Genetics Lectures 19–23.doc

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22 Apr 2012
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Lecture 19 – Large-scale chromosomal changes in
plants
View of a chromosome:
3 parts to it: p, q, and a centromere that connects the short and long
arm.
P = petit
Q = next letter
How to visualize chromosomes:
1. Take a mammalian blood sample. Centrifuge the sample to
separate the white blood cell (nucleated) from the remaining
blood. Put the white cells in culture. Most normal mammalian cell
will die eventually in culture.
2. Stimulate the cells to divide in vitro by giving growth factor (fetal
bovine serum). Stop them when they are dividing. Stop the mitotic
spindles with drugs (Taxol), and the cells will be stuck in mitosis.
3. Add hypotonic solution to expand the cells, and then squash the
cells on he slide, fix them, and stain.
4. Using a radioactive or fluorescent biotin probe, one can see the
location of the probes on the chromosomes.
Ploidy: the number of chromosomes.
For an organism or a cell:
euploid : complete normal sets of chromosomes
aneuploid: chromosome is over- or under-represented (trisomy;
genetic imbalance)
polyploid: extra set of chromosomes (plants 3n, 4n, 5n)
diploid : two sets of chromosomes (human, mammals, n=23)
tetraploids: four sets of chromosomes
Rearrangements: an abnormal chromosome (deletion,
translocation, duplication)
- Some species exist in monoploidy (male bees, wasps and ants)
-Monosomic = one less chromosome, NOT one less set. Like a
diploid - 1 chromosome.
-Trisomic = cell will have an EXTRA chromosome. Increase the
total number of chromosomes by 1.
- Humans have 46 chromosomes to start with, so can have 45 or 47
(or 48, 49, 50).
Polyploidy: Very common in plants but rarer in animals
Autopolyploids: multiple chromosome set from within one species
(most triploids)
Allopolyploids: have chromosome sets from two or more species
Sterility in a triploid: Meiosis pairing in a triploid
They have an extra chromosome that cannot pair with the other.
Meiosis will generate three things: trivalent or bivalent and
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univalent.
Almost impossible for a triploid organism to produce gametes with
an equal number of chromosomes. For this reason, TRIPLOID
PLANTS ARE STERILE.
Meiosis uncertainty in Tetraploidy:
Tetraploidy is less intense, but can still have univalent and
trivalent segregation. Trivalent and Univalent cells will be
sterile.
What will drive the migration of the chromosomes in the daughter
cells is the centromere! Makes the chromosomes migrate.
If the segregation is equal (lined as two bivalent or one
quadravalent), it will still be fertile with diploid gametes!
Most tetraploid plants will generate unfertile gametes, but some
(like the bread wheat and Raphanobrassica) are fertile!
Allopolyploidy: The wonder of hybridization
Radish are 2n (diploid) = 18 chromosomes, so the gametes will have 9
chromosomes each.
Fuse 9 and 9 from cabbage (diploid 2n=18) together and make 2n=18.
But it will be sterile! It won't exit meiosis because it won't segregate the
chromosomes properly.
Spontaneous doubling of the chromosomes (4n = 36) causes
amphidiploidy. These plants WERE fertile. This is because although they
have the same number of chromosomes and although they were close
enough to make an individual from the two gametes, all the
chromosomes from the radish could not pair with the chromosomes
from the cabbage. The pairing was not happening, so the chromosome
would not segregate one from the other during meiosis.
A tetraploid is more of a double diploid this can make a fertile
amphidiploid.
Origin of hexaploid wheat by sequential bybridization of
different species :
Wheat is a hexaploid plant! It took many years do this. AA and BB
gametes from two diploid species that fuse. They needed to be a double
diploid (AABB) for it to be fertile (a tetraploid individual).
Plants are quite tolerant to the duplication of chromosomes.
Many years later, AABB individual was mixed with another plant and this
became an AA BB DD plant. ABD was sterile until the duplication the
chromosomes (through a sterile intermediate A B C D). The wheat we
have now is hexaploid.
In cell culture, the use of colchicine allows the generation of tetraploid
cells:
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- It disrupts spindle fiber formation and leaves the chromosomes
unsegregated. We can generate the tetraploid cells, which are
useful for studying different genetic traits.
- Tetraploid grapes are twice as big.
Generation of monoploid plants:
Plant breeders have generated monoploid plants to select favorable
recessive mutations. (Want to keep these recessive mutations.) These
are easier to keep in a monoploid plant than a diploid plant (where they
have to be homozygous recessive).
1. Take a diploid plant and plate the immature pollen cells.
Bring them to have a cold treatment in a fridge.
2. Under cold treatment, the cold cells well start growing as
EMBRYOIDS.
3. This embyroid can be put into a medium to support growth
of a plantlet. This will give rise to a monoploid plant.
Aneuploidy: trisomy in plants:
Datura plant has 12 different set of chromosome (n=12). Any of the 12
chromosomes can be trisomic.
Each of the plants with an extra chromosome has a different
phenotype not present in the parental plant. This extra
chromosome will affect gene balance.
Gene balance
In monoploid and diploid individuals, the gene ratio from different
chromosomes (chromosome 2 /chromosome 3) =1
In monosomic or trisomic individuals this gene ratio from different
chromosomes
[(chromosome 2+ chromosome 2) / chromosome 3] = 2 (or 0.5)
1:1 ratio. The ratio is changed in trisomic plants.
True except for the X-chromosome in mammals. Men have X and
Y, and females have XX. They have dosage compensations and
inactivate the X.
Chromosome replication occurs without cell division= endomitosis:
- If chromosome replication occurred over and over again without
cell division, this is endomitosis.
- The daughter will have 4 sets of chromosome.
-Sometimes cells in the human liver and kidneys are tetraploid!
Polytene Chromosomes:
- Occurs in salivary glands of Drosophila melanogaster larvae and in
other insects (flies and mosquitoes).
- Salivary chromosomes have 9 rounds of DNA replication without
cell division! This makes a polytene chromosomes. Polytene
chromosomes stays together and can see this under the
microscope.
- After 9 rounds of replication, it has 512 chromosomes per cell
(doubled each round) and they can be seen under low
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