chapter 09

Prokaryotes reproduce by DNA followed by fission.
- • Binary fission.
  To reproduce, a prokaryotic cell initiates replication of its circular DNA at the origin of replication, copying the DNA in both directions to yield two DNA molecules.
  The cell elongates and grows a new membrane between the DNA molecules, then undergoes binary fission and divides into two "daughter" cells.
  The new cells are genetically identical to the "parent" cell.
  Exercise:
Eukaryotic DNA is packaged into composed of pairs: one chromosome on each pair is inherited from each parent.
- • Human karyotype.
  Each species has a specific number of chromosomes with sizes and shapes that can be arranged in a karyotype.
  Human somatic cells (cells other than sex cells) have 46 chromosomes.
  44 of the 46 are autosomes that occur in homologous pairs: each member of the pair have the same genes, inherited from the parents.
  The other 2 chromosomes are sex chromosomes: females have 2 X chromosomes; males have 1 X and 1 Y chromosome.
  Thus somatic cells are diploid: chromosomes occur in pairs. One in each pair is inherited from the mother, the other from the father.
Each chromosome may consist of sister attached by a .
- • Homologous chromosomes occur in pairs; one chromosome on each pair is inherited from each parent. Before a cell divides, each chromosome replicates its DNA molecule to produce 2 sister chromatids held together by a centromere.
  Exercise:
There are two type of eukaryotic cell division: , which occurs in cells, and , which occurs in cells.
- • Sexual life cycle.
  Germ-line cells (2n = diploid) are produced in organs called gonads (ovaries and testes).
  These cells undergo meiosis to form gametes (eggs and sperm, n = haploid).
  The rest of the diploid body cells are called somatic cells and reproduce by mitosis.
  Fertilization from the fusion of gametes creates the diploid zygote, which undergoes many rounds of mitosis to produce the organism.
Mitosis is part of a cell cycle, where cell growth and DNA occurs during an , when chromosomes are extended and not easily visible.
- • Cell cycle. The cell prepares to divide during Interphase, which is composed of 3 sub-phases:
  G₁ phase: primary growth; S phase: DNA synthesis (replication); G₂ phase: microtubule synthesis.
  During mitosis (M phase), the nucleus divides. Then the cytoplasm divides during cytokinesis (C phase).
  Review and mitosis preview:
Cell division takes place in two steps: nuclear division occurs during and cytoplasmic division occurs during .
- • Cell division.
  1. interphase
  Mitosis:
  1. prophase
  2. metaphase
  3. anaphase
  4. telophase
  Cytokinesis:
  1. division of cytoplasm
  Mitosis and cytokinesis result in two genetically identical "daughter" cells.
  Review:
- • Interphase.
  Cell grows and makes copies of its components.
  DNA replicates and condenses.
  Centrioles (in animal cells) also replicate. Microtubule subunits are synthesized.
  The cell prepares for division.
- • Prophase.
  The nuclear envelope breaks down.
  DNA condenses into visible chromosomes.
  A network of protein cables called the spindle forms from microtubule subunits.
- • Metaphase.
  The chromosomes align along the equatorial (metaphase) plane (plate) in the center of the cell.
  Spindle fibers attach to kinetochores on opposite sides of the centromeres.
- • Anaphase.
  The centromeres replicate.
  Sister chromatids separate and move to opposite poles by shortening of the microtubules attached to them.
  Each chromatid is now a full-fledged chromosome.
- • Telophase.
  Spindle apparatus is disassembled.
  The chromosomes de-condense.
  The nuclear envelope reappears.
  Mitosis is complete and the cell enters cytokinesis.
- • Cytokinesis.
  The cytoplasm divides and two "daughter" cells form.
  Each cell is a replicate of the "parent" cell and is diploid.
- • Cytokinesis.
  1. In animal cells, a cleavage furrow forms, pinching the cell in two.
  2. In plant cells, a cell plate grows between the "daughter" cells, making a new plasma membrane.
    Then new cell walls made of cellulose are formed along the new membrane.
- • Interphase.
  Cell grows and makes copies of its components.
  DNA replicates and condenses.
  Centrioles (in animal cells) also replicate. Microtubule subunits are synthesized.
  The cell prepares for division.
- • Prophase.
  The nuclear envelope breaks down.
  DNA condenses into visible chromosomes.
  A network of protein cables called the spindle forms from microtubule subunits.
- • Metaphase.
  The chromosomes align along the equatorial (metaphase) plane (plate) in the center of the cell.
  Spindle fibers attach to kinetochores on opposite sides of the centromeres.
- • Anaphase.
  The centromeres replicate.
  Sister chromatids separate and move to opposite poles by shortening of the microtubules attached to them.
  Each chromatid is now a full-fledged chromosome.
- • Telophase.
  Spindle apparatus is disassembled.
  The chromosomes de-condense.
  The nuclear envelope reappears.
  Mitosis is complete and the cell enters cytokinesis.
- • Cytokinesis.
  The cytoplasm divides and two "daughter" cells form.
  Each cell is a replicate of the "parent" cell and is diploid.
- • Cytokinesis.
  1. In animal cells, a cleavage furrow forms, pinching the cell in two.
  2. In plant cells, a cell plate grows between the "daughter" cells, making a new plasma membrane.
    Then new cell walls made of cellulose are formed along the new membrane.