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Review Chromosome Patterns
 At the end of mitosis’
telophase and
cytokinesis each
new cell has an
unreplicated copy of
each chromosome.
 The chromosomes
look like sticks.
 During interphase each chromosome is replicated.
The replicated chromosomes look like X’s and
have 2 sister chromatids joined at a centromere.
 This diagram reviews the shapes of the
chromosomes during mitosis.
Meiosis – V2
Meiosis
 Meiosis is the process by which sex cells or
gametes are made.
 This process is also called gametogenesis.
 During meiosis the number of chromosomes
per cell is reduced by half.
 Meiosis has two distinct stages
 Meiosis I
 Meiosis II
 The starting cells in the ovary or testes are
diploid and the chromosomes have
replicated.
Overview
Note that the big
difference from mitosis is
during the first meiosis I
metaphase, when the
replicated chromosomes
line up on the equator with
their homologous
chromosome – IN PAIRS!
Prophase I
 Nuclear membrane dissolves, and the centrioles move
to opposite poles.
 Each chromosome pairs with its corresponding
homologous chromosome to form a tetrad.
 There are 4 sister chromatids in a tetrad.
 The pairing of homologous chromosomes is the key to
understanding meiosis.
 Crossing-over probably occurs here.
 Crossing-over is when homologous chromosomes
overlap and exchange portions
of their chromatids.
Metaphase I
 Homologous pairs of chromosomes line
up at the equatorial plate.
 Spindle fibers from the centrioles attach
to the centromeres of the chromosomes.
Anaphase I
 The spindle fibers pull the homologous
chromosomes toward opposite ends of
the cell.
Telophase I
 Nuclear membranes might reform.
 The cell separates into 2 cells.
Cytoplasm divides after telophase,
resulting in 2 haploid daughter cells.
Prophase II
 The centrioles are replicated.
 Centrioles in the 2 new cells move to opposite
poles.
 New spindle fibres form and attach to
centromeres.
Metaphase II
 The chromosomes are in a single line on
the equatorial plate.
 This is similar to metaphase in mitosis.
Anaphase II
 Sister chromatids separate and move to
opposite ends of the cell.
Telophase II
 Cytoplasm separates, resulting in 4
haploid cells.
Gamete Formation
 In males, meiosis or spermatogenesis
results in 4 sperm cells
 In females, meiosis or oogenesis results in
1 egg cell for reproduction and three polar
bodies, which are not used in
reproduction.
 The one egg cell has all the cytoplasm,
while the polar bodies only have a haploid
copy of the chromosomes.
Summary
Comparing Mitosis and Meiosis
Mitosis
Starting # cells?
Where does this
occur?
Are ending cells
genetically identical?
Ending # of cells?
Ending N?
Ending cells
haploid or diploid?
Meiosis
Comparing Mitosis and Meiosis
Starting # cells?
Mitosis
1 diploid
Meiosis
1 diploid
Starting N?
4N
4N
Where does this
occur?
Body or
somatic cells
Are ending cells
Genetically
genetically identical? identical
Ending # of cells?
2
Ending N?
2N
Ending cells haploid Diploid
or diploid?
Ovary or testes
Genetically
different
4
N
Haploid