11/22/2010
Meiosis
Why Meiosis?
Bellringer- (write on page 28)
• Each type of organism has a specific number
of chromosomes (humans have 46
chromosomes)
• Because a each parent passes along a
chromosome, there has to be some way for
offspring to have the same number of
chromosomes as their parents and not double
the number.
Why do we need diversity?
What type of reproduction do
humans use?
Why Meiosis?
• Gametes that are produced are sperm cells for
males and egg cells for females
• Sexual reproduction: when haploid egg and
sperm cells join together to form a diploid
zygote
• In humans, each sperm and egg cell has 23
chromosomes (which is half of 46, the normal
number of chromosomes) or Sperm (23) +
Egg (23) = Zygote (46)
Before Meiosis
• Homologous chromosomes are attached to
each other
Why Meiosis?
• For the most part, healthy human zygotes
cannot have more than 46 chromosomes
• Once formed, zygotes undergo mitosis to grow
and develop (asexual reproduction)
Before Meiosis
• Tetrad: 2 homologous chromosomes attached
together (= 4 sister chromatids)
– Homologous chromosomes: 2 of the same type of
chromosome
• Chromosomes copy themselves so that they
can be passed along to the daughter cells
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Genetic Variation
Genetic variation: rearrangement of alleles
that can produce completely new
combinations (essential for evolution to
occur)
Genetic Variation
• Genetic Recombination: the reassortment of
chromosomes and genetic material, either by
crossing over or independent assortment,
which increases genetic variety
Meiosis I—2 new cells
are formed
Prophase I:
• Tetrads condense(appear)
• Crossing over occurs between chromosomes
• Nuclear membrane disappears
Metaphase I:
• Tetrads line up in the middle of the cell
Genetic Variation
• Crossing Over- When the
homologous
chromosomes pair so
tightly that a piece of the
chromatid breaks off. The
piece changes place with a
piece from the other
homologous tetrad. This
scrambles the traits
around = genetic variety.
Genetic Variation
• Both crossing over and independent
assortment are completely RANDOM…so this
is why humans can sometimes look
completely different from other family
members
Meiosis I
Anaphase I:
• Tetrads split and each set of sister
chromatids moves to opposite ends of
the cell.
Telophase I:
• Cytokinesis (division of the cytoplasm)
results in 2 new cells, each with
homologous chromosomes.
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Meiosis II
4 new cells formed- *Like Mitosis
Prophase II:
• Centrioles on the chromosomes form
spindle fibers.
Metaphase II:
Chromosomes (sister chromatids) line
up randomly in the middle.
Mistakes in Meiosis
• Nondisjunction: when homologous
chromosomes don’t separate from each other
during meiosis
• So homologous chromosomes move together
into a new gamete giving that gamete too
many chromosomes, while the other new
gamete is missing the chromosome
Meiosis II
Anaphase II:
• Sister chromatids separate and move
to opposite ends of the cell
Telophase II/Cytokinesis:
• New nuclei are formed and four
daughter cells are produced, each
containing ONE chromosome from
each homologous pair
Mistakes in Meiosis
• Organisms with extra chromosomes can
survive
• Organisms with too few chromosomes usually
do not survive
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Mistakes in Meiosis
Conditions that Result from Extra
Chromosomes
• Polyploidy: when
organisms have more
than the normal
number of
chromosome sets
Examples of Nondisjunction
Turner’s Syndrome
– instead of 46
chromosomes, they
might have double or
triple that number
– Can be lethal for
animals but not
plants (plants just
usually get bigger)
Genotype (X=Female sex
chromosome,
Y= male sex chromosome)
XO
Trisomy X (metafemales)
XXX
Klinefelter’s syndrome
XXY
Jacob’s syndrome
XYY
Down syndrome
TRISOMY 21 (3 copies)
Patau’s syndrome
TRISOMY 13
Lethal condition in males
OY
Examples of Nondisjunction
• Turner’s Syndrome
• Edward’s Syndrome
– Genotype: Females-X
– Trisomy 18
– About 95% die before birth
– Of liveborn infants, only 50% live to 2
months, and only 5–10% will survive their
first year of life
• Down’s Syndrome
Comparing Mitosis and Meiosis
Mitosis
DNA replicates in interphase
Meiosis
DNA replicates in interphase
ONE division of chromosomes
Separate sister chromatids
TWO divisions of chromosomes
Separate homologous chromosomes
(meiosis I)
Separate sister chromatids (meiosis
II)
Makes 2 identical DIPLOID cells
Makes 4 HAPLOID cells
Chromosome # is identical to parent
cell
Chromosome # is HALF of the parent
cell
Used for replacing old cells, growth,
and repair
Used to make gametes
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