Meiosis
http://www.youtube.com/watch?
v=Fq9ho9wEpI8
Let’s review...7 characteristics of life?
1. All made of cells
2. All reproduce
3. All have a genetic code (DNA)
4. All grow and develop
5. All use energy
6. All respond to environment
7. All maintain homeostasis
Sexual Reproduction
Egg and sperm cells
-these 2 combine to form one cell, so each
only need HALF the amount of DNA.
Meiosis is cell division to produce these
gametes (egg/sperm)
Chromosomes
Each person has 1 chromosome set from mom
and 1 set from dad
-you have 2 of each chromosome =
diploid
-those 2 chromosomes = homologous pair
Egg/sperm cells need only 1 set each = haploid
Phases of Meiosis
1. Interphase
-like all other cells, it starts in interphase
-growth and copying of DNA
-unduplicated → duplicated chromosomes
Starting cell….
Cell Membrane
Nuclear membrane
Chromosomes
(unduplicated)
= “mom”
= “dad”
Interphase….
Cell has grown in
size!
Nuclear membrane
Chromosomes
(duplicated)
Meiosis I
2. Prophase I
-nuclear membrane breaks down
-centriole and spindle fibers form
*chromosomes find their homologous pair!
(“mom” finds its “dad” half) - this is called a tetrad
*crossing-over can occur → chromosomes swap
their information → everyone looks different!
(adds diversity to species)
Nuclear membrane is
disappearing!
Spindle
Chromosomes
(duplicated)
Homologous pairs
(tetrads)
Centrioles
Crossing-over
“mom” and “dad”
chromosomes can
swap information!
3. Metaphase I
-chromosomes line-up in the middle
-spindle fibers attach to centromere of
each chromosome
*separate the homologous pairs from each
other (mom and dad chromosomes
separate)
(in mitosis - separating sister chromatids, in meiosis this doesn’t happen
yet!)
Homologous Pairs
Chromosomes
(duplicated)
Still as tetrads!
Spindle
Centrioles
Centromere
4. Anaphase I
-homologous pairs get pulled to poles
*still duplicated chromosomes!
Chromosomes
(duplicated)
Spindle
Centrioles
5. Telophase I/Cytokinesis
-nuclear membrane reforms
*still duplicated chromosomes!
-cells separate into 2
*only have HALF the number of
chromosomes it started with
(Mitosis - same amount at the end because it is making identical copies.
Meiosis - new cells are all different!)
Nuclear
membrane
Cell membrane
(splitting in 2)
Chromosomes
(duplicated)
HALF the chromosomes!
Both cells are DIFFERENT!
We started with…
Ended Meiosis I with…
1 cell
6 chromosomes
2 cells (haploid)
3 chromosomes each
(unduplicated)
(duplicated)
Meiosis II → just like regular mitosis
6. Prophase II
-nuclear membrane breaks down in
each cell
-centrioles and spindle fibers reform
Nuclear membrane
is disappearing!
Chromosomes
(duplicated)
7. Metaphase II
-chromosomes line-up in middle
-spindle attaches to centromere
Chromosomes
(duplicated)
Spindle
Centromere
Sister Chromatids
8. Anaphase II
-sister chromatids get ripped apart
-now they become unduplicated again!
9. Telophase II/Cytokinesis
-nuclear membrane reforms
-centrioles/spindles break down
-cytoplasm divides and separates cells
*End results of meiosis in 4 HAPLOID cells!
(mitosis = 2 identical, diploid cells)
4 cells with 3
chromosomes
each!
Haploid
Unduplicated
At the end of meiosis = cells are NOT identical!
-if our pictures showed crossing-over, all
the cells would be completely different!
● grow/repair
● identical cells
● 1 cell → 2 cells
MEIOSIS
● make gametes
● different cells
● 1 cell → 4 cells
● diploid → diploid
● diploid → haploid
● 1 cycle of division
● 2 cycles of division
MITOSIS