Download Prophase I (cont`d)

Cell Reproduction
Part 2
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Meiosis
 Nuclear division that occurs
only in reproductive cells of an
organism.
 A variation of mitosis, in which
daughter cells receive half of
the usual number of
chromosomes of the parent.
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Meiosis (cont’d)
 Only one chromosome of each
homologous pair goes into the
daughter cell.
 Results in a haploid number
(1n).
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Meiosis (cont’d)
 Involves 2 successive cell
divisions:
 Meiosis I
 Meiosis II
 Produces 4 cells.
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Meiosis (cont’d)
 Each division has stages similar
to those of mitotic division, but
with certain important
differences.
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Interphase
 Cells still
undergo G1, S,
and G2 phases
and begin
meiosis with a
duplicate set of
chromosomes.
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Meiosis I
 4 phases: Prophase I,
Metaphase I, Anaphase I, and
Telophase I
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Prophase I
 DNA coils into chromosomes.
 Spindle fibers appear.
 Nucleus and nucleolus
disappear.
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Prophase I (cont’d)
 Every
chromosome
lines up next to
its homologue
and joins at the
centromere.

This pairing is
called synapsis.
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Prophase I (cont’d)


Since each
chromosome has
replicated before this,
this combination has 4
chromatid strands.
This group of
chromatid strands is
call a tetrad.
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Prophase I (cont’d)
 During synapsis, the chromatids
twist around each other and
sometimes exchange segments.
 Crossing over
 Results in genetic
recombination – new mixture
of genetic material.
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Crossing Over
Metaphase I
 Tetrads of homologous
chromosomes become attached
to the spindle fibers at their
centromere.
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Metaphase I (cont’d)
 Move to the
central plane.
 Orientation of
the homologous
pairs is random
with respect to
the poles.
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Independent Assortment
Anaphase I
 Each homologous
chromosome (2
chromatids
attached at a
centromere)
moves to the
opposite pole.
 This is called
disjunction.
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Anaphase I (cont’d)
 The random separation of
homologous chromosomes is
called independent assortment.
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Anaphase I (cont’d)

Results in the random
separation of maternal and
paternal chromosomes.
 Results in genetic
recombination.
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Anaphase I (cont’d)
 This is the phase where
mistakes can happen.
 Nondisjunction
 When chromosomes do not
separate properly.
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Telophase I
 Chromosomes
reach the opposite
ends of the cell.
 New nuclear
membranes form.
 Cytokinesis begins,
dividing the cell
into 2 cells.
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Interphase
 A brief interphase occurs with
no DNA replication.
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Meiosis II
 Follows the same course as
mitotic division.
 4 phases: Prophase II,
Metaphase II, Anaphase II, and
Telophase II
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Prophase II
 Spindle fibers
form.
 Chromosomes
recoil.
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Metaphase II
 Chromosomes
move to the
central plane.
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Anaphase II
 Centromere
divide.
 Chromatids
separate and
move to the
opposite poles of
the cell.
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Telophase II
 Chromosomes uncoil.
 Cytokinesis occurs.
 4 cells formed.
 Each contain half (haploid
number) of the original
number of chromosomes
(diploid number).
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Telophase II (cont’d)
 Nuclear
membrane forms
around the
chromosomes in
each of the 4 new
cells.
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Formation of Gametes
 In animals, meiosis produces
haploid reproductive cells called
gametes.
 This occurs only in the testes
and ovaries.
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Spermatogenesis
 Occurs in the testes.
 Produces 4 male gametes called
sperm cells or spermatozoa.
 Meiosis forms 4 haploid cells
called spermatids which then
develop into sperm cells.
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Oogenesis
 Occurs in the ovaries.
 Produces mature egg cells or
ova.
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Oogenesis (cont’d)
 In cytokinesis, the cytoplasm is
unequally divided.
 One egg is formed.
 3 polar bodies are formed,
which degenerate.
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Asexual Reproduction
 Production of offspring by one
parent.
 New organisms are usually
produced by mitotic division of
the parent cell.
 Offspring are genetically
identical to the parent.
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Asexual Reproduction
(cont’d)
 Unicellular organisms: new
organisms are produced by
binary fission or mitosis.
 Multicellular organisms: new
organisms produced by budding
of portions of parent.
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Sexual Reproduction
 Production of offspring through
meiosis and the union of sperm
and egg.
 Offspring are genetically
different from the parents.
 Genes were combined in new
ways in meiosis.
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