Meiosis

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Meiosis

• Biology
• Mrs. Volin

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Meiosis

• Cell division in which gametes are produced
  (sperm and egg cells)
• A reduction division
• The number of chromosomes per cell is cut in half

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• Body cells contain 2 sets of chromosomes.
• One chromosome from each parent
• The two sets are homologous (each chromosome from
  male parent has a corresponding chromosome from
  the female parent)

Body cells are diploid.
X and y chromosomes
Cell with both sets of chromosomes is
diploid. Cell with both sets of chromosomes
also termed 2N.
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• In
  humans, 2N 46
• In fruit flies, 2N 8

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• Cells with only one set of chromosomes are
  haploid or 1N
• For human cells, 1N23
• This occurs in egg and sperm cells

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2 Stages of Meiosis

• Meiosis I, the first meiotic division
• Meiosis II, the second meiotic division

See textbook pgs. 276-277
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Interphase DNA is copied to make 2 identical
strands Prophase I Chromosomes pair up to form
tetrad 4 chromatids crossing over
occurs ---alleles are exchanged
cytokinesis
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Metaphase I Spindle fibers attach to
chromosomes. Anaphase I Fibers pull homologous
chromosomes to opposite poles Telophase I Cell
divides in 2 chromosomes and alleles of
daughter cells are different from each other and
different from parent
cytokinesis
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• Prophase II There are now 2 cells. Each cell
  has half the number of chromosomes as original
• Metaphase II Chromosomes line up at middle of
  cell
• Anaphase II Sister chromatids separate and move
  toward poles
• Telophase II Each cell splits in half

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Results 4 daughter cells from one
parent Each daughter has haploid (N) number
of chromosomes Daughter cells genetically
different from each other
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• Meiosis
• 4 genetically different haploid cells
• Daughter cells different from parent cell and
  from each other
• Gametes

• Mitosis
• 2 genetically identical diploid cells
• Daughter cells identical to parent cell
• Body cells

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Linkage

• Genes that tend to be inherited together are
  said to be linked
• Discovered by Thomas Hunt Morgan, 1910, while
  studying fruit flies

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Linkage

• If genes are farther apart on a chromosome, the
  more likely they will be separated by a
  crossover.
• Alfred Sturtevant studied rates that crossing
  over occurs between genes
• Developed the first gene map
• Relative location of genes can be mapped.

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• Fruit fly gene map showing locations of some genes

Chromosome 2
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Genetic Diversity

• Contributes to evolutionary change.
• If an offspring inherits a combination of genes
  that gives it a survival advantage, it is better
  able to survive and pass on its genes.
• This means the chance that the combination is
  passed on increases. As a result, there is an
  accumulation of favorable characteristics.

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Events contributing to Genetic Diversity
Meiosis provides new genetic combinations

•  Independent Assortment
• The orientation of homologous chromosomes on one
  side of the metaphase plate or the other in
  Meiosis I is random.
• The number of possible orientations is 2n, where
  n is the haploid number. For humans, the number
  is 223 8.4 million

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Events contributing to Genetic Diversity

• Crossing over
• When crossing over is considered, the number
  of combinations is nearly infinite.

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Events contributing to Genetic Diversity

• Random fertilization
• Random fertilization of an egg by one sperm cell

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