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Making Haploid Gametes
 Both
chromosomal
problems
(monosomy/trisomy &
breakage) are result of
improper haploid cell
formation/division in
process known as meiosis
 Two phases: meiosis I &
meiosis II
 Goal: 2n (diploid)  n
(haploid)

Body or somatic cell
division is called mitosis,
which generates the
EXACT same cell
 Mitosis: cell division
which produces cells
containing SAME number
of chromosomes as
parent’s cell
○ Start: One 2n skin
cell
○ End: Two 2n skin
cells
 Gamete
or sex cell
n
division is called
meiosis, which
generates cell with HALF
number of chromosomes
as original
 Start: One 2n cell
 End: Four n sperm
(spermatogenesis) or
egg (oogenesis) cells
Video of Mitosis vs. Meiosis
n
n
oogenesis
2n
spermatogenesis
n
MITOSIS
MEIOSIS
Growth & repair
In body/somatic cells
Making gametes
In sex cells/gametes
One 2n cell  Two 2n cells One 2n cell  Four n cells
One diploid  two diploids One diploid  four haploids
Exact copy of parent
Half copy of parent
2n
2n
2n
2n
n
n
n
n
Steps of meiosis
 Two
repeating stages with precursor stage:
Interphase
Meiosis
I
Meiosis
II
MEIOSIS II – 2n  n MEIOSIS I = 4n  2n

1.
2.
3.
4.
Interphase – DNA replication (2n  4n)
Prophase I – homologous chromosomes visible;
crossing over occurs to add genetic variety
Metaphase I – homologs move to equator
Anaphase I – homologs move to opposite poles
Telophase I – 2n sets move to poles; cytokinesis
Prophase II – new spindle forms around
chromosomes
6. Metaphase II – chromosomes move to equator
7. Anaphase II – centromeres divide; n chromatids
to poles
8. Telophase II – n sets move to poles; cytokinesis
5.
Interphase
 Doubling of
chromosomes (2n  4n)
 Cell looks same
 Nucleus is darker
because of chromosomal
duplication
AA aa
B B b b “tetrad”
CC cc
A Aa a
BB b b
CC c c
Prophase I




centrioles
spindle
fibers
Nuclear envelope disappears
Spindle fibers form from centrioles
Chromosomes in homologous pairs
Crossing-over occurs (genetic
diversity!)
A A a a Same gene,
A aA a
BB b b
CC c c
BB b b
CC c c
different variety
Crossing over
Homologous pair
Nuclear envelope
Metaphase I
 Homologs move to
equator
 How they line up varies
(genetic diversity!) = law
of independent
assortment
 Spindle fibers attach to
centromeres
Anaphase I
 Spindle fibers contract
 Homologs pulled to poles
Anaphase Mistakes
 What
happens when
anaphase goes
wrong?
 If tetrads are not
pulled apart evenly,
chromosomal
abnormalities result
 Trisomy (extra
chromosome
 Monosomy (missing)
Telophase I
 Chromosomes go to poles
 Cytokinesis occurs; cells split in two
 4n  2n (halfway to goal!)
Prophase II
 New spindle fibers form
 2n (2 sets of 23 = 46)
Metaphase II
 Same as metaphase I
 Homologs move to equator
 Spindle fibers attach to centromeres
Anaphase II
 Sister chromatids pulled apart at centromere
 Chromatids move to opposite poles
Anaphase Mistakes – A Second Chance

What happens when
anaphase goes wrong?
 If chromatids are not pulled
apart evenly, chromosomal
abnormalities result
 Trisomy (extra
chromosome
 Deletion/Monosomy
(missing)
Telophase II
 Cytokinesis takes place, splitting two cells
into four (sperm or eggs)
 2n  n
Meiosis Overview
Watch Meiosis Video
Overview
Precursor cell
2n
diploid
INTERPHASE
4n
polyploid
MEIOSIS I
2n
2n
diploid
MEIOSIS II
n
n
n
n
haploid