Chapter Five: Cell Reproduction

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Chapter outline

1. Organization of DNA
   1. A cell's DNA is not simply cut in half during reproduction, because then each daughter cell would receive only half of the genetic code.
   2. DNA in prokaryotes
      1. In prokaryotes, the DNA is arranged in a large circle.
      2. The DNA is not wrapped around protein clusters called histones as it is in eukaryotes.
   3. DNA in eukaryotes
      1. In eukaryotes, the DNA is linear, and it is wrapped around histones.
      2. When a eukaryote is not dividing, its DNA appears as a large mass called chromatin.
      3. As the eukaryote prepares to divide, the chromatin condenses into structures called sister chromatids, attached together at a region called the centromere.
2. Prokaryotic cell division
   1. Division in prokaryotes is simpler than in eukaryotes because prokaryotes have shorter DNA and do not have many organelles.
   2. Before the cell divides, the DNA replicates, and each copy attaches to a point on the cell membrane.
   3. As the cell expands before dividing, each copy of DNA is pulled toward one side of the cell.
   4. When the cell is about double its original size, the cell membrane pinches inward in the middle of the cell, forming two new cells.
3. The cell cycle
   1. Mitosis
      1. The division of a cell's nucleus and its DNA.
      2. This stage is much shorter than the other stage of the cell cycle, interphase, which occurs between each cell division.
   2. Interphase
      1. G₁ phase
         1. The G₁ phase is a period of growth which follows a cell division.
         2. The cell grows to normal size and synthesizes new organelles.
      2. S phase
         1. The cell's DNA is replicated in preparation for a division.
         2. It is currently unknown exactly what causes the transition between the G₁ phase and the S phase.
      3. G₂ phase
         1. The cell synthesizes the structures required for cell division.
         2. After the G₂ phase, the cell undergoes mitosis.
4. Mitosis
   1. Mitosis is the process through which a cell gives each of its daughter cells identical copies of its DNA.
   2. Prophase
      1. The chromatin in the nucleus condenses into sister chromatid structures.
      2. The nuclear membrane and the nucleolus disappear.
      3. The centrioles begin moving toward opposite ends of the nucleus, and the spindle fibers begin to form between them.
   3. Metaphase
      1. The spindle fibers attach to the centromeres of the sister chromatid pairs.
      2. The sister chromatids are pulled to the center of the cell.
   4. Anaphase
      1. The centromeres of all of the sister chromatids break simultaneously. Each chromatid is now called a chromosome.
      2. The spindle fibers pull one chromosome from each pair toward opposite ends of the cell.
   5. Telophase
      1. The spindle fibers break apart.
      2. New nuclear membranes form around each set of chromosomes, and nucleoli form.
      3. The chromosomes begin to disperse back into the mass of chromatin.
      4. Each centriole replicates so that each daughter cell receives two centrioles.
5. Cytokinesis
   1. Cytokinesis is the division of the cytoplasm into nearly equal halves. It begins in telophase.
   2. Cytokinesis in cells without a cell wall
      1. The cell membrane begins to pinch inward, caused by a ring of contractile proteins called actin and myosin.
      2. The groove formed by the pinching inward is called the cleavage furrow.
      3. When the two sides of the furrow meet, the cell splits into two daughter cells.
   3. Cytokinesis in cells with a cell wall
      1. Vesicles from the Golgi bodies fuse in the center of the cell, forming a structure called the cell plate.
      2. The cell plate is built outward as more vesicles are added.
      3. When the cell plate reaches the cell membrane, the cell has been divided into two daughter cells.
      4. The cell plate is used as a frame to build a cell wall for each daughter cell.

Terms to know

• anaphase - The third stage of mitosis during which all of the sister chromatid pairs break simultaneously and are tugged toward opposite ends of the cell by the spindle fibers.
• cell cycle - A description of the general stages of life of a eukaryotic cell. It is divided into mitosis and interphase.
• cell plate - A structure made of flattened vesicles which is built from the center toward the cell membrane during cytokinesis in cells which have a cell wall.
• centrioles - Two structures which, during mitosis, move to opposite ends of the cell and direct the action of the spindle fibers.
• centromere - A region at which a pair of sister chromatids are attached to one another.
• chromatin - The organization of a eukaroytic cell's DNA when it is not dividing. Chromatin is simply a large, dense mass of DNA.
• chromosome - A term which refers to each half of the sister chromatids after they split during mitosis.
• cleavage furrow - The deep groove formed when the cell membrane pinches inward during cytokinesis in cells without a cell wall.
• cytokinesis - The division of the cytoplasm after mitosis, resulting in an approximately equal distribution of organelles in each of the daughter cells.
• DNA - Deoxyribonucleic acid. DNA is a long molecule composed of deoxyribose, phosphate groups, and nitrogenous bases which indirectly dictates the production of proteins in a cell.
• interphase - The stage of the cell cycle between each cell division. Interphase is divided into three phases: the G₁ phase, the S phase, and the G₂ phase.
• metaphase - The second stage of mitosis during which the spindle fibers attach to the kinetochore of each sister chromatid structure and pull them to the center of the cell.
• mitosis - The process by which a cell's DNA is copied and then distributed so that each daughter cell receives an identical copy of the original DNA.
• prophase - The first stage of mitosis, during which the nuclear membrane and nucleolus disappear, the chromatin condenses into sister chromatid structures, the centrioles begin to move apart, and the spindle fibers begint to form.
• sister chromatids - The individual copies of portions of the DNA molecule formed after the chromatin condenses.
• spindle fibers - The structures which, during mitosis, direct the movement of the chromosomes for proper cell division.
• telophase - The final stage of mitosis during which the spindle fibers break apart, new membranes begin to form around each set of chromosomes, the nuclear membrane and nucleoli reappear, and the chromosomes begin to disperse back into chromatin.