Transcript MEIOSIS

MEIOSIS
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CHROMOSOME NUMBER (FRUIT FLIES)
Homologous: corresponding chromosomes from male & female
Homologs: the actual chromosomes of the set
Diploid: A cell that contains both sets of homologs
2N: the diploid number; Drosophila 2N=8
So, a diploid cell contains 2 complete sets of chromosomes
and complete sets of genes.
Gametes have only a single set of genes due to segregation
Haploid: cells that contain a single set of chromosomes
N: the haploid number; Drosophila N=4
In order for gametes to be produced there must be a
process that divides the diploid number of chromosomes in half
PHASES OF MEIOSIS
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Meiosis: Process of reduction division in which the number of
chromosomes per cell is cut in half and the homologous
chromosomes that exist in a diploid cell are separated.
The names of the phases are the same for mitosis/meiosis
There are two stages in meiosis division (See fig. 9-17)
MEIOSIS I
DNA replication in reproductive organs (CAREFUL)
Prophase I: each chromosome seeks out its corresponding
homologous chromosome for form a TETRAD.
Metaphase I: the tetrads line up in the center of the cell
Crossing Over: The exchange of chromatid pieces between
homologous chromo., resulting in new combinations of gene info.
This results in an exchange of genes between homologous chromo.
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What happens next?
The homologous chromo separate and two new cells form
Each cell now has 8 chromatids like mitosis w/ a difference
The two cells no longer have two complete sets of 4 chromo.
Instead maternal and paternal chromo have been shuffled
The two cells have sets of chromo that are
MEIOSIS II
No DNA replication, so each cell’s chromo has 2 chromatids
Metaphase II: 4 chromo line up in the center of each cell
Anaphase II: the paired chromatids separate; each of the 4
daughter cells receives 4 chromatids w/ N-4 chromo each
The amount of genetic material has been reduced
REVIEW
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PURPOSE: The production of haploid gametes
PROPHASE I: Homologous chromo pair off; forms tetrads
METAPHSE I: Tetrads line up in the center of the cell
ANAPHASE I: Cell divides the tetrads; it moves half the
chromo to one end, the other half to the opposite end
TELOPHASE I: A membrane forms between the cell’s two ends;
the result is 2 haploid cells
MEIOSIS II: It is identical to mitotic division of chpt. 8. The
only difference is that in meiosis II, the parent cell is haploid.
THE END PRODUCT: 4 haploid gametes
MEIOSIS AND GENETICS
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Chromosomes pair and separate during meiosis exactly as
Mendel predicted for the structures that carry genes.
MEIOSIS I: results in segregation and independent assortment
During Anaphase I the homologous chromo separate and are
segregated to different cells, which also segregates the
homologous forms of the genes (alleles).
All this is completely random and a matter of chance
GAMETE FORMATION:
In males: animals-sperm, plants-pollen; contain haploid cells
In females: animals-egg, plants ovule; contain haploid cells
In female animals, the cells divisions at the end of Meiosis
I and II are uneven, so that the egg receives most of the
cytoplasm. The other 3 cells, known as polar bodies, usually
don’t participate in reproduction
COMPARING MITOSIS AND MEIOSIS
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MITOSIS: Results in the production of two genetically
identical cells.
A diploid cell that divides by mitosis gives rise to two diploid
daughter cells.
The daughter cells have sets of chromosomes and genes
identical to each other and to the original parent cell.
MEIOSIS: Begins with a diploid cell, but produces four haploid
cells which are genetically different from the diploid cell and
from one another.
This is because the homologous chromo are separated during
the first meiotic division.
This is also because of cross-over resulting in the production
of new gene combinations on the chromosomes.
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