I. Genes.

    A. Definition.
        1. Mendel: a factor that causes a particular trait.
        2.  Current: a section of DNA that codes for a particular polypeptide chain.
        3.  Located in the chromosomes.

    B. Chromosome structure.
        1. DNA double helix.
        2. Double helix wrapped around clusters of 8 proteins called histones, like beads on a string.
        3. This bead-like chain is coiled.
        4. This coil is coiled again - a supercoil.
        5. Each leg of a chromosome is called a chromatid.
        6. When the chromosome replicates, the two copies are joined in the middle by a centromere.

II. Mitosis.

    A. Purpose: produce two new cells identical to the original.

    B. Interphase.
        1. When cell is not dividing.
        2. Chromosomes are uncoiled, invisible.
        3. Transcription is occurring.
        4. DNA replicates and duplicate chromosomes are formed.

    C. Prophase.
        1. Centrioles divide and migrate to opposite sides of the nucleus.
        2. Aster becomes visible around each centriole.
        3. Spindle forms between the centrioles - made of microtubules.
        4. Chromosomes coil up, getting short and thick.
        5. Nuclear membrane disintegrates and nucleolus disappears.

    D. Metaphase.
        1. Chromosomes line up on the equatorial plane.
        2. Sister chromatids repel each other.
        3. Spindle fibers attach to the centromeres.

    E. Anaphase.
        1. Centromeres split.
        2. Spindle fibers pull the chromatids toward either end of the cell.

    F. Telophase.
        1. Nuclear envelope reforms.
        2. Spindle disintegrates.
        3. Chromosomes uncoil.
        4. Cytoplasm divides.

III. Meiosis.

    A. Purpose: produce gametes, with half the number of chromosomes as the original.
        1. Only occurs in diploid cells.
        2. Each cell has one set of chromosomes from the father, and one set from the mother.
        3. The new cell must have a complete set of chromosomes, but only one set.
        4. The new set may be composed of some chromosomes from the father and some from the mother.

    B. First Division.
        1. At metaphase, homologous chromosomes pair up on the equatorial plate, forming a tetrad.
        2. The centromeres do not divide, and the chromatids do not separate.
        3. Instead, the homologous chromosomes separate intact, with their sister chromatids still joined at the centromere.
        4. The cell divides, but the chromosomes do not uncoil.
        5. Now each cell is haploid, but with a copy of the chromosomes.