Genetics BIOL 2120 - Lecture Notes

I. Chromosomal Theory of Inheritance
    A. How do Mendel's laws fit into physical reality?
    B. What molecules 'carry' the information of inheritance?
    C. How do we explain segregation of alleles?
    D. How do we explain independent assortment?
II. Discovery of Chromosomes
    A. Chromosome - colored body
        1. identified when staining techniques were developed
    B. During most of cell cycle, individual chromosomes indistinguishable
    C. When coil for mitosis or meiosis, distinct chromosomes visible
    D. Banding patterns
        1. depend on dye used
        2. characteristic pattern for each chromosome
        3. helpful in identifying
        4. associated with how tightly packed the DNA is
III. Anatomy of a Chromosome
    A. Centromere
        1. attachment site for spindle fibers
        2. fibers used to move chromosomes during cell division
    B. Kinetochore
        1. structure on surface of centromere
        2. spindle fibers actually attach
        3. proteinaceous
    C. Short arm - p arm
    D. Long arm - q arm
    E. Numbering of arms based on banding patterns
IV. Identifying chromosomes
    A. Metacentric 'meta' = among; in the midst - equate with middle
    B. Submetacentric
    C. Acrocentric 'acro' = edge - equate with not at the center
    D. Subtelocentric
    E. Telocentric 'telo' = end
V. Karyotype
    A. Capture cells in mitosis - chromosomes visible
    B. Total chromosomal complement of cell
    C. Idiogram
        1. Cut out karyotype
    2. Align in homomorphic chromosome pairs
    3. Note: X and Y are heteromorphic chromosome pair
    4. Check out website to do an idiogram
            a. http://www.biology.washington.edu/karyotyping/
    D. Purpose: identify chromosomal abnormalities and sex
VI. Mitosis 'thread'
    A. Function
        1. Exact copy of each chromosome into daughter cells
        2. Location - somatic (body) cells
    B. Characteristics
        1. Diploid to diploid
        2. Daughter cells identical to mother cell
        3. One cycle
        4. One cell to two cells
        5. Nonreductional division
VII. Stages of mitosis
    A. Interphase
        1. "Between phases" - chromatin like plate of tangled spaghetti
        2. G₁ phase
            a. Acquires food
            b. Performs specialized function
            c. Differentiates
            d. Grows
        3. S phase
            a. DNA synthesis
        4. G₂ phase
            a. Committed to division
            b. Synthesis of organelles necessary for division
    B. Prophase
        1. Pro - before
        2. Chromosome condensation (visible in light microscope)
        3. Each chromosome has two chromatids (DNA synthesis in S phase)
        4. Nucleoli disappear
        5. Nuclear membrane disappears (at end of prophase)
        6. Spindle formation
            a. Microtubule organizing center near nucleus
            b. Animals: centrioles, asters, spindle apparatus
            c. Plants: only spindle apparatus; no centrioles or asters
            d. In both: movement of MOC to opposite poles
            e. Formation of spindle apparatus
        7. What holds chromatids together??
            a. Twisted DNA?
            b. Protein?
    C. Metaphase
        1. Meta - mid
        2. Chromosomes line up along equator of cell
        3. Spindle fibers run pole to pole, not attached to centrioles
    D. Anaphase
        1. Ana - back
        2. Sister chromatids pulled to opposite poles by kinetochore microtubules
            a. Kinetochore microtubules shorten
            b. Spindle microtubules elongate - push chromosomes further apart
        3. Now called 'daughter chromosomes'
        4. Identical copies of original chromosome
        5. Arms trailing (look like J's or V's)
    E. Telophase (return to interphase conditions)
        1. Telo - end
        2. Spindle disintegrates
        3. Chromosomes uncoil and elongate
        4. Nuclear envelope reforms
        5. Nucleoli reappear
        6. Cytokinesis
    F. Cytokinesis
        1. Animal
            a. Microfilaments attach to plasma membrane
            b. Pull at equator, dividing cell into two new daughter cells
            c. "Cleavage furrow"
        2. Plant
            a. Golgi complex buds off carbohydrate-filled vesicles
            b. Line up along equator, fusing together to form cell plate
VIII. Meiosis
    A. Function
        1. Reductional division
        2. Production of gametes
    B. Location - sex cells
    C. Characteristics
        1. Diploid to haploid
        2. Daughter cells differ from each other and from parent
        3. Two cycles
        4. One cell to four cells
        5. Reductional division
IX. Overview of Meiosis
    A. Homologous chromosomes contain same types of information
        1. But not identical
X. Sexual reproduction in diploid individuals
        1. If no reduction in chromosome number, ever increasing number of
            chromosomes
        2. Requires that each parent give half of chromosomes
            a. Formation of gametes
            b. Union of gametes to return to diploid zygote - grows into diploid individual
        3. Daughter cells different from each other and from parent cell
    A. Meiosis - two nuclear divisions
        1. Meiosis I "to diminish"
            a. Reduction division
            b. Each daughter receives one homologue of each pair of chromosomes
            c. Sister chromatids do not separate
            d. Cells go from diploid to haploid
        2. Meiosis II
            a. Similar to mitosis
            b. Sister chromatids split into two chromosomes
            c. Haploid cells go to haploid cells (gametes)
        3. Fusion of haploid gametes to form diploid zygote
    B. Occurs in limited number of cells in multicellular organisms
        1. Sex organs in animals
        2. Gametophyte in plants
XI. Meiosis I - reduction division
    A. Prophase I: (Note: the textbook gets bogged down in this section - the following information is CORRECT!)
        1. Leptonema
            a. Lepto - thin
            b. Tips of chromosomes attached to nuclear membrane
        2. Zygonema
            a. Zygo - yoke shaped: pairing of homologous chromosomes
            b. Synapsis - proteins zip up chromosomes
                (1) Synaptonemal complex
                (2) Each chromosome consists of two 'bivalents'
        3. Pachynema
            a. Pachy - thick
            b. Crossing over
            c. Increases genetic variability
        4. Diplonema
            a. Diplo - double
            b. Visualize sister chromatids
            c. Called tetrads at this stage
            d. Chiasmata - X shaped places where crossovers occur
            e. Rest of synaptonemal complex disappears
            f. Chiasmata help hold tetrads together
            g. Can decondense and express at this point
            h. Point at which human female reproduction stalls from fetus to puberty
        5. Diakinesis
            a. Maternal and paternal chromosomes intertwine
            b. Form chiasmata
            c. Exchange genetic material at crossovers
    B. Metaphase I
        1. Two homologues of each pair align at equator of cell
        2. Attach to kinetochore microtubules of opposite poles
        3. Sister chromatids remain together
    C. Anaphase I
        1. Homologous chromosomes separate and go to opposite poles
        2. The two clusters of chromosomes are now haploid
    D. Telophase I
        1. Most cells go immediately into meiosis II
        2. No chromatid replication between the phases
        3. Often no decondensation of chromosomes or reforming nuclear envelope
XII. Meiosis II - like mitosis
    A. Prophase II
        1. Two sister chromatids become attached to spindle fibers
        2. Extend to opposite poles
    B. Metaphase II
        1. Migration of chromosomes to equator of cell
    C. Anaphase II
        1. Centromeres split and sister chromatids are pulled to opposite poles
    D. Telophase II
        1. Cytokinesis
        2. Nuclear membrane reforms
        3. Chromosomes relax
        4. Four haploid daughter cells
XIII. Eukaryotic Life Cycles
    A. Compare and contrast mitosis and meiosis
    B. Life cycles
        1. Haploid
            a. Lower organisms, microbes and fungi
        2. Diploid
            a. Most animals
            b. Diploid to haploid; fusion of gametes to form zygote
        3. Alternation of generations
            a. Most plants
            b. Have multicellular diploid and haploid forms
XIV. Meiosis and Sexual Reproduction as Methods of Creating Genetic Variability
    A. In meiosis I, maternal and paternal homologues are shuffled and redealt
    B. How many possibilities?? 223 = 8388608 for rearranging chromosomes
    C. Crossing over creates new chromosome combinations
        1. How many genes per chromosome?? Perhaps 1000 to 106
        2. Crossovers anywhere on chromosome
    D. Combination from each of two parents -
        1. 83886082 or 7 x 1013 possibilities, even without crossovers!
        2. This is just from two individuals - unless identical twins, never two alike
XV. Chromosome Theory of Inheritance
    A. Genes are part of chromosomes
    B. Physical proof only after sex linkage was shown

For questions, comments and additional information, contact mfhicks@pstcc.edu
Last Updated: June 22, 2001
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