Ch. 16

• Genotype


  The Genetic makeup of an organism
• Diploid


  Cells containing two sets of chromosomes
• All of our cells are diploid, the only exception are the gametes
• Karyotype


  • Reveals number, size, and form of chromosomes in an actively dividing cell
• Humans have 23 pairs of chromosomes (or 46 total chromosomes)
• Chromosome types in humans

  • Autosomes (22 pairs)
  • Sex chromosomes (1 pair - XX or XY)
• Females have 2 X chromosomes. Males have X and Y chromosomes


  Y determines the gender
• Haploid cells


  • Gametes in plants and animals
  • Egg or sperm
• Cell cycle


  1. Interphase (G1, S, G2)

  2. Mitosis

  3. Cytokinesis
• Interphase


  The resting stage of the cell cycle
• G1 Stage


  Cell Grows
• S phase


  DNA replication/synthesis takes place. DNA content doubles. Each chromosome makes an identical copy of itself by process of replication. The chromosome appears as an X. Each line in the X is called sister chromatids.
• Sister chromatids


  Two identical copies with associated proteins. Chromatids tightly associated at centromere.
• G2 Stage


  Cell synthesizes proteins needed by performing mitosis and cytokinesis. The number of cell organelles double during this phase.
• Mitosis


  1. Nuclear division (Prophase, Metaphase, Anaphase, Telophase)

  2. Cytokinesis
• Prometaphase


  • Nuclear envelope completely fragments
  • Mitotic spindle is fully formed
  • Centrosomes move apart and demarcate the two poles
  • Spindle fibers interact with sister chromatids
  • Two kinetochores on each pair of sister chromatids are attached to kinetochore microtubules from opposite poles
• Metaphase


  • Pairs of sister chromatids are aligned along a plane halfway between the poles called the metaphase plate
  • Organized into a single row
• Anaphase


  • Connections broken between sister chromatids
  • Each individual chromatid is linked to only one pole by kinetochore microtubules
  • Kinetochore microtubules shorten, pulling chromosomes toward the pole to which they are attached
  • The two poles move away from each other as overlapping polar microtubules lengthen and push against each other
  • Chromosomes pulled apart
• Telophase


  • Chromosomes have reached their respective poles and decondense
  • Nuclear membranes now re-form to produce two separate nuclei
• Cytokinesis


  1. In animals - cleavage furrow constricts like a drawstring to separate the cells

  2. In plants - cell plate forms a cell wall between the two daughter cells
• After cytokinesis 2 identical daughter cells having the same number of chromosomes are formed
• Mitosis


  The dividing stage of the cell cycle. It has two substages which are Nuclear division and cytoplasmic division.
• In Mitosis from one cell, we get 2 cells. These cells are identical and have the same chromosome number. Mitosis is needed for Growth and repair. In some unicellular organisms it can be used for reproduction (asexual) as well. This type of division occurs throughout the lifetime of an organism.
• Meiosis


  The process by which haploid cells are produced from a cell that was originally diploid. We get 4 different cells from one cell. This type of division happens in reproductive tissues like testis and ovaries or in flowers in plants.
• Example: Diploid human cell has 46 chromosomes, but after meiosis the haploid egg or sperm has 23 chromosomes
• Meiosis


  1. Meiosis I

  2. Meiosis II
• Meiosis vs Mitosis


  • Homologous pairs form a bivalent or tetrad
  • Crossing over
• Meiosis is a different type of cell division. Here we get 4 cells from one cell. Cells formed by meiosis are not identical. They are unique and different from the parent cell. This type of division happens in reproductive tissues like testis and ovaries or in flowers in plants. It has two cycles of division. During first division, 2 cells are formed and the chromosome number goes by half. It has same stages as that of Mitosis. Here in the prophase (prophase1) crossing over takes place in the homologous chromosomes that makes them different from the parental chromosomes. Due to crossing over and the independent assortment of chromosomes during metaphase, the cells that are formed after meiosis I division become unique. These two events, crossing over and independent assortment make the cells unique. During the meiosis II division the sister chromatids separate and the two cells divide into 4. Meiosis II is identical to that of Mitosis.
• Main function of Meiosis

  Sexual reproduction that leads to the biodiversity in the nature. This type of division occurs only in reproductive phase/time/period of the organism.
• Nondisjunction


  Unequal distribution of chromosomes in meiosis. Some cells will have extra, some will not have enough chromosomes. Chromosomes do not sort properly during cell division. During meiosis can produce aneuploid gametes (too many or too few chromosomes).
• What's Phenotype?


  Appearance of an organism.
• Karyotype: the number and visual appearance of the chromosomes in the cell nuclei of an organism or species.
• Meiosis happen in the reproductive tissue of organisms. The ovaries and testies of animals, and flower of plants. Will produce haploid cells.
• Resting stage of the cell cycle is the interphase.
• Replicate happens in the S phase of the cell cycle.
• Sister Chromatids are identical.
• Homologous chromosomes are NOT identical.
• Point of interception is called the centromere! Diagram will be given on exam.
• During G2 Phase, proteins for cell division are made and the number of cell organelles double.
• Prophase(Mitosis): Chromosomes can be seen in the cytoplasm.