Study guide, Chapter 18 pt 1 PDF

Preview

Summary

Download Study guide, Chapter 18 pt 1 PDF

Description

GCD 3033

Spring 2012

Cell Biology Study Guide ECB Chapter 18: The cell division cycle The term cell cycle refers to the series of events in which the cell duplicates its contents and divides in two. For a cell to remain in equilibrium, it must double its mass each division cycle, including its organelles, proteins, and other biomolecules. This chapter focuses on the process of division and how the steps of the cell cycle are regulated and coordinated. The cell cycle can be broadly divided into M phase, in which the nucleus and cell divide, and interphase, the time between M phases. Most synthesis and duplication of cell components occurs during interphase, which includes S phase, the period of DNA replication. Interphase also includes the gaps between mitosis and S phase (G1 phase) and between S phase and mitosis (G2 phase).

Cell cycle control It is helpful to think of the proteins that guide the cell through the cell cycle as acting in three different groups. One group is the machinery that duplicates new cell components; another group is the machinery that partitions these into two new cells; and a third group is in charge of regulating the first two groups so that cell cycle events proceed in the proper order. Cyclin dependent protein kinases (CDKs) are master regulators of the cell cycle. The activity of each of the four major CDKs (see Table 18-2) is cyclically switched on and off at distinct phases of the cell cycle. Each CDK phosphorylates and regulates a distinct set of proteins that act at different steps in the cell cycle. CDKs are activated in part through the cyclical accumulation of cyclins, which bind to and activate specific CDKs. The concentration of different cyclins gradually rise and then rapidly fall at specific times in the cell cycle. The abrupt drop in cyclin levels occurs through their ubiquitination and proteosomedependent proteolysis, which ensures that the cell cycle is irreversible and proceeds only in a forward direction. In addition to cyclin binding, CDKs can also be activated or inhibited by phosphorylation, through the activity of protein phosphatases and other protein kinases (see Figures 18-9 and 18-17). Both activation and inactivation of CDKs are necessary for normal cell cycle progression. Cyclins and CDKs were identified through parallel experiments involving the large eggs of frogs, clams and sea urchins, and from genetic experiments in yeast. It was found that injection of cytoplasm from a dividing frog egg can cause an unfertilized egg to enter mitosis; fractionation of this cytoplasmic activity led to the identification of CDKs. Cyclins were first identified by their patterns of oscillation in egg extracts at different stages of the cell cycle. At about the same time, cyclins and CDKs were independently discovered through genetic screens

for yeast mutants with specific cell cycle arrest phenotypes. Remarkably, mutations in the yeast CDK gene can be rescued by expression of a human CDK gene. The Nobel prize in medicine was awarded in 2001 to three researchers working in these different model systems for their discoveries of key regulators of the cell cycle. To ensure that the cell cycle proceeds in the proper order, the cell cycle control system monitors for damage, environmental signals, and for successful completion of one event before allowing the cycle to proceed to the next. These regulatory steps are known as checkpoints, and they prevent the cell from entering the cell cycle before it has grown enough, or from dividing before its chromosomes are properly aligned. The major checkpoints are shown in Figure 18-13. The G1 checkpoint (also known as Start) is particularly important, because the cell will normally progress through the entire cell cycle after passing Start. Checkpoints can function by affecting cyclin levels, CDK phosphorylation, or through the expression of CDK inhibitors, which bind and inactivate specific CDKs. Some cells such as nerve and muscle permanently exit the cell cycle when they differentiate. This state is sometimes referred to as G0, and it differs from G1 in that many of the cell cycle control proteins are no longer present in these cells.

S phase We will consider the details and mechanisms of DNA replication and repair in Chapter 6. Here we focus on how this process is regulated within the cell cycle. A key point is that chromosomes must be accurately replicated once and only once in each cell cycle. DNA replication begins at multiple sites along each chromosome known as origins of replication. A multiprotein complex called the origin recognition complex (ORC) remains bound to these sites throughout the cell cycle. Just prior to S phase, the ORC recruits other proteins necessary for the initiation of DNA replication, to form the pre-replicative complex (PRC). Once the PRC is fully assembled, “firing” of the origin in response to active S-Cdk is what drives the cycle into S phase. Cdc6 is a key protein in the PRC, because its phosphorylation by S-Cdk causes the PRC to disassemble, and also marks Cdc6 for degradation. This prevents reassembly of PRCs during S-phase, ensuring that each origin of replication is used only once during each S phase (Fig. 18-14). During S phase, proteins called cohesins form rings around the length of each sister chromatid pair. This ties the chromatids together until they separate late in mitosis. DNA can be damaged by several environmental insults: X rays, UV irradiation, harmful chemicals, etc. DNA damage checkpoints act both before and during S

phase, to shut off the cell cycle and prevent replication of damaged DNA. The tumor suppressor protein p53 is an important component of these checkpoints. In response to DNA damage, p53 activates transcription of the CDK inhibitor p21, which binds and inhibits G1/S-Cdk and S-Cdk (Figure 18-16). This allows time for the damaged DNA to be repaired before it is replicated. Damaged or incompletely replicated DNA can also trigger a checkpoint that prevents cells from entering mitosis. This checkpoint acts by blocking a protein phosphatase, called Cdc25, which is required to remove an inhibitory phosphate on M-Cdk (see Fig. 18-17). This prevents M-Cdk from becoming active when DNA damage is sensed.

M phase During mitosis, replicated chromosomes are separated. This is usually followed immediately by cytokinesis, in which the cell divides in two. Together, mitosis and cytokinesis make up the M phase of the cell cycle. The major events of M phase are controlled, either directly or indirectly, by M-Cdk. As M-Cdk activity begins to increase at the start of M phase, it further activates itself through a positive feedback loop, resulting in a rapid and sustained activation of M-Cdk (see Figure 18-18). One of the first visible signs of M phase is the condensation of replicated chromosomes. This is driven by M-Cdk-dependent phosphorylation and activation of another group of ring-like structures called condensins, which help to coil individual chromatids into a more compact state. The cytoskeleton also plays a major role during M phase. - The microtubule-based mitotic spindle aligns and separates the replicated chromosomes during mitosis. The spindle is organized from a pair of centrosomes, which duplicate during interphase, and migrate to opposite sides of the nucleus during prophase to form the poles of the mitotic spindle. Different sets of microtubules with different functions project outward from the poles: kinetochore microtubules bind to the chromosomes, astral microtubules project toward the cell cortex, and interpolar microtubules interact with microtubules from the opposite pole. - At the end of mitosis, the actin-based contractile ring assembles beneath the plasma membrane, perpendicular to the mitotic spindle. Dyneindependent sliding provides the force to pinch the cell in two. M phase can be divided into six stages: 5 stages of mitosis, plus cytokinesis. Study panel 18-1 as a an excellent overall summary of these events. 1. Prophase: the major events here are chromosome condensation and formation of the mitotic spindle. As the duplicated centrosomes migrate along the surface of the nucleus, they form a radial array of highly dynamic microtubules called an aster. The mitotic spindle forms through

2.

3.

4.

5.

6.

selective stabilization of interacting microtubules from the opposite centrosomes, now called spindle poles (Figure 18-23). The beginning of prometaphase is marked by the breakdown of the nuclear envelope, which is triggered by phosphorylation of nuclear lamins by M-Cdk. Nuclear envelope breakdown allows the spindle microtubules to contact the chromosomes at special structures called kinetochores. Kinetochores form at a specific chromosome sequence called the centromere, usually one on each sister chromatid, facing toward the spindle poles. This results in bi-orientation, in which each chromatid is attached to the pole opposite its sister. Tension caused by the pulling of multiple microtubules on each kinetochore in an opposite direction acts as an important signal that the two chromatids are properly aligned. Lack of tension on a single chromosome is sufficient to act as a checkpoint (spindle assembly checkpoint) preventing further progression through mitosis. Alignment of chromosomes at the center of the mitotic spindle marks the beginning of metaphase. This arrangement of chromosomes along the spindle equator is called the metaphase plate. During metaphase, the chromosomes are under tension and can be seen to oscillate slightly back and forth. This tension is caused by microtubule motors and by growth and shrinkage of individual microtubules. Anaphase, the separation of sister chromatids to opposite poles, is triggered by the sudden degradation of the cohesin rings that link sister chromatids. Cohesin is degraded by a protease called separase, which itself is regulated indirectly by proteolysis: a protein complex called the anaphase-promoting complex (APC) causes ubiquitination and degradation of the protein securin, which normally acts to inhibit separase. The APC also ubiquitinates M-cyclin, leading to inactivation of M-Cdk; this step is necessary for exit from mitosis. Chromosomes move to opposite sides of the cell through two simultaneous processes, referred to as anaphase A and anaphase B. During anaphase A, shortening of spindle microtubules causes the chromosomes to move toward each spindle pole. This is driven by a microtubule motor-like protein that links the kinetochore to the spindle microtubule and removes tubulin subunits from the end of the microtubule. Anaphase B occurs through lengthening of the mitotic spindle, pushing the two poles apart. Both dynein and kinesin motor proteins act to push apart the interpolar microtubules and pull on the astral microtubules connected to the cell cortex. During telophase, processes that occurred during prophase and prometaphase are reversed: the nuclear envelope is reassembled, chromosomes begin to decondense, and the mitotic spindle disassembles. This marks the end of mitosis. The final step of M phase occurs after mitosis has finished. During cytokinesis, a ring of actin and myosin filaments forms around the equator of the cell in response to the position of the mitotic spindle.

Contraction of this ring results in formation of a cleavage furrow the bisects the cell between the two nuclei. During mitosis, cell-cell and cellsubstratum contacts are often disrupted, and dividing cells often adopt a rounded shape. These contacts and normal cell morphologies are regained following cytokinesis. Other components of the cell, including soluble proteins and organelles, are also partitioned during cytokinesis. Protein complexes, ribosomes, and most small organelles are simply distributed at random. The endoplasmic reticulum is divided by the contractile ring, whereas the Golgi apparatus disassembles into fragments that associate with microtubules of the mitotic spindle. Plant cells use a different mechanism for cytokinesis. Rather than a contractile band of actin-myosin, plants form a new cell wall between the two daughter cells, via vesicle-mediated fusion with a microtubular structure called a phragmoplast....