Chapter 12 The Cell Cycle

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

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

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

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

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 that condenses during cell division Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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, which separate during cell division The centromere is 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

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

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

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

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

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

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

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

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

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

Chromosome, consisting of two sister chromatids Fig. 12-6b 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

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

Telophase and Cytokinesis Fig. 12-6d 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

The Mitotic Spindle: A Closer Look 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. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

An aster (a radial array of short microtubules) extends from each centrosome The spindle includes the centrosomes, the spindle microtubules, and the asters Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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

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

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

EXPERIMENT RESULTS CONCLUSION Kinetochore Spindle pole Mark Chromosome Fig. 12-8 EXPERIMENT Kinetochore Spindle pole Mark RESULTS Figure 12.8 At which end do kinetochore microtubules shorten during anaphase? CONCLUSION Chromosome movement Kinetochore Motor protein Tubulin subunits Microtubule Chromosome

EXPERIMENT Kinetochore Spindle pole Mark RESULTS Fig. 12-8a Figure 12.8 At which end do kinetochore microtubules shorten during anaphase? RESULTS

Chromosome movement Kinetochore Tubulin Motor Subunits Microtubule Fig. 12-8b CONCLUSION Chromosome movement Kinetochore Tubulin Subunits Motor protein Microtubule Figure 12.8 At which end do kinetochore microtubules shorten during anaphase? Chromosome

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

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

Video: Sea Urchin (Time Lapse) Video: Animal Mitosis Video: Sea Urchin (Time Lapse) For the Cell Biology Video Nuclear Envelope Formation, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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)

(a) Cleavage of an animal cell (SEM) Fig. 12-9a 100 µm Cleavage furrow Figure 12.9a Cytokinesis in animal and plant cells Contractile ring of microfilaments Daughter cells (a) Cleavage of an animal cell (SEM)

(b) Cell plate formation in a plant cell (TEM) Fig. 12-9b Vesicles forming cell plate Wall of parent cell 1 µm Cell plate New cell wall Figure 12.9b Cytokinesis in animal and plant cells Daughter cells (b) Cell plate formation in a plant cell (TEM)

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

Cell wall Origin of replication Plasma membrane E. coli cell Bacterial Fig. 12-11-1 Cell wall Origin of replication Plasma membrane E. coli cell Bacterial chromosome Two copies of origin Figure 12.11 Bacterial cell division by binary fission

Cell wall Origin of replication Plasma membrane E. coli cell Bacterial Fig. 12-11-2 Cell wall Origin of replication Plasma membrane E. coli cell Bacterial chromosome Two copies of origin Origin Origin Figure 12.11 Bacterial cell division by binary fission

Cell wall Origin of replication Plasma membrane E. coli cell Bacterial Fig. 12-11-3 Cell wall Origin of replication Plasma membrane E. coli cell Bacterial chromosome Two copies of origin Origin Origin Figure 12.11 Bacterial cell division by binary fission

Cell wall Origin of replication Plasma membrane E. coli cell Bacterial Fig. 12-11-4 Cell wall Origin of replication Plasma membrane E. coli cell Bacterial chromosome Two copies of origin Origin Origin Figure 12.11 Bacterial cell division by binary fission

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

Fig. 12-12 Bacterial chromosome (a) Bacteria Chromosomes Microtubules Intact nuclear envelope (b) Dinoflagellates Kinetochore microtubule Intact nuclear envelope Figure 12.12 A hypothetical sequence for the evolution of mitosis (c) Diatoms and yeasts Kinetochore microtubule Fragments of nuclear envelope (d) Most eukaryotes

Bacterial chromosome (a) Bacteria Chromosomes Microtubules Fig. 12-12ab Bacterial chromosome (a) Bacteria Chromosomes Microtubules Figure 12.12 A hypothetical sequence for the evolution of mitosis Intact nuclear envelope (b) Dinoflagellates

Kinetochore microtubule Intact nuclear envelope (c) Diatoms and yeasts Fig. 12-12cd Kinetochore microtubule Intact nuclear envelope (c) Diatoms and yeasts Kinetochore microtubule Figure 12.12 A hypothetical sequence for the evolution of mitosis Fragments of nuclear envelope (d) Most eukaryotes

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

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

EXPERIMENT RESULTS Experiment 1 Experiment 2 S G1 M G1 S S M M Fig. 12-13 EXPERIMENT Experiment 1 Experiment 2 S G1 M G1 RESULTS 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 12.13 Do molecular signals in the cytoplasm regulate the cell cycle?

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

G1 checkpoint Control system S G1 G2 M M checkpoint G2 checkpoint Fig. 12-14 G1 checkpoint Control system S G1 G2 M Figure 12.14 Mechanical analogy for the cell cycle control system M checkpoint G2 checkpoint

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

G0 G1 G1 G1 checkpoint (b) Cell does not receive a go-ahead signal Fig. 12-15 G0 G1 checkpoint Figure 12.15 The G1 checkpoint G1 G1 Cell receives a go-ahead signal (b) Cell does not receive a go-ahead signal

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) The activity of cyclins and Cdks fluctuates during the cell cycle 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

Fig. 12-16 RESULTS 5 30 4 20 3 % of dividing cells (– ) Protein kinase activity (– ) 2 10 1 Figure 12.16 How does the activity of a protein kinase essential for mitosis vary during the cell cycle? 100 200 300 400 500 Time (min)

M G1 S G2 M G1 S G2 M G1 Fig. 12-17 MPF activity Cyclin concentration Time (a) Fluctuation of MPF activity and cyclin concentration during the cell cycle G1 S Cdk Figure 12.17 Molecular control of the cell cycle at the G2 checkpoint Cyclin accumulation M Degraded cyclin G2 G2 Cdk Cyclin is degraded checkpoint Cyclin MPF (b) Molecular mechanisms that help regulate the cell cycle

(a) Fluctuation of MPF activity and cyclin concentration during Fig. 12-17a M G1 S G2 M G1 S G2 M G1 MPF activity Cyclin concentration Figure 12.17 Molecular control of the cell cycle at the G2 checkpoint Time (a) Fluctuation of MPF activity and cyclin concentration during the cell cycle

(b) Molecular mechanisms that help regulate the cell cycle Fig. 12-17b G1 S Cdk Cyclin accumulation M Degraded cyclin G2 G2 checkpoint Cdk Figure 12.17 Molecular control of the cell cycle at the G2 checkpoint Cyclin is degraded Cyclin MPF (b) Molecular mechanisms that help regulate the cell cycle

Stop and Go Signs: Internal and External Signals at the Checkpoints 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 For example, platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Scalpels Petri plate Without PDGF cells fail to divide With PDGF Fig. 12-18 Scalpels Petri plate Without PDGF cells fail to divide Figure 12.18 The effect of a growth factor on cell division With PDGF cells prolifer- ate Cultured fibroblasts 10 µm

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

Density-dependent inhibition Fig. 12-19 Anchorage dependence Density-dependent inhibition Density-dependent inhibition Figure 12.19 Density-dependent inhibition and anchorage dependence of cell division 25 µm 25 µm (a) Normal mammalian cells (b) Cancer cells

Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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

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

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