Cell Division M4

• Cell cycle/cell division


  Cyclical process of growth and mitosis

  
• Phases of cell cycle

  • Interphase
  • M-phase

  
• Types of cellular reproduction

  • Mitosis
  • Meiosis

  
• Somatic cells


  • All body cells except sex cells
  • Contain diploid (2n) number of chromosomes
  • Undergo mitosis
• Sex cells


  • Gametes, sperm and egg cells
  • Contain haploid (n) number of chromosomes
  • Undergo mitosis and meiosis

  
• Interphase


  Preparatory stage for mitosis, not a resting stage

  
• Subdivisions of interphase


  • G1 period (pre-synthesis)
  • S period (synthesis)
  • G2 period (post-synthesis)
• G1 period


  Growth of the cell, RNA and protein synthesis, building of new protoplasm and organelles
• S period


  DNA synthesis and replication, RNA and protein synthesis, synthesis of histones and other proteins
• G2 period


  Completion of DNA synthesis and replication, continuation of RNA and protein synthesis
• Mitosis


  Somatic cell division, produces two daughter cells with the same quantity and quality of chromosomes as the parent cell

  
• Mitosis


  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase

  
• Prophase


  • Chromosomes appear as thin threads becoming shorter and thicker
  • Each chromosome is visible as two chromatids held together by the centromere
  • Chromosomes move toward the equator of the cell
  • Centrioles move to opposite sides of the cell
  • Nucleolus disappears and nuclear membrane starts to disappear
  • Mitotic apparatus (asters and spindle fibers) are nearly formed
• Metaphase


  • Double-stranded chromosomes are aligned at the equator of the cell
  • Centromeres of each chromosome are attached to the spindle fibers
  • Mitotic apparatus is completely formed
  • Nuclear membrane completely disappears
• Anaphase


  • Centromeres of each chromosome divide
  • Two sets of single-stranded chromatids (daughter chromosomes) separate and move towards opposite poles of the cell
  • Cytokinesis begins (formation of cell plate in plant cell and cleavage furrow in animal cell)

  
• Telophase


  • Daughter chromosomes reach the opposite poles of the cell
  • Chromosomes become longer, thinner, and less distinct
  • Centrioles are replicated
  • Nucleolus reappears and new nuclear membrane forms
  • Mitotic apparatus disappears
  • Cytokinesis completed resulting in two daughter cells with the same quantity and quality of chromosomes as the parent cell
• Meiosis


  Sex cell division, produces four daughter cells with haploid number of chromosomes as the parent cell
• Meiosis I


  1. Prophase I

  2. Anaphase I

  3. Telophase I

  
• Prophase I


  • Longest duration of meiosis
  • Subdivided into 5 substages: Leptonema, Zygoneme, Pachynema, Diplonema, Diakinesis

  
• Anaphase I


  • Spindle fibers pull the tetrads apart, pulling the maternal and paternal chromosomes toward opposite sides of the cell
  • The whole chromosomes from each tetrad separate and migrate toward the opposite poles of the cell
  • The centromeres of each bivalent do not divide, the chromatids (dyads) remain attached at respective centromere
• Telophase I


  • The dyads reach the poles of the cell
  • New nuclear membrane may form
  • New nucleolus may form
  • Cytokinesis completed resulting in 2 daughter cells with haploid number of chromosomes
  • End of first meiotic division

  
• Meiosis II


  1. Prophase II

  2. Metaphase II

  3. Anaphase II

  4. Telophase II

  
• Prophase II

  • Centrioles radiate spindle fibers as they move to opposite sides of the cell
  • The dyads become thicker and shorter
• Metaphase II


  • The centromeres of each dyad are directed to the equator of the cell
  • Then the centromere divide
• Anaphase II


  • Single stranded chromosomes (monads) separate and migrate towards the opposite poles of the cell
• Telophase II


  • The monads reach the poles of the cell
  • New nuclear membrane may form
  • Cytokinesis occurs resulting in daughter cells with the same haploid number of chromosomes
  • The chromosomes uncoil and become thinner and invisible again