cell cycle

• Reproduction

  Every living thing undergoes reproduction
• The nutrients taken by an individual provide energy for metabolic processes, growth, development, and reproduction
• Cell division

  Offers the backdrop for the organismal level of reproduction
• Cell division

  Chromosomes of a cell change form as the cell transitions from one stage to another in a typical cell cycle
• Stages of the cell cycle

  • Interphase
  • Cell division phase
• Interphase

  The period that follows one cell division and precedes another; the cell grows and the chromosome produces an exact copy of itself
• Interphase

  Divided into three sub stages: G1, S1, G2
• G1 stage


  From the formation of a new cell until it begins to replicate its DNA; characterized by protein and RNA synthesis
• S1 stage


  The period of DNA synthesis or replication
• G2 stage


  Period of rapid cell growth to prepare for cell division
• During interphase, the nucleus is visible as a distinct membrane-bound organelle, and chromosomes appear as an irregular mass that is grainy in appearance
• Mitosis


  The process in which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two daughter nuclei
• Mitosis is followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles, and cell membrane into two daughter cells containing roughly equal shares of these cellular components
• Mitosis and cytokinesis together define the mitotic phase of the cell cycle
• Mitosis is complex and highly regulated, divided into phases: prophase, prometaphase, metaphase, anaphase, and telophase
• During mitosis, pairs of chromosomes condense and attach to fibers that pull the sister chromatids to opposite sides of the cell, followed by cytokinesis to produce two identical daughter cells
• Meiosis


  The first of the two separate divisions during which the diploid cell separates into two diploid cells; genetic variation is created by recombination
• Meiosis is often called the reduction division
• Meiosis

  1. First of the two separate divisions during which the diploid cell separates into two diploid cells

  2. Genetic variation is created by recombination

  3. Often called the reduction division as the chromosome complement is reduced from diploid to haploid
• Interphase in meiosis

  • Identical to interphase in mitosis
  • No way to determine the type of division the cell will undergo when it divides
• Meiotic division

  • Occurs only in cells associated with male or female sex organs
• Prophase I

  Similar to prophase in mitosis involving appearance of chromosomes, development of spindle apparatus, and breakdown of nuclear membrane
• Metaphase I

  1. Chromosome pairs are aligned on either side of the metaphase plate

  2. Chromatid arms may overlap and temporarily fuse, resulting in crossovers
• Anaphase I

  Spindle fibers contract, pulling homologous pairs away from each other and toward each pole of the cell
• Telophase I

  1. Cleavage furrow forms, followed by cytokinesis

  2. Each daughter cell has a single set of chromosomes, half the total number in the original cell
• Meiosis II

  1. Two diploid cells are split into four haploid cells during the second set of stages of meiosis

  2. A mitotic division of each of the haploid cells produced in Meiosis I

  3. No Interphase between Meiosis I and Meiosis II
• Prophase II

  New set of spindle fibers forms, chromosomes begin to move toward the equator of the cell
• Metaphase II


  All the chromosomes in the two cells align with the metaphase plate
• Anaphase II


  Centromeres split, spindle fibers shorten, drawing the chromosomes toward each pole of the cell
• Telophase II

  1. Cleavage furrow develops, followed by cytokinesis and formation of the nuclear membrane

  2. Chromosomes begin to fade and are replaced by granular chromatin
• After Meiosis II, there will be a total of four daughter cells, each with half the total number of chromosomes as the original cell
• In male structures

  All four cells will eventually develop into sperm cells
• In female life cycles in higher organisms

  Three of the cells will typically abort, leaving a single cell to develop into an egg cell, which is much larger than a sperm cell