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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