Chapter 10: Chromosomes, Mitosis, and Meiosis

Eukaryotic Chromosomes

  • Genes are informational units made of DNA.
    • In eukaryotes, DNA associates with protein to form the chromatin fibers that make up chromosomes.
  • The organization of eukaryotic DNA into chromosomes allows the DNA to be accurately replicated and sorted into daughter cells without tangling.
    • In eukaryotic cells DNA is associated with histones (basic proteins) to form nucleosomes, each of which consists of a histone bead with DNA wrapped around it.
    • Nucleosomes are organized into large, coiled loops held together by nonhistone scaffolding proteins.

The Cell Cycle and Mitosis

  • The eukaryotic cell cycle is the period from the beginning of one division to the beginning of the next.
    • The cell cycle consists of interphase and M phase.
  • Interphase consists of the first gap phase (G1), the synthesis phase (S), and the second gap phase (G2).
    • During the G1 phase, the cell grows and prepares for the S phase.
    • During the S phase, DNA and the chromosome proteins are synthesized, and chromosome duplication occurs.
    • During the G2 phase, protein synthesis increases in preparation for cell division.
  • M phase consists of mitosis, the nuclear division that produces two nuclei identical to the parental nucleus, and cytokinesis, the division of the cytoplasm to yield two daughter cells.
  • A duplicated chromosome consists of a pair of sister chromatids, which contain identical DNA sequences.
    • Each chromatid includes a constricted region called a centromere.
    • Sister chromatids are tightly associated in the region of their centromeres.
    • Attached to each centromere is a kinetochore, a protein structure to which microtubules can bind.
  • Mitosis assures that the chromosome number is preserved when one eukaryotic cell divides to form two.
    • In mitosis, identical chromosomes are distributed to each pole of the cell, and a nuclear envelope forms around each set.
  • During prophase, the structure of the duplicated chromosomes becomes apparent as the chromatin condenses; each is composed of a pair of identical sister chromatids.
    • The nuclear envelope begins to disassemble, and the mitotic spindle begins to form.
  • During prometaphase, spindle microtubules attach to kinetochores of chromosomes, and chromosomes begin to move toward the cell’s midplane.
  • During metaphase, the chromosomes are aligned on the cell’s midplane, or metaphase plate; the mitotic spindle is complete, and the kinetochores of the sister chromatids are attached by microtubules to opposite poles of the cell.
  • During anaphase, the sister chromatids separate and move to opposite poles.
    • Each former chromatid is now a chromosome.
  • During telophase, a nuclear envelope re-forms around each set of chromosomes, nucleoli become apparent, the chromosomes uncoil, and the spindle disappears.
    • Cytokinesis generally begins in telophase.

Regulation of the Cell Cycle

  • Control mechanisms, called cell-cycle checkpoints, temporarily block key events from being initiated during the cell cycle.
    • Cyclin-dependent kinases (Cdks) are protein kinases involved in regulating the cell cycle.
    • Cdks are active only when they bind tightly to regulatory proteins called cyclins.
    • Cyclin levels fluctuate predictably during the cell cycle.

Sexual Reproduction and Meiosis

  • Offspring produced by asexual reproduction usually have hereditary traits identical to those of the single parent.
    • Mitosis is the basis for asexual reproduction in eukaryotic organisms.
  • In sexual reproduction two haploid sex cells, or gametes, fuse to form a single diploid zygote.
    • In a sexual life cycle, meiosis must occur before gametes can be produced.
  • A diploid cell has a characteristic number of chromosome pairs per cell.
    • The members of each pair, called homologous chromosomes, are similar in length, shape, and other features and carry genes affecting the same kinds of attributes of the organism.
  • A haploid cell contains only one member of each homologous chromosome pair.
  • A diploid cell undergoing meiosis completes two successive cell divisions, yielding four haploid cells.
    • Sexual life cycles in eukaryotes require meiosis, which makes it possible for each gamete to contain only half the number of chromosomes in the parent cell.
  • Meiosis I begins with prophase I, in which the members of a homologous pair of chromosomes physically join by the process of synapsis.
    • Crossing-over is a process of genetic recombination during which homologous (nonsister) chromatids exchange segments of DNA strands.
  • At metaphase I, tetrads—each consisting of a pair of homologous chromosomes held together by one or more chiasmata— line up on the metaphase plate.
    • The members of each pair of homologous chromosomes separate during meiotic anaphase I and are distributed to different nuclei.
    • Each nucleus contains the haploid number of chromosomes; each chromosome consists of two chromatids.
  • During meiosis II, the two chromatids of each chromosome separate, and one is distributed to each daughter cell.
    • Each former chromatid is now a chromosome.
  • Mitosis involves a single nuclear division in which the two daughter cells formed are genetically identical to each other and to the original cell.
    • Synapsis of homologous chromosomes does not occur during mitosis.
  • Meiosis involves two successive nuclear divisions and forms four haploid cells.
    • Synapsis of homologous chromosomes occurs during prophase I of meiosis.

Sexual Life Cycles

  • Somatic cells of animals are diploid and are produced by mitosis.
  • The only haploid cells are gametes, produced by gametogenesis, which in animals occurs by meiosis.
  • Most fungi and many protists are haploid and are produced by mitosis.
    • The only diploid stage is the zygote, which undergoes meiosis to restore the haploid state.
  • The life cycle of plants, some algae, and some fungi includes an alternation of generations.
    • The multicellular diploid sporophyte generation forms haploid spores by meiosis.
    • Each spore divides mitotically to form a multicellular haploid gametophyte generation, which produces gametes by mitosis.
    • Two haploid gametes then fuse to form a diploid zygote, which divides mitotically to produce a new sporophyte generation.