Quick review: identify this stage of the diploid
cell cycle.
Quick review: identify this stage of the diploid
cell cycle.
Quick review: identify this stage of the diploid
cell cycle.
Quick review: identify this stage of the diploid
cell cycle.
Quick review: identify this stage of the diploid
cell cycle.
Quick review: identify this stage of the diploid
cell cycle.
• Defined: Process
where the
chromosome number
is reduced by half
• Gametes created
(egg, sperm, pollen)
• End Result: Four
Haploid Cells
• Same process
–G1: Growth,
organelle
replication, normal
functions
–S: Chromatin
duplicated
–G2: Growth and
normal functions
Synthesis (S stage)
Chromatin duplicated
Mom 1
Dad 1
• Similar to mitosis, except…
• Two Events:
– 1) Synapsis: Pairing of homologous
chromosomes
– Dad’s #1 chromosome pairs with Mom’s #1, etc…
– The four paired chromatids equals a TETRAD
– 2) Crossing Over: Exchange of genes
Early Prophase
Chromosomes
form
2
1
Nucleus
dissolves
3
2
3
1
Synapsis
tetrad
Spindle fibers
pull
homologous
chromosomes
together
tetrad
2
1
3
2
3
1
tetrad
• Homologous chromosomes
(tetrads) align along cell
equator
1 1
22
3 3
• Homologous chromosomes
are pulled apart
–Tetrads separated
11
33
2 2
• Cytokinesis divides the cells
– Not apart of telophase
• Each cell contains 1 of the
homologous chromosomes
– In humans = 23 chromosomes
1
1
3
3
2
2
Let’s review…starting with the S stage
Chromatin duplicated
Prophase 1
Chromosomes
form
2
1
Nucleus
dissolves
3
2
3
1
Prophase 1 (Synapsis)
tetrad
Spindle fibers
pull
homologous
chromosomes
together
tetrad
2
1
3
2
3
1
tetrad
Metaphase 1
1 1
22
3 3
Tetrads pulled to middle of cell
Anaphase 1
11
33
2 2
Tetrads pulled apart… one entire
chromosome to each end of the cell
Telophase 1
1
1
3
3
2
2
Nucleus reforms… Cell Plate forms
Cytokinesis splits the cell into two…
• Same as mitosis
• Note: No new DNA was
created (no 2nd interphase)
1
1
3
3
2
2
• Remaining chromosomes are
aligned at cell equator
1
1
3
3
2
2
• Chromatids are pulled to
opposite ends
• Nucleus reforms on the four new cells
• Cell plate forms
• Cytokinesis divides the cells
– Not apart of telophase
• Chromatids uncoil
• End Result: Four haploid cells
Spermatogonium
(diploid)
Meiosis 1
• Defined: Production
of sperm cells
• Location: male testes
• Makes final changes
to sperm cell
• Four haploid sperm
cells at a time
Meiosis 2
Spermatogenesis
Oocyte
(diploid)
• Defined: Production of
the egg cell (ovum)
• Location: female
ovaries
• During meiosis I:
Cytoplasm distributed
unequally
• During meiosis II: cells
split again
• End result: 1 mature
cell, 3 polar bodies
Meiosis 1
Meiosis 2
Oogenesis
• Division of somatic
cells
• Ex: Blood cells, skin
cells, nerve cells…
• End Result: 2 Diploid
Cells
• Importance: Replaces
aging cells
• Importance: Form of
asexual reproduction
• Division of gametes
• Ex: Sperm and Egg
cells
• End Result: 4
Haploid cells
• Importance: Reduces
the gamete
chromosome number
by half
Let’s review…starting with the S stage
Chromatin duplicated
Prophase 1
Chromosomes
form
2
1
Nucleus
dissolves
3
2
3
1
Prophase 1 (Synapsis)
tetrad
Spindle fibers
pull
homologous
chromosomes
together
tetrad
2
1
3
2
3
1
tetrad
Metaphase 1
1 1
22
3 3
Tetrads pulled to middle of cell
Anaphase 1
11
33
2 2
Tetrads pulled apart… one entire
chromosome to each end of the cell
Telophase 1
1
1
3
3
2
2
Cytokinesis splits the cell into two…
Nucleus reforms
Prophase 2
1
1
3
3
2
Nucleus dissolves….
Spindle fibers form
2
Metaphase 2
1
1
3
3
2
2
Chromosomes are pulled to middle
of each cell
Anaphase 2
Chromosomes split apart…
One chromatid pulled to each end of the cell
Telophase 2
Nucleus reforms… cell plate created
Cytokinesis splits the two cells into four cells…
Chromatids unwind into chromatin
REVIEW
1) Name three ways that meiosis and mitosis differ.
2) Pick a stage of meiosis…any stage. Describe what happens
in it.
3) What type of cells does meiosis create?
4) What are homologous chromosomes?
5) When do homologous chromosomes move to opposite ends
of the cell?
6) When do chromatids move to opposite ends of the cell?
7) Why is no DNA created in meiosis 2?
8) How does meiosis differ in males vs. females?