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Fundamentals of Biology

Lesson 16

Advanced Genetics, Plant Physiology



I. Plant Hormone Experiments.

    A. Charles and Francis Darwin.
        1. 1880, The Power of Movement in Plants.
        2. Grew oat seeds on the windowsill.
        3. Covered the tip of some of the coleoptiles.
            (A coleoptile is the sheath covering the embryonic shoot.)
            a. Some uncovered.
            b. Some covered with glass sheath.
            c. Some covered with a black cap.
            d. Cut some tips cut off.
        4. The coleoptiles covered in black or with the tips removed did not turn toward the light.
        5. Conclusion:
            a. Only the coleoptile tip can respond to light.
            b. The tip transmits a signal or produces a substance which causes the stem to turn toward the light.

    B. Fritz W. Went.
        1. Mid 1920's.
        2. Reasoned that a signal must move from the tip of a seedling to the stem.
        3. Cut off the tips of oat seedlings and placed on agar.
        4. Results of seedlings with tips removed:
            a. No further growth without the tip.
            b. An agar block exposed to a tip caused growth to continue.
            c. An agar block exposed to a tip placed on the side of the tip caused the seedling to turn away from the block.
            d. An agar block not exposed to a tip did not cause the seedling to resume growing.
        5. Conclusions:
            a. The coleoptile tip contains a diffusible substance capable of stimulating growth in the lower regions of the seedling.
            b. Light affects the transmission of this substance. The side away from the light receives more of it and therefore grows faster.

II. Auxins.

    A. Promote growth by cell elongation.

    B. Inhibit axillary bud development.
        1. Remove the main shoot, axillary buds grow.
        2. Put auxin paste on the cut - the main shoot will continue growing and the axillary shoots will stop.

    C. Speed fruit development.
        1. Produced by the developing embryo.
        2. The amount of auxin often determines the ultimate size of the fruit.

    D. Gravitropism.
        1. Cause stem to grow up.
        2. Cause roots to grow down.

    E. Inhibit the process of abscission.
        1. When the leaf ceases sending auxin to the stem, the abscission layer begins to form.
        2. When the cell walls in this area break down, it becomes a weak spot and the leaf can easily blow off.

III. Tropisms.

    A. Phototropism: response to light.
    B. Geotropism: response to gravity.
    C. Thigmotropism: response to touch.
    D. Chemotropism: response to chemicals (may not be a true tropism).

IV. Gibberelins.

    A. Increase both the size and the number of cells.
    B. Over 70 gibberellins have been isolated.
    C. Dwarf plants grow to normal height when treated with gibberellins.
    D. Induce internodal stem elongation in cabbage - 10 ft. plants.
    E. Promote fruit enlargement in some plants.

V. Abscisic Acid.

    A. Suppresses plant growth.
        1. Promotes dormancy.
        2. Contributes to abscission of leaves, fruits, and flowers.

    B. Specific effects
        1. Stimulates buds to produce tough outer leaves to protect the bud during the winter.
        2. Increases in drought which causes the closure of the stomata.
        3. Suppresses root and shoot elongation in the embryo.

VI. Aberrations in the number of chromosomes.

    A. Terms.
        1. Genome: a single complete set of genes (and therefore of chromosomes).
        2. Parthenogenesis: development of an unfertilized egg into an adult.

    B. Euploidy: the addition or loss of an entire genome.
        1. Haploidy.
            a. Created by parthenogenesis.
            b. Organisms smaller and weaker than normal.
        2. Polyploidy: 3 or more genomes.
        3. Triploidy.
            a. A diploid gamete fertilized by a haploid gamete.
            b. The odd number of homologs makes meiosis impossible, so these are sterile.
            c. No such animals.
            d. Some plants exist, but must be reproduced vegetatively.
        4. Tetraploidy.
            a. Human liver cells.
            b. Irish white potato.
            c. American upland cotton: tetraploid of 2 different genomes.

    C. Aneuploidy.
        1. Have too many or too few of a particular chromosome.
        2. Occurs during meiosis.
            a. One pair of chromosomes fails to separate (nondisjunction).
            b. One gamete has an extra chromosome, the other has none.
        3. Usually lethal in animals.
        4. Down’s Syndrome is one nonlethal case - Trisomy 21.
        5. Aneuploidy is common in plants.

VII. Aberrations within the chromosomes.

    A. Crossing over.
        1. Exchange of a piece of a chromatid with another in homologous chromosomes.
        2. Harmless if whole genes are transferred.
        3. Results in greater genetic variation.

    B. Translocation.
        1. Exchange of a piece of a chromatid with another in nonhomologous chromosomes.
        2. Some gametes will be normal, some will lack genes, and others will have extra genes.

VIII. Mutations.

    A. Alteration of the DNA.

    B. Types of mutations.
        1. Substitution: one nucleotide is substituted for another.
        2. Addition: an extra nucleotide in inserted in the gene.
        3. Deletion: a nucleotide is lost from the gene.

    C. Molecular Effects:
        1. No effect.
            a. The DNA code is degenerative.
            b. Three nucleotides can be arranged in 64 (43) ways.
            c. But there are only 21 amino acids.
            d. Thus some amino acids are coded by several different codons. In fact, only two amino acids are coded by only one codon.
            e. If the substitution creates a codon for the same amino acid, there will be no change in the polypeptide produced,
                and thus no change in the effect on the organism.
        2. Minor effects: The protein is only slightly different, or is of minor importance to the organism.
        3. Major effects: The gene no longer codes for a real protein, or it codes for a different protein, which can be lethal to the organism.

    D. Biological Effects.
        1. Somatic mutations in mature organisms.
            a. Produce an odd protein.
                i. In a diploid organism, the other allele will produce the correct protein.
                ii. The odd protein may decompose or be expelled as waste.
            b. No effect, because the gene never gets turned on (is never used in the cell where the mutation occurred).
            c. Kill the cell; but the loss of one cell is inconsequential.
        2. Somatic mutations in developing organisms.
            a. Killing one cell may result in part of the organism developing improperly.
            b. As the organism differentiates, some of the cells may form tissue that will need the protein.
        3. Germ mutations (in the sex cells).
            a. A mutation here will affect every cell in the new organism, including its sex cells.
            b. Thus these are far more significant.

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