The Cell Cycle
8.1 The Balance Between Cell Division and Cell Death
1. Cell division (mitosis) and cell death (apoptosis) shape organs, select certain cell types to persist in development, and protect tissues by ridding the body of cells that could become cancerous.
8.2 The Cell Cycle
2. The cell cycle is a sequence of events that describes whether a cell is dividing its genetic material (mitosis or karyokinesis); dividing its cytoplasm, organelles, and macromolecules (cytokinesis); or preparing to divide (interphase).
3. Interphase includes two gap periods, G1 and G2, when the cell makes proteins, carbohydrates, and lipids, and a synthesis period (S), when it replicates genetic material. G1 includes a checkpoint that sets a cell’s fate—to divide, stay specialized but not divide, or die. G0 is a “time out” from the cell cycle.
4. Each replicated chromosome consists of two complete sets of genetic information, called chromatids, attached at a section of DNA called a centromere that also replicates.
5. Microtubules assemble to build the mitotic spindle. It arises from paired centrosomes. Chromosomes attach to the mitotic spindle with microtubule assemblies called kinetochores.
6. Mitosis consists of five stages. In prophase, the chromosomes condense and become visible when stained, the nuclear membrane disassembles, and the mitotic spindle forms. In prometaphase, the nuclear membrane fragments and moves out of the way. In metaphase, spindle fibers align replicated chromosomes down the cell’s equator. In anaphase, the chromatids of each replicated chromosome separate, sending a complete set of genetic instructions to each end of the cell. In telophase, the spindle breaks down and nuclear membranes form.
7. In cytokinesis in an animal cell, a cleavage furrow forms and a contractile band draws the two cells apart. In a plant cell, a phragmoplast provides space for a new cell wall to be laid down, separating the cells. Cytokinesis usually begins during anaphase or telophase. Lack of cytokinesis results in a huge cell with many nuclei.
8.3 How Is the Cell Cycle Controlled?
8. Checkpoints are interacting proteins that maintain the sequence of cell cycle events and control cell fate.
9. Shrinking telomeres track the number of divisions a cell has undergone, and when telomeres reach a certain length, division ceases. Cancer cells and certain rapidly dividing cells retain long telomeres and divide continually.
10. Contact inhibition prevents normal cells from dividing. Extracellular signals (hormones and growth factors) and intracellular signals (cyclins and kinases) control cell division rate and number.
11. Stem cells actively divide, replenishing tissues. Different cell populations include specific proportions of cells in different stages of the cell cycle.
8.4 Cancer - When the Cell Cycle Goes Awry
12. Apoptosis is a form of programmed cell death. Caspases carry out the destruction. Apoptosis is neat compared to necrosis. An apoptotic cell rounds up, the cell membrane forms blebs, the nuclear membrane breaks down, chromatin condenses, and DNA is cut into many equal-sized pieces.
13. Apoptosis shapes structures, and protects by getting rid of cells that could become cancerous.
8.5 Cancer-When the Cell Cycle Goes Awry
14. Cancer can result from excess cell division or deficient apoptosis. A cancer cell divides more often or more times than surrounding cells, has an altered surface, loses specialization, and divides to yield other cancer cells. A malignant tumor infiltrates nearby tissues and metastasizes if it reaches the bloodstream.
15. Cancer can result from an overexpressed oncogene or an inactivated tumor suppressor gene and may be sensitive to environmental triggers.