 Mitosis in a plant cell 3 Metaphase 4 Anaphase 5 Telophase 54

Nucleus Chromosomes condensing Nucleolus 10 m Prophase 1 Figure 9.11a
Figure 9.11a Mitosis in a plant cell (part 1: prophase) 10 m 1 Prophase 55

Chromosomes 10 m Prometaphase 2 Figure 9.11b
Figure 9.11b Mitosis in a plant cell (part 2: prometaphase) 10 m 2 Prometaphase 56

Figure 9.11c Figure 9.11c Mitosis in a plant cell (part 3: metaphase) 10 m 3 Metaphase 57

Figure 9.11d Figure 9.11d Mitosis in a plant cell (part 4: anaphase) 10 m 4 Anaphase 58

Cell plate 10 m Telophase 5 Figure 9.11e
Figure 9.11e Mitosis in a plant cell (part 5: telophase) 10 m 5 Telophase 59

Binary Fission in Bacteria
Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission In E. coli, the single chromosome replicates, beginning at the origin of replication The two daughter chromosomes actively move apart while the cell elongates The plasma membrane pinches inward, dividing the cell into two © 2014 Pearson Education, Inc. 60

Origin of Cell wall replication Plasma membrane E. coli cell Bacterial
Figure Origin of replication Cell wall Plasma membrane E. coli cell Bacterial chromosome 1 Chromosome replication begins. Two copies of origin Figure Bacterial cell division by binary fission (step 1) 61

Origin of replication Cell wall Plasma membrane E. coli cell Bacterial
Figure Origin of replication Cell wall Plasma membrane E. coli cell Bacterial chromosome 1 Chromosome replication begins. Two copies of origin 2 One copy of the origin is now at each end of the cell. Origin Origin Figure Bacterial cell division by binary fission (step 2) 62

Figure Origin of replication Cell wall Plasma membrane E. coli cell Bacterial chromosome 1 Chromosome replication begins. Two copies of origin 2 One copy of the origin is now at each end of the cell. Origin Origin 3 Replication finishes. Figure Bacterial cell division by binary fission (step 3) 63

Figure Origin of replication Cell wall Plasma membrane E. coli cell Bacterial chromosome 1 Chromosome replication begins. Two copies of origin 2 One copy of the origin is now at each end of the cell. Origin Origin 3 Replication finishes. Figure Bacterial cell division by binary fission (step 4) 4 Two daughter cells result. 64

The Evolution of Mitosis
Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission Certain protists (dinoflagellates, diatoms, and some yeasts) exhibit types of cell division that seem intermediate between binary fission and mitosis © 2014 Pearson Education, Inc. 65

(b) Diatoms and some yeasts
Figure 9.13 Chromosomes Microtubules Intact nuclear envelope (a) Dinoflagellates Kinetochore microtubule Figure 9.13 Mechanisms of cell division Intact nuclear envelope (b) Diatoms and some yeasts 66

Concept 9.3: The eukaryotic cell cycle is regulated by a molecular control system
The frequency of cell division varies with the type of cell These differences result from regulation at the molecular level Cancer cells manage to escape the usual controls on the cell cycle © 2014 Pearson Education, Inc. 67

Evidence for Cytoplasmic Signals
The cell cycle is driven by specific signaling molecules present in the cytoplasm Some evidence for this hypothesis comes from experiments with cultured mammalian cells Cells at different phases of the cell cycle were fused to form a single cell with two nuclei at different stages Cytoplasmic signals from one of the cells could cause the nucleus from the second cell to enter the “wrong” stage of the cell cycle © 2014 Pearson Education, Inc. 68

Conclusion Molecules present in the cytoplasm
Figure 9.14 Experiment Experiment 1 Experiment 2 S G1 M G1 Results S S M M Figure 9.14 Inquiry: Do molecular signals in the cytoplasm regulate the cell cycle? G1 nucleus immediately entered S phase and DNA was synthesized. G1 nucleus began mitosis without chromosome duplication. Conclusion Molecules present in the cytoplasm control the progression to S and M phases. 69

Checkpoints of 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 timing device of a washing machine The cell cycle control system is regulated by both internal and external controls The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received © 2014 Pearson Education, Inc. 70

G1 checkpoint Control system S G1 G2 M M checkpoint G2 checkpoint
Figure 9.15 G1 checkpoint Control system S G1 G2 M Figure 9.15 Mechanical analogy for the cell cycle control system M checkpoint G2 checkpoint 71

For many cells, the G1 checkpoint seems to be the most important
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 © 2014 Pearson Education, Inc. 72

Without go-ahead signal, cell enters G0. With go-ahead signal,
Figure 9.16 G1 checkpoint G0 G1 G1 Without go-ahead signal, cell enters G0. With go-ahead signal, cell continues cell cycle. G1 S (a) G1 checkpoint M G2 G1 G1 M G2 M G2 Figure 9.16 Two important checkpoints M checkpoint G2 checkpoint Anaphase Prometaphase Metaphase Without full chromosome attachment, stop signal is received. With full chromosome attachment, go-ahead signal is received. (b) M checkpoint 73

Without go-ahead signal, cell enters G0. With go-ahead signal,
Figure 9.16a G1 checkpoint G0 G1 G1 Without go-ahead signal, cell enters G0. With go-ahead signal, cell continues cell cycle. Figure 9.16a Two important checkpoints (part 1: G1 checkpoint) (a) G1 checkpoint 74

Without full chromosome attachment, stop signal is received.
Figure 9.16b G1 G1 M G2 M G2 M checkpoint G2 checkpoint Anaphase Prometaphase Metaphase Without full chromosome attachment, stop signal is received. Figure 9.16b Two important checkpoints (part 2: M checkpoint) With full chromosome attachment, go-ahead signal is received. (b) M checkpoint 75

An example of an internal signal occurs at the M phase checkpoint
In this case, anaphase does not begin if any kinetochores remain unattached to spindle microtubules Attachment of all of the kinetochores activates a regulatory complex, which then activates the enzyme separase Separase allows sister chromatids to separate, triggering the onset of anaphase © 2014 Pearson Education, Inc. 77

Some external signals are 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 © 2014 Pearson Education, Inc. 78

Scalpels A sample of human connective tissue is cut up into small
Figure Scalpels 1 A sample of human connective tissue is cut up into small pieces. Petri dish Figure The effect of platelet-derived growth factor (PDGF) on cell division (step 1) 79

Figure Scalpels 1 A sample of human connective tissue is cut up into small pieces. Petri dish 2 Enzymes digest the extracellular matrix, resulting in a suspension of free fibroblasts. Figure The effect of platelet-derived growth factor (PDGF) on cell division (step 2) 80

Figure Scalpels 1 A sample of human connective tissue is cut up into small pieces. Petri dish 2 Enzymes digest the extracellular matrix, resulting in a suspension of free fibroblasts. 3 Cells are transferred to culture vessels. 4 PDGF is added to half the vessels. Figure The effect of platelet-derived growth factor (PDGF) on cell division (step 3) 81

Figure Scalpels 1 A sample of human connective tissue is cut up into small pieces. Petri dish 2 Enzymes digest the extracellular matrix, resulting in a suspension of free fibroblasts. 3 Cells are transferred to culture vessels. 4 PDGF is added to half the vessels. Figure The effect of platelet-derived growth factor (PDGF) on cell division (step 4) Cultured fibroblasts (SEM) Without PDGF With PDGF 10 m 82

Cultured fibroblasts (SEM) 10 m Figure 9.17a
Figure 9.17a The effect of platelet-derived growth factor (PDGF) on cell division (SEM) 10 m 83

Another 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 © 2014 Pearson Education, Inc. 84

Anchorage dependence: cells require a surface for division
Figure 9.18 Anchorage dependence: cells require a surface for division Density-dependent inhibition: cells form a single layer Density-dependent inhibition: cells divide to fill a gap and then stop Figure 9.18 Density-dependent inhibition and anchorage dependence of cell division 20 m 20 m (a) Normal mammalian cells (b) Cancer cells 85

(a) Normal mammalian cells
Figure 9.18a 20 m (a) Normal mammalian cells Figure 9.18a Density-dependent inhibition and anchorage dependence of cell division (part 1: normal cells) 86

20 m (b) Cancer cells Figure 9.18b
Figure 9.18b Density-dependent inhibition and anchorage dependence of cell division (part 2: cancer cells) 87

Loss of Cell Cycle Controls in Cancer Cells
Cancer cells do not respond to signals that normally regulate the cell cycle 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 © 2014 Pearson Education, Inc. 88

A normal cell is converted to a cancerous cell by a process called transformation
Cancer cells that are not eliminated by the immune system form tumors, masses of abnormal cells within otherwise normal tissue If abnormal cells remain only 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 additional tumors © 2014 Pearson Education, Inc. 89

5 m Breast cancer cell (colorized SEM) Metastatic tumor Lymph vessel
Figure 9.19 5 m Breast cancer cell (colorized SEM) Metastatic tumor Lymph vessel Tumor Blood vessel Figure 9.19 The growth and metastasis of a malignant breast tumor Glandular tissue Cancer cell 1 A tumor grows from a single cancer cell. 2 Cancer cells invade neighboring tissue. 3 Cancer cells spread through lymph and blood vessels to other parts of the body. 4 A small percentage of cancer cells may metastasize to another part of the body. 90

Tumor Glandular tissue A tumor grows from a single cancer cell.
Figure 9.19a Tumor Glandular tissue 1 A tumor grows from a single cancer cell. 2 Cancer cells invade neighboring tissue. 3 Cancer cells spread through lymph and blood vessels to other parts of the body. Figure 9.19a The growth and metastasis of a malignant breast tumor (part 1) 91

Metastatic tumor Lymph vessel Blood vessel Cancer cell
Figure 9.19b Metastatic tumor Lymph vessel Blood vessel Cancer cell Figure 9.19b The growth and metastasis of a malignant breast tumor (part 2) 3 Cancer cells spread through lymph and blood vessels to other parts of the body. 4 A small percentage of cancer cells may metastasize to another part of the body. 92

5 m Breast cancer cell (colorized SEM) Figure 9.19c
Figure 9.19c The growth and metastasis of a malignant breast tumor (SEM) 93

One of the big lessons in cancer research is how complex cancer is
Recent advances in understanding the cell cycle and cell cycle signaling have led to advances in cancer treatment Medical treatments for cancer are becoming more “personalized” to an individual patient’s tumor One of the big lessons in cancer research is how complex cancer is © 2014 Pearson Education, Inc. 94

Amount of fluorescence per cell (fluorescence units)
Figure 9.UN01 Control Treated 200 A B C A B C 160 120 Number of cells 80 40 Figure 9.UN01 Skills exercise: interpreting histograms 200 400 600 200 400 600 Amount of fluorescence per cell (fluorescence units) 95

Figure 9.UN02 G1 S Cytokinesis Mitosis G2 MITOTIC (M) PHASE Prophase Figure 9.UN02 Summary of key concepts: mitotic phase and interphase Telophase and Cytokinesis Prometaphase Anaphase Metaphase 96

Figure 9.UN03 Figure 9.UN03 Test your understanding, question 6 97

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