Lecture Outline - Chapter 5

5.1. Maintaining the Chromosome Number

1. Cell division is necessary for growth and repair of multicellular organisms, and for reproduction of all organisms.
2. When not undergoing cell division, a cell's DNA is tangled mass of thin threads called chromatin.
3. At cell division, long threads of DNA and protein called chromatin coils, loops, and condenses into highly compact structures called chromosomes.
4. Each organism usually has a characteristic number of chromosomes: humans (46), corn (20), crayfish (200), etc.
5. The full number of paired chromosomes, found in regular body cells, is called the diploid number.
6. Half the diploid number, or one chromosome from each pair, is found in the sperm or egg of animals and is called the haploid number.
7. Cell division of eukaryotes involves both nuclear division and cytokinesis (division of cytoplasm).
8. Prior to cell division, each chromosome duplicates to form two identical parts called sister chromatids attached to each other at a region called the centromere.
9. Division of centromeres releases two daughter chromosomes, one for each daughter cell.
10. What's in a Chromosome
    • a. Eukaryotic chromosome is over 50% protein.
    • b. Some proteins involved in DNA and RNA synthesis.
    • c. Large proportion of protein are histones (H1, H2A, H2B, H3, H4)
    • d. Histones package over two meters of DNA molecules in small space.
    • e. DNA double helix is wound around core of eight histones forming a bead called a nucleosome.
11. The Cell Cycle
    • a. Before 1950s, lack of chromosomal activity between cell divisions led scientists to consider this a resting state termed interphase.
    • b. Interphase is now known to include DNA replication, changing perspective to an ongoing cell cycle concept.
    • c. The M stage is the entire cell division stage, including both mitosis and cytokinesis.
    • d. The S stage is the period of DNA synthesis when replication occurs; the chromosomal proteins are also synthesized at this stage.
    • e. The G₁ stage ("G" for "gap") prior to S stage is a time the cell grows in size.
    • f. The G₂ stage prior to M stage involves metabolic events preparing for mitosis.
12. Cell Cycle Control
    • a. Some cells such as skin cells divide continuously.
    • b. Skeletal muscle cells and nerve cells are arrested in the G₁ stage.
    • c. Experiments fusing cells at different stages reveal two critical checkpoints:
      
      G₁ stage --> S stage
      G₂ stage --> M stage

      d. Activation of kinase, an enzyme that removes a phosphate group from ATP, is a method to turn on various metabolic pathways and regulate the cell cycle.

      e. Cyclin is a protein that activates kinases, but is destroyed by resultant enzymes.

      f. The cell cycle is ultimately controlled by these cyclin-dependent kinase interactions. (Fig. 5.3)

      g. Cyclins that have gone awry are one reason for cancer tumors.

1. Mitosis is nuclear division that produces two daughter nuclei with same chromosomes as the parent nucleus.
2. Before mitosis, a spindle forms to bring about an orderly distribution of chromosomes to the daughter cell nuclei; the spindle consists of fibers made of microtubules.
3. Tubulin protein dimers join and split to assemble and disassemble the microtubules.
4. Animal cells contain organizing centrosomes made of a pair of centrioles made of microtubules; plant cells lack centrioles.
5. Animal cell mitosis involves no cell wall, has centrioles present in the centrosomes, and a cleavage furrow forms to divide cells.
6. Stages of Mitosis (PMAT)
   • a. Prophase (Fig. 5.4a)
     • i. Centrosomes move toward opposing ends.
     • ii. Spindle fibers appear.
     • iii. Nuclear envelope fragments.
     • iv. Nucleolus disappears.
     • v. Chromosomes are visible.
   • b. Prometaphase (Fig. 5.4b)
     • i. Spindle develops poles, asters, and fibers.
     • ii. Centromeres of chromosomes attach to spindle fibers without orientation.
   • c. Metaphase (Fig. 5.5a)
     • i. Chromosomes attached to centromeric spindle fibers are aligned at the metaphase plate or equator of spindle.
     • ii. At close of metaphase, centromeres uniting sister chromatids split.
   • d. Anaphase (Fig. 5.5b)
     • i. Daughter chromosomes move to the opposite poles.
     • ii. Dyenin, present in microtubules in flagella, is likely motor molecule in centromeres.
     • iii. Spindle lengthens, extending distance between poles.
   • e. Telophase (Fig. 5.6)
     • i. Spindle disappears, chromosomes arrive at the poles.
     • ii. The spindle apparatus disappears as microtubules disassemble.
     • iii. New nuclear envelope forms around daughter chromosomes.
     • iv. Chromosomes diffuse into chromatin.
     • v. Nucleolus appears in each daughter nucleus.
7. Cytokinesis (cytoplasmic division)
   • a. In animal cells, a cleavage furrow forms (indentation of cell membrane) as anaphase draws to a close.
   • b. Actin filaments form a contractile ring; as the ring gets smaller, the cleavage furrow pinches the cell and forms two daughter cells. (Fig. 5.7)
8. Plant mitosis (Fig. 5.8) occurs primarily in meristematic tissue at tips of roots and stems and edge of trunk; has same stages as animal mitosis with two main differences:
   • a. Plants lack centrioles and asters; however, spindle fibers and centrosomes are present.
   • b. Rigid cell wall does not permit furrowing; instead, a cell plate is formed by Golgi-produced vesicles that fuse at equator; cellulose fibrils are added for strength later.

1. Meiosis reduces the chromosome number by half.
   • a. Used in sexual reproduction in animals; produces egg and sperm.
   • b. Gametes contain half the number of chromosomes found in other (diploid) body cells and are called haploid; human haploid number is 23, diploid is 46 or 23 pairs.
   • c. Functions to keep chromosome number constant generation after generation and to ensure next generation has a different genetic makeup.
2. Overview of Meiosis (Fig. 5.10)
   • a. Requires two nuclear divisions (meiosis I and II) and produces four haploid daughter cells.
   • b. DNA replication occurs prior to meiosis I.
   • c. During meiosis I, homologous chromosomes come together and line up side-by- side; this is called synapsis where four chromatids are in close proximity during the first two phases of meiosis I; they form a tetrad.
   • d. Crossing-over (Fig. 5.11) occurs between nonsister chromatids during synapsis and allows genetic exchange between chromosomes, providing a new combination of genes different from either parent.
   • e. All possible combinations of chromosomes can occur in gametes after meiosis.

1. Same four stages seen in mitosis (prophase, metaphase, anaphase, and telophase) occur during both meiosis I and meiosis II.
   • a. First Meiotic Division (Fig. 5.12) (p. 90)
     • i. Prophase I events are same as in prophase of mitosis except homologous chromosomes undergo synapsis and crossover.
     • ii. Metaphase I, tetrads line up at the equator.
     • iii. Anaphase I, homologous chromosomes separate.
     • iv. Telophase I may occur where the nuclear envelope reforms and nucleoli reappear; this phase may or may not be accompanied by cytokinesis.
     • v. Chromosomes are still duplicated and each has two sister chromatids.
     • vi. No replication of DNA occurs during a period called interkinesis.
   • b. Second Meiotic Division (Fig. 5.13)
     • i. Prophase II, a spindle apparatus appears and dyads attach to them independently.
     • ii. Metaphase II, chromosomes are lined up at equator; at end of metaphase, the centromeres split.
     • iii. Anaphase II, sister chromatids move toward each pole.
     • iv. Telophase II, the spindle disappears, nuclear envelope reforms, and cytokinesis occurs.
2. Oogenesis and Spermatogenesis Produce the Gametes (Fig. 5.14)
   • a. Spermatogenesis
     • i. Production of sperm in testes of males via meiosis.
     • ii. Results in four viable haploid sperm from each original cell.
     • iii. Primary spermatocyte with 46 chromosomes divides to form two secondary spermatocytes with 23 duplicated chromosomes each.
     • iv. The two secondary spermatocytes each divide to produce a total of four spermatids, with 23 single strand chromosomes each.
     • v. Spermatogenesis, once started, moves to completion and four mature sperm result.
   • b. Oogenesis
     • i. Production of eggs in ovaries of females via meiosis.
     • ii. Results in unequal cytoplasmic division producing only one functional egg and three nonfunctional polar bodies that later degenerate.
     • iii. Oogenesis does not necessarily go to completion; only if a sperm fertilizes the secondary oocyte does it undergo meiosis II and become an egg.
3. Sperm and Egg Differ
   • a. Sperm is small and flagellated; egg is stationary and quite large (extra cytoplasm provides nutrients for developing embryo).
   • b. Meiosis produces four sperm; meiosis produces one egg and three polar bodies.
4. Meiosis is Important
   • a. Without meiosis, the chromosome of sexually reproducing organisms would continually increase with each fusion of sperm and egg.
   • b. Precise division of chromosome pair ensures each daughter cell receives one of each kind of gene.
   • c. Meiosis provides for genetic recombination each generation as a result of independent assortment of chromosomes; 8,388,608 combinations are possible for humans on 23 independent chromosomes.
   • d. Crossing-over introduces additional variation in offspring; for humans, this can result in over 70 trillion different gametes from the one parent.
   • e. Fertilization combines the variation from each parent, providing over 70 trillion different zygotes without crossing over, and over 4²³ different zygotes for every couple.
   • f. Meiosis provides a sexually-reproducing organism with a tremendous storehouse of genetic recombinations for evolution.
   • g. Mitotic or asexual organisms rely on mutations for variation which is sufficient due to the huge numbers of offspring produced.