Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 9

Cell division 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.
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). The cell cycle has checkpoints to ensure the appropriate sequence of events.
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 special checkpoint that sets a cell's fate---to divide, stay specialized but not divide, or die.
Microtubules synthesized during S phase assemble to build the mitotic spindle. It consists of microtubules that aggregate to form a centriole, enclosed in a centrosome. Astral rays extend from the centriole. Chromosomes attach to the mitotic spindle with microtubule assemblies called kinetochores.
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.
Mitosis consists of four stages. In prophase, the chromosomes condense and become visible when stained, the nuclear membrane disassembles, and the mitotic spindle forms. 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.
In cytokinesis in an animal cell, a cleavage furrow forms at the site of astral rays, marking formation of a band of microfilaments that will draw the two progeny cells apart. In plant cells, a phragmoplast provides space for a new cell wall to be laid down, separating the progeny cells. Cytokinesis usually begins during anaphase or telophase. Lack of cytokinesis results in a huge cell with many nuclei.
Human cells usually divide up to 50 times. 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 somatic cells retain long telomeres and divide continually.
Crowding prevents normal cells from dividing. Extracellular signals (hormones and growth factors) and intracellular signals (cyclins and kinases) control cell division rate and number.
Apoptosis is a form of programmed cell death. It selects certain cells to die during prenatal development, particularly in the thymus and brain. 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.
Stem cells actively divide, replenishing tissues. Different cell populations include specific proportions of cells in different stages of the cell cycle.
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, lacks specialization, and divides to yield other cancer cells. A malignant tumor infiltrates nearby tissues and metastasizes if it reaches the bloodstream.
Cancer can result from an overexpressed oncogene or an inactivated tumor suppressor gene and may be sensitive to environmental triggers.