《生物学》课程参考书籍文献（英文版，Chapter 11 - Chapter 19）Part IV Reproduction and Heredity、Part V Molecular Genetics

Cell nucleus Nucleoli Nucleus Microtubules Cytoplasm Cell wall Condensed chromosomes Microtubule CYTOKINESIS Prophase plant cells: cell plate forms, dividing nuclear membrane daughter cells disintegr ates animal cells: cleavage furrow forms cleolus disappears hter nucle at equator of cell and pinches inward chromosomes condense Cell plate and nucleoli until cell divides in two between centrioles kinetochores begin to mature and attach to spindle Telophase chromosomes reach poles of cell kinetochores disappear polar microtubules continue to elongate, preparing cell for nuclear membrane re-forms · nucleolus reappears to mitotic spindle and align them separating chromosomes to microtubules metaphase plate microtubules FIGURE 11.11 Mitosis and cytokinesis Mitosis(separation of the two genomes)occurs in four stages--prophase, metaphase, anaphase, and telophase- and is followed by cytokinesis(division into two separate cells). In this depiction, the chromosomes of the African blood lily, Haemanthus katharine are stained blue and microtubules are stained red. Chapter 11 How Cells Divide 215

Chapter 11 How Cells Divide 215 CYTOKINESIS • plant cells: cell plate forms, dividing daughter cells • animal cells: cleavage furrow forms at equator of cell and pinches inward until cell divides in two Prophase • nuclear membrane disintegrates • nucleolus disappears • chromosomes condense • mitotic spindle begins to form between centrioles • kinetochores begin to mature and attach to spindle Metaphase • kinetochores attach chromosomes to mitotic spindle and align them along metaphase plate at equator of cell Anaphase • kinetochore microtubules shorten, separating chromosomes to opposite poles • polar microtubules elongate, preparing cell for cytokinesis Telophase • chromosomes reach poles of cell • kinetochores disappear • polar microtubules continue to elongate, preparing cell for cytokinesis • nuclear membrane re-forms • nucleolus reappears • chromosomes decondense Nucleolus Nucleus Cytoplasm Cell wall Microtubules Cell nucleus Condensed chromosomes Chromosomes Centromere and kinetochore Mitotic spindle Mitotic spindle microtubules Chromosomes aligned on metaphase plate Kinetochore microtubules Polar microtubules Chromatids Spindle microtubules (pink) Daughter nuclei Cell plate and nucleoli Microtubule FIGURE 11.11 Mitosis and cytokinesis. Mitosis (separation of the two genomes) occurs in four stages—prophase, metaphase, anaphase, and telophase— and is followed by cytokinesis (division into two separate cells). In this depiction, the chromosomes of the African blood lily, Haemanthus katharinae, are stained blue, and microtubules are stained red

Overlapping microtubules 2 um Pole Late anaphase FIGURE 11.12 microtubules slide past each other as the chromosomes separate. In these electron micrographs of dividing diatoms, the overlap of the microtubules lessens markedly during spindle elongation as the cell passes from metaphase to anal Anaphase and Telophase: Separation of the When the sister chromatids separate in anaphase, the Chromatids and reformation of the nuclei accurate partitioning of the replicated genomethe es- Of all the stages of mitosis, anaphase is the shortest and sential element of mitosis--is complete. In telophase, the spindle apparatus disassembles, as the microtubules are the most beautiful to watch. It starts when the centromeres broken down into tubulin monomers that can be used to divide. Each centromere splits in two, freeing the two sister chromatids from each other. The centromeres of all the construct the cytoskeletons of the daughter cells. A nu clear envelope forms around each set of sister chromatids chromosomes separate simultaneously, but the mechanism which can now be called chromosomes because each has its own centromere. The chromos Freed from each other, the sister chromatids are pulled coil into the more extended form that permits gene ex- rapidly toward the poles to which their kinetochores are at tached. In the process, two forms of movement take place pression. One of the early group of genes expressed are simultaneously, each driven by microtubules he rRNA genes, resulting in the reappearance of the nucleolus First, the poles move apart as microtubular spindle fibers physically anchored to opposite poles slide past each other, away from the center of the cell (figure 11. 12). Because an During prophase, microtubules attach the other group of microtubules attach the chromosomes to centromeres joining pairs of sister chromatids t the poles, the chromosomes move apart, too. If a flexible opposite poles of the spindle apparatus. During metaphase, each chromosome is drawn to a ring along membrane surrounds the cell, it becomes visibly elongated the inner circumference of the cell by the Second, the centromeres move toward the poles as the mi microtubules extending from the centromere to the crotubules that connect them to the poles shorten. This two poles of the spindle apparatus. During anaphase, shortening process is not a contraction; the microtubules the poles of the cell are pushed apart by microtubular do not get any thicker. Instead, tubulin subunits are re- sliding, and the sister chromatids are drawn to moved from the kinetochore ends of the microtubules by opposite poles by the shortening of the microtubule e ol ganizing center. As more subunits are remove attached to them. During telophase, the spindle is chromatid-bearing microtubules are progressively disas disassembled, nuclear envelopes are reestablished, and sembled, and the chromatids are pulled ever closer to the the normal expression of genes present in the oles of the cell chromosomes is reinitiated 216 Part Iv Reproduction and Heredity

Anaphase and Telophase: Separation of the Chromatids and Reformation of the Nuclei Of all the stages of mitosis, anaphase is the shortest and the most beautiful to watch. It starts when the centromeres divide. Each centromere splits in two, freeing the two sister chromatids from each other. The centromeres of all the chromosomes separate simultaneously, but the mechanism that achieves this synchrony is not known. Freed from each other, the sister chromatids are pulled rapidly toward the poles to which their kinetochores are at￾tached. In the process, two forms of movement take place simultaneously, each driven by microtubules. First, the poles move apart as microtubular spindle fibers physically anchored to opposite poles slide past each other, away from the center of the cell (figure 11.12). Because an￾other group of microtubules attach the chromosomes to the poles, the chromosomes move apart, too. If a flexible membrane surrounds the cell, it becomes visibly elongated. Second, the centromeres move toward the poles as the mi￾crotubules that connect them to the poles shorten. This shortening process is not a contraction; the microtubules do not get any thicker. Instead, tubulin subunits are re￾moved from the kinetochore ends of the microtubules by the organizing center. As more subunits are removed, the chromatid-bearing microtubules are progressively disas￾sembled, and the chromatids are pulled ever closer to the poles of the cell. When the sister chromatids separate in anaphase, the accurate partitioning of the replicated genome—the es￾sential element of mitosis—is complete. In telophase, the spindle apparatus disassembles, as the microtubules are broken down into tubulin monomers that can be used to construct the cytoskeletons of the daughter cells. A nu￾clear envelope forms around each set of sister chromatids, which can now be called chromosomes because each has its own centromere. The chromosomes soon begin to un￾coil into the more extended form that permits gene ex￾pression. One of the early group of genes expressed are the rRNA genes, resulting in the reappearance of the nucleolus. During prophase, microtubules attach the centromeres joining pairs of sister chromatids to opposite poles of the spindle apparatus. During metaphase, each chromosome is drawn to a ring along the inner circumference of the cell by the microtubules extending from the centromere to the two poles of the spindle apparatus. During anaphase, the poles of the cell are pushed apart by microtubular sliding, and the sister chromatids are drawn to opposite poles by the shortening of the microtubules attached to them. During telophase, the spindle is disassembled, nuclear envelopes are reestablished, and the normal expression of genes present in the chromosomes is reinitiated. 216 Part IV Reproduction and Heredity Metaphase Late anaphase Pole Overlapping microtubules Pole Pole Overlapping microtubules Pole 2 µm FIGURE 11.12 Microtubules slide past each other as the chromosomes separate. In these electron micrographs of dividing diatoms, the overlap of the microtubules lessens markedly during spindle elongation as the cell passes from metaphase to anaphase

Cytokinesis cell has partitioned its replicated genome into two nuclei positioned at opposite ends of the cell. While mitosis was going on, the cytoplasmic organelles, including mitochon dria and chloroplasts(if present), were reassorted to areas that will separate and become the daughter cells. The repli tion of organelles takes place before cytokinesis, often in the S or g2 phase. Cell division is still not complete at the end of mitosis. however because the division of the cell has not yet begun. The phase of the cell cycle wh the cell actually divides is called cytokinesis. It gener involves the cleavage of the cell into roughly equal halves Cytokinesis in Animal Cells FIGURE 11.13 In animal cells and the cells of all other eukaryotes that lack Cytokinesis in animal cells cell walls, cytokinesis is achieved by means of a constricting (a)A cleavage furrow forms around a dividing sea urchin egg belt of actin filaments. As these filaments slide past one an-(30x).(b)The completion of cytokinesis in an animal cell. The other, the diameter of the belt decreases, pinching the cell and creating a cleavage furrow around the cell,'s circumfer ence(figure 1113a). As constriction proceeds, the furrow deepens until it eventually slices all the way into the center of the cell. At this point, the cell is divided in two(figure 1113b) Cytokinesis in Plant Cells Plant cells possess a cell wall far too rigid to be squeezed in two by actin filaments. Instead, these cells assemble mem- brane components in their interior, at right angles to the spindle apparatus(figure 11. 14). This expanding membrane partition, called a cell plate, continues to grow outward until it reaches the interior surface of the plasma mem- brane and fuses with it, effectively dividing the cell in two Cellulose is then laid down on the new membranes creat ing two new cell walls. The space between the daughter cells becomes impregnated with pectins and is called a middle lamella Vesicles containing membrane omponents fusing to form cell plate FIGURE 11.14 Cytokinesis in Fungi and Protists Cytokinesis in plant cells. In this photograph and companion In fungi and some groups of protists, the nuclear mem- brane does not dissolve and as a result, all the events of m the plate is complete, there will be two cells. tosis occurs entirely within the nucleus. Only after mitosis is complete in these organisms does the nucleus then divide into two daughter nuclei, and daughter cell during cytokinesis. This separate nuclear di- plasts are distributed equally between the daughter cells vision phase of the cell cycle does not occur in plants, ani- However, as long as some of each organelle are present in als, or most protists each cell, the organelles can replicate to reach the number After cytokinesis in any eukaryotic cell, the two daug appropriate for that cell cells contain all of the components of a complete cell While mitosis ensures that both daughter cells contain a full complement of chromosomes, no similar mechanism a eukaryotic cell into two daughter cells. ' te Cytokinesis is the physical division of the cytoplasm of ensures that organelles such as mitochondria and chloro- Chapter 11 How Cells Divide 217

Cytokinesis Mitosis is complete at the end of telophase. The eukaryotic cell has partitioned its replicated genome into two nuclei positioned at opposite ends of the cell. While mitosis was going on, the cytoplasmic organelles, including mitochon￾dria and chloroplasts (if present), were reassorted to areas that will separate and become the daughter cells. The repli￾cation of organelles takes place before cytokinesis, often in the S or G2 phase. Cell division is still not complete at the end of mitosis, however, because the division of the cell proper has not yet begun. The phase of the cell cycle when the cell actually divides is called cytokinesis. It generally involves the cleavage of the cell into roughly equal halves. Cytokinesis in Animal Cells In animal cells and the cells of all other eukaryotes that lack cell walls, cytokinesis is achieved by means of a constricting belt of actin filaments. As these filaments slide past one an￾other, the diameter of the belt decreases, pinching the cell and creating a cleavage furrow around the cell’s circumfer￾ence (figure 11.13a). As constriction proceeds, the furrow deepens until it eventually slices all the way into the center of the cell. At this point, the cell is divided in two (figure 11.13b). Cytokinesis in Plant Cells Plant cells possess a cell wall far too rigid to be squeezed in two by actin filaments. Instead, these cells assemble mem￾brane components in their interior, at right angles to the spindle apparatus (figure 11.14). This expanding membrane partition, called a cell plate, continues to grow outward until it reaches the interior surface of the plasma mem￾brane and fuses with it, effectively dividing the cell in two. Cellulose is then laid down on the new membranes, creat￾ing two new cell walls. The space between the daughter cells becomes impregnated with pectins and is called a middle lamella. Cytokinesis in Fungi and Protists In fungi and some groups of protists, the nuclear mem￾brane does not dissolve and, as a result, all the events of mi￾tosis occurs entirely within the nucleus. Only after mitosis is complete in these organisms does the nucleus then divide into two daughter nuclei, and one nucleus goes to each daughter cell during cytokinesis. This separate nuclear di￾vision phase of the cell cycle does not occur in plants, ani￾mals, or most protists. After cytokinesis in any eukaryotic cell, the two daughter cells contain all of the components of a complete cell. While mitosis ensures that both daughter cells contain a full complement of chromosomes, no similar mechanism ensures that organelles such as mitochondria and chloro￾plasts are distributed equally between the daughter cells. However, as long as some of each organelle are present in each cell, the organelles can replicate to reach the number appropriate for that cell. C`ytokinesis is the physical division of the cytoplasm of a eukaryotic cell into two daughter cells. Chapter 11 How Cells Divide 217 FIGURE 11.13 (b) Cytokinesis in animal cells. (a) A cleavage furrow forms around a dividing sea urchin egg (30×). (b) The completion of cytokinesis in an animal cell. The two daughter cells are still joined by a thin band of cytoplasm occupied largely by microtubules. Cell wall Nuclei Vesicles containing membrane components fusing to form cell plate FIGURE 11.14 Cytokinesis in plant cells. In this photograph and companion drawing, a cell plate is forming between daughter nuclei. Once the plate is complete, there will be two cells

1.4 The cell cycle is carefully controlled General Strategy of Cell Cycle Control The events of the cell cycle are coordinated in much the same way in all eukaryotes. The control system human cells utilize first evolved among the protists over a billion years ago; today, it operates in essentially the same way in fungi The goal of controlling any cyclic process is to adjust the duration of the cycle to allow sufficient time for all events to occur. In ple, a variety of methods can achieve this goal. For example, an internal"clock "can be employed to allow adequate time for each phase of the cycle to be completed. This is how many organisms con- trol their daily activity cycles. The disadvantage of using such a clock to control the cell cycle is that it is not very flexible. One way to achieve a more flexible and sensitive egulation of a cycle is simply to let the completion of each phase of the cycle trigger the beginning of the next FIGURE 11.15 phase, as a runner passing a baton starts the next leg in a Control of the cell cycle. Cells use a centralized control system relay race. Until recently, biologists thought this type of to check whether proper conditions have been achieved before mechanism controlled the cell division cycle. However, passing three key"checkpoints"in the cell cycle we now know that eukaryotic cells employ a separate, cen- tralized controller to regulate the process: at critical points in the cell cycle, further progress depends upon a central set of"go/no-go"switches that are regulated by feedback from the cell FIGURE 11.16 The gi checkpoin This mechanism is the same one engineers use to con Feedback from the trol many processes. For example, the furnace that heats withdraw to Go cell determines a home in the winter typically goes through a daily heat whether the cell cycle ing cycle. When the daily cycle reaches the morning will proceed to the S turn on"checkpoint, sensors report whether the house hase, pause,or temperature is below the set point(for example, 70%F).If withdraw into Go for it is, the thermostat triggers the furnace which warms an extended rest the house. If the house is already at least that warm, the thermostat does not start up the furnace. Similarly, the ell cycle has key checkpoints where feed back signals from the cell about its size and the condition of its chro- mosomes can either trigger subsequent phases of the cycle, or delay them to allow more time for the current initiating S phase. The Gi checkpoint is where the more hase to be completed complex eukaryotes typically arrest the cell cycle if envi ronmental conditions make cell division impossible, or if Architecture of the Control System the cell passes into go for an extended perio The success of DNA replication is assessed at the Three principal checkpoints control the cell cycle in eu- Gz checkpoint. The second checkpoint, which occurs karyotes(figure 11. 15) at the end of G2, triggers the start of M phase. If this Cell growth is assessed at the gi checkpoint. LO- checkpoint is passed, the cell initiates the many molecu- cated near the end of gl, just before entry into S phase, lar processes that signal the beginning of mitosis this checkpoint makes the key decision of whether the Mitosis is assessed at the M checkpoint. Occurring cell should divide, delay division, or enter a resting stage at metaphase, the third checkpoint triggers the exit from (figure 11. 16). In yeasts, where researchers first studied mitosis and cytokinesis and the beginning of o this checkpoint, it is called START. If conditions are fa vorable for division, the cell begins to copy its DNA, The cell cycle is controlled at three checkpoints 218 Part IV Reproduction and Heredity

General Strategy of Cell Cycle Control The events of the cell cycle are coordinated in much the same way in all eukaryotes. The control system human cells utilize first evolved among the protists over a billion years ago; today, it operates in essentially the same way in fungi as it does in humans. The goal of controlling any cyclic process is to adjust the duration of the cycle to allow sufficient time for all events to occur. In principle, a variety of methods can achieve this goal. For example, an internal “clock” can be employed to allow adequate time for each phase of the cycle to be completed. This is how many organisms con￾trol their daily activity cycles. The disadvantage of using such a clock to control the cell cycle is that it is not very flexible. One way to achieve a more flexible and sensitive regulation of a cycle is simply to let the completion of each phase of the cycle trigger the beginning of the next phase, as a runner passing a baton starts the next leg in a relay race. Until recently, biologists thought this type of mechanism controlled the cell division cycle. However, we now know that eukaryotic cells employ a separate, cen￾tralized controller to regulate the process: at critical points in the cell cycle, further progress depends upon a central set of “go/no-go” switches that are regulated by feedback from the cell. This mechanism is the same one engineers use to con￾trol many processes. For example, the furnace that heats a home in the winter typically goes through a daily heat￾ing cycle. When the daily cycle reaches the morning “turn on” checkpoint, sensors report whether the house temperature is below the set point (for example, 70°F). If it is, the thermostat triggers the furnace, which warms the house. If the house is already at least that warm, the thermostat does not start up the furnace. Similarly, the cell cycle has key checkpoints where feedback signals from the cell about its size and the condition of its chro￾mosomes can either trigger subsequent phases of the cycle, or delay them to allow more time for the current phase to be completed. Architecture of the Control System Three principal checkpoints control the cell cycle in eu￾karyotes (figure 11.15): Cell growth is assessed at the G1 checkpoint. Lo￾cated near the end of G1, just before entry into S phase, this checkpoint makes the key decision of whether the cell should divide, delay division, or enter a resting stage (figure 11.16). In yeasts, where researchers first studied this checkpoint, it is called START. If conditions are fa￾vorable for division, the cell begins to copy its DNA, initiating S phase. The G1 checkpoint is where the more complex eukaryotes typically arrest the cell cycle if envi￾ronmental conditions make cell division impossible, or if the cell passes into G0 for an extended period. The success of DNA replication is assessed at the G2 checkpoint. The second checkpoint, which occurs at the end of G2, triggers the start of M phase. If this checkpoint is passed, the cell initiates the many molecu￾lar processes that signal the beginning of mitosis. Mitosis is assessed at the M checkpoint. Occurring at metaphase, the third checkpoint triggers the exit from mitosis and cytokinesis and the beginning of G1. The cell cycle is controlled at three checkpoints. 218 Part IV Reproduction and Heredity 11.4 The cell cycle is carefully controlled. G2 M S G2 checkpoint M checkpoint G1 checkpoint G1 C FIGURE 11.15 Control of the cell cycle. Cells use a centralized control system to check whether proper conditions have been achieved before passing three key “checkpoints” in the cell cycle. proceed to S? pause? withdraw to Go? FIGURE 11.16 The G1 checkpoint. Feedback from the cell determines whether the cell cycle will proceed to the S phase, pause, or withdraw into G0 for an extended rest period

Molecular mechanisms of cell FIGURE 11.17 Cycle control A complex of two proteins triggers passage through kpoints Cdk Exactly how does a cell achieve central control of the divi- is a protein kinase that The b activates numerous cell proteins sensitive to the condition of the cell interact at the proteins by phosphorylating checkpoints to trigger the next events in the cycle. Two key them. Cyclin is a regulatory types of proteins participate in this interaction: cochin protein required to activate dependent protein kinases and cyclins(figure 11.17) does not function unless cyclin is bound to it. The Cyclin Control System Cyclin-dependent protein kinases(Cdks) are enzymes that phosphorylate(add phosphate groups to)the serine and threonine amino acids of key cellular enzymes and other proteins. At the G2 checkpoint, for example, Cdks phosphorylate histones, nuclear membrane filaments, and Trigger mitosis the microtubule-associated proteins that form the mitotic spindle. Phosphorylation of these components of the cell M-phase-promoting factor division machinery initiates activities that carry the cycle past the checkpoint into mitosis MPF Cyclins are proteins that bind to Cdks, enabling the Mitotic Cdks to function as enzymes. Cyclins are so named because they are destroyed and resynthesized during each turn of G2 checkpoint the cell cycle(figure 11. 18). Different cyclins regulate the Gi and g cell cycle checkpoints The G2 Checkpoint. During G2, the cell gradually accu mulates G cyclin(also called mitotic cyclin). This cyclin binds to Cdk to form a complex called MPF (mitosis-pro- moting factor). At first, MPF is not active in carrying the cycle past the G2 checkpoint. But eventually, other cellular enzymes phosphorylate and so activate a few molecules of MPF. These activated MPFs in turn increase the activity of the enzymes that phosphorylate MPF, setting up a positive PcA feedback that leads to a very rapid increase in the cellular concentration of activated mPf. when the level of acti Start kinase vated MPF exceeds the threshold necessary to trigger mito- Trigger DNA replication MPF sows the seeds of its own destruction. The FIGURE 11.18 by the activity of mpe. for one of its many functions is to How cell cycle control works. As the cell cycle passes through activate proteins that destroy cyclin. As mitosis proceeds different cyclins and, as a result, activates different cellular to the end of metaphase, Cak levels stay relatively con- processes. At the completion of each phase, the cyclins are stant,but increasing amounts of G2 cyclin are degraded, degraded, bringing Cdk activity to a halt until the next set of causing progressively less MPf to be available and so ini- cyclins appears tiating the events that end mitosis. After mitosis, the gradual accumulation of new cyclin starts the next turn of the cell cycle The GI Checkpoint. The Gi checkpoint is thought to be regulated in a similar fashion. In unicellular eukaryotes cell grows, its cytoplasm increases in size, while the such as yeasts, the main factor triggering DNA replication amount of DNA remains constant. Eventually a threshold is cell size. Yeast cells grow and divide as rapidly as ratio is reached that promotes the production of cyclins ble,and they make the START decision by comparing and thus triggers the next round of DNA replication and the volume of cytoplasm to the size of the genome. As a cell division Chapter 11 How Cells Divide 219

Molecular Mechanisms of Cell Cycle Control Exactly how does a cell achieve central control of the divi￾sion cycle? The basic mechanism is quite simple. A set of proteins sensitive to the condition of the cell interact at the checkpoints to trigger the next events in the cycle. Two key types of proteins participate in this interaction: cyclin￾dependent protein kinases and cyclins (figure 11.17). The Cyclin Control System Cyclin-dependent protein kinases (Cdks) are enzymes that phosphorylate (add phosphate groups to) the serine and threonine amino acids of key cellular enzymes and other proteins. At the G2 checkpoint, for example, Cdks phosphorylate histones, nuclear membrane filaments, and the microtubule-associated proteins that form the mitotic spindle. Phosphorylation of these components of the cell division machinery initiates activities that carry the cycle past the checkpoint into mitosis. Cyclins are proteins that bind to Cdks, enabling the Cdks to function as enzymes. Cyclins are so named because they are destroyed and resynthesized during each turn of the cell cycle (figure 11.18). Different cyclins regulate the G1 and G2 cell cycle checkpoints. The G2 Checkpoint. During G2, the cell gradually accu￾mulates G2 cyclin (also called mitotic cyclin). This cyclin binds to Cdk to form a complex called MPF (mitosis-pro￾moting factor). At first, MPF is not active in carrying the cycle past the G2 checkpoint. But eventually, other cellular enzymes phosphorylate and so activate a few molecules of MPF. These activated MPFs in turn increase the activity of the enzymes that phosphorylate MPF, setting up a positive feedback that leads to a very rapid increase in the cellular concentration of activated MPF. When the level of acti￾vated MPF exceeds the threshold necessary to trigger mito￾sis, G2 phase ends. MPF sows the seeds of its own destruction. The length of time the cell spends in M phase is determined by the activity of MPF, for one of its many functions is to activate proteins that destroy cyclin. As mitosis proceeds to the end of metaphase, Cdk levels stay relatively con￾stant, but increasing amounts of G2 cyclin are degraded, causing progressively less MPF to be available and so ini￾tiating the events that end mitosis. After mitosis, the gradual accumulation of new cyclin starts the next turn of the cell cycle. The G1 Checkpoint. The G1 checkpoint is thought to be regulated in a similar fashion. In unicellular eukaryotes such as yeasts, the main factor triggering DNA replication is cell size. Yeast cells grow and divide as rapidly as possi￾ble, and they make the START decision by comparing the volume of cytoplasm to the size of the genome. As a cell grows, its cytoplasm increases in size, while the amount of DNA remains constant. Eventually a threshold ratio is reached that promotes the production of cyclins and thus triggers the next round of DNA replication and cell division. Chapter 11 How Cells Divide 219 Cyclin Cyclin-dependent kinase (Cdk) FIGURE 11.17 A complex of two proteins triggers passage through cell cycle checkpoints. Cdk is a protein kinase that activates numerous cell proteins by phosphorylating them. Cyclin is a regulatory protein required to activate Cdk; in other words, Cdk does not function unless cyclin is bound to it. Trigger mitosis MPF G2 checkpoint G1 checkpoint G1 cyclin Mitotic cyclin Cdk Trigger DNA replication G1 G2 S M Start kinase M-phase-promoting factor C P P FIGURE 11.18 How cell cycle control works. As the cell cycle passes through the G1 and G2 checkpoints, Cdk becomes associated with different cyclins and, as a result, activates different cellular processes. At the completion of each phase, the cyclins are degraded, bringing Cdk activity to a halt until the next set of cyclins appears