Download Meiosis II

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 The
form of cell division by which
GAMETES (SEX CELLS) with
HALF the number of
CHROMOSOMES are produced.
 DIPLOID (2n)  HAPLOID (n)
 Meiosis is SEXUAL reproduction.
 TWO divisions (MEIOSIS I and
MEIOSIS II).
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Occurs
only in GONADS (testes or
ovaries).
 Male: SPERMATOGENESIS –
creates 4 sperm

Female: OOGENESIS – creates 1
egg or ova and 3 polar bodies that die
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n=23
human
sex cell
sperm
n=23
n=23
2n=46
diploid (2n)
n=23
n=23
haploid (n)
n=23
Meiosis I
Meiosis II
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Haploid
(1n)
n=23
human
sex cell
egg
n=23
2n=46
diploid (2n)
Meiosis I
Polar
Bodies
(die)
n=23
Meiosis II
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Similar to mitosis interphase.
 G1 phase- Cell growth
 S phase- Chromosomes are replicated
 Each duplicated chromosome consists of two
identical SISTER CHROMATIDS attached at
their CENTROMERES.
 G2 phase- Cell prepares
for cell division.

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Nucleus
chromatin
and nucleolus visible.
Nucleus
cell
membrane
nucleolus
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 Pair
of chromosomes (maternal and
paternal) that are similar in shape and
size.
 Homologous pairs (tetrads) carry GENES
controlling the SAME inherited traits.
 Each locus (position of a gene) is in the
same position on homologues.
 Humans have 23 pairs of homologous
chromosomes:
a. First 22 pairs are called autosomes
b. Last pair are sex chromosomes
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eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
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XX chromosome - female
XY chromosome - male
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Cell
division that reduces the
chromosome number by onehalf.
Four phases:
a. Prophase I
b. Metaphase I
c. Anaphase I
Prophase I
d. Telophase I
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Chromosomes
condense.
Synapsis occurs - Homologous
chromosomes come together
to form a tetrad.
Tetrad is two chromosomes
or four chromatids (sister and
non-sister chromatids).
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Homologous chromosomes
sister chromatids
Tetrad
sister chromatids
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Crossing
over may occur between
non-sister chromatids only during
Prophase I
Crossing over: segments of
nonsister chromatids break and
reattach to the other chromatid.
Causes Genetic Recombination or
gene exchange.
Creates genetic variation.
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nonsister chromatids
site of crossing over
Tetrad
variation
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Meiosis I
Homologs
separate
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•
•
•
•
Nucleolus & nuclear membrane disappear
Spindle fibers forms
Chromosomes coil & Synapsis (pairing) occurs
Tetrads form & Crossing over Occurs
spindle fiber
centrioles
aster
fibers
TETRAD
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•Tetrads align on the equator.
•Independent assortment occurs – chromosomes
separate randomly causing GENETIC
RECOMBINATION
Homologs line up at equator or
metaphase plate
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•
•
Homologous chromosomes separate
and move towards the poles.
Sister chromatids remain
attached at their centromeres.
Homologs separate
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 Each
pole now has haploid (1n) set
of chromosomes.
 Cytokinesis occurs and two haploid
daughter cells are formed.
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cytokinesis
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Meiosis II
Sister
Chromatids
Separate
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No Interphase or very short
 No DNA Replication
 Remember: Meiosis II is similar
to mitosis but 2 cells go through
the process now.

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Same
as Prophase in mitosis
Nuclear membrane & nucleolus
disappear
 Chromosomes condense
 Spindle fibers forms

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Same
as Metaphase in mitosis
Chromosomes (not homologs) line up at equator
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Same
as Anaphase in mitosis
SISTER CHROMATIDS
separate and move toward
ends of each cell.
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 Same
as Telophase in mitosis.
 Nuclei and Nucleoli reform, spindle
disappears
 CYTOKINESIS occurs.
 Remember: FOUR HAPLOID
DAUGHTER cells are produced.
 Called GAMETES (eggs and sperm)
1n Sperm cell
fertilizes 1n egg
to form 2n zygote
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