cell division

• Cell Cycle


  The regular pattern of growth, DNA duplication, and cell division that occurs in eukaryotic cells
• Cell Cycle stages


  • Gap 1 (G1)
  • Synthesis (S)
  • Gap 2 (G2)
  • Mitosis (M)
• Interphase


  Longest part of a cell's life cycle, called the "resting stage" because the cell isn't dividing
• Interphase stages


  • Gap 1 (G1)
  • Synthesis (S)
  • Gap 2 (G2)
• G1 (Gap 1)


  Cells mature & increase in size, normal metabolic activities, must pass through a checkpoint before it can go to S phase
• S (Synthesis) Phase


  DNA is replicated (copied)
• G2 (Gap 2)


  Additional growth, has to pass a checkpoint, everything must be in order-adequate cell size, undamaged DNA- before the cell goes through mitosis/meiosis and division
• Cell Division


  Includes two processes: Mitosis and Cytokinesis
• Mitosis


  The process in which the nucleus is divided into 2 new nuclei, occurs in our somatic (body cells)
• Somatic (body) Cells


  Contains both sets of homologous chromosomes (one from each parent), diploid (2n) "two sets- 2 chromosomes at each pair- 23 pairs = 46 chromosomes total
• Chromosomes


  One long continuous thread of DNA that consists of numerous genes along with regulatory information
• Chromosomes


  • During interphase, DNA is loosely organized
  • During mitosis, chromosomes are tightly condensed
• Histones


  Proteins that DNA wraps around, forming chromatin
• As a cell progresses into mitosis, chromatin further condenses
• The left and right halves of the chromosome are two identical DNA double helixes
• Chromosome Numbers


  Each organism has a specific number of chromosomes, humans have 23 pairs (46 total)
• Human Chromosome Types


  • Autosomes (1-22)
  • Sex chromosomes (23)
• Human Sex Chromosomes


  • Male - XY
  • Female - XX
• Phases of Mitosis


  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• Prophase


  • Chromosomes become visible when they condense into sister chromatids
  • Centrioles (animal cells) move to opposite ends of cell
  • Spindle Fibers grow from the centrioles and radiate toward the center of the cell
  • Nuclear envelope breaks down, nucleolus disappears
• Metaphase


  • Spindle fibers attach to a protein structure on the centromere of each chromosome
  • Chromosomes line up in center/middle or equator of the cell moved by the spindle
• Anaphase


  • Spindle fibers attached to the centromere pull the sister chromatids apart, spindle fibers shorten
  • Chromosomes move toward opposite ends of cell, centrioles
• Telophase


  • Nuclear membrane forms at each end of the cell around the chromosomes
  • Nucleolus reform
  • Chromosomes become less tightly coiled & appear as chromatin again
  • Signifies completion of nuclear division
• In plant cell mitosis, a cell plate is formed during cytokinesis
• In animal cell mitosis, a cleavage furrow forms from outside to inside, pinching the parent cell in two
• Meiosis


  Sexual reproduction: fusion of two gametes resulting in genetically different offspring
• Gametes


  Sex cells (either egg or sperm), haploid (1n) which means "one set" of chromosomes
• Zygote


  Formed by the fusion of egg and sperm, inherits one chromosome from each parent to become diploid
• Meiosis Stages


  • Meiosis I
  • Meiosis II
• Meiosis I


  • Prophase I: Crossing over occurs
  • Metaphase I: Homologous chromosomes line up
  • Anaphase I: Homologous chromosomes separate
  • Telophase I: Cell goes through cytokinesis
• There is no DNA replication between Meiosis I and Meiosis II
• Meiosis II


  • Prophase II: Chromatids shorten and thicken, centrioles move
  • Metaphase II: Chromosomes line up
  • Anaphase II: Sister chromatids separate
  • Telophase II: Nuclear membrane forms, cell divides
• Meiosis reduces the chromosome number by half
• Gametogenesis


  Production of gametes, diploid cells undergo meiosis to form haploid gametes
• Spermatogenesis


  Haploid sperm cell is produced from a diploid primordial cell (spermatogonia)
• Oogenesis


  Haploid egg cell is produced from a diploid primordial cell (oogonia), only one of the four cells from meiosis becomes the egg
• Sperm


  • Main contribution is DNA, has a whip-like flagellum and connecting neck filled with mitochondria to enable movement
• Egg


  • More complicated process, begins at birth and continues until fertilization, contributes organelles, molecular building blocks, and other materials the embryo needs
• In many species, polar bodies do not undergo meiosis II