1. Meiosis
Day 2

3. Objectives Summarize the events of Meiosis I and Meiosis II.
Explain crossing-over and how it contributes to the production of unique individuals.

4. Meiosis
Meiosis occurs in sexual reproduction when a diploid germ cell (2n) produces four haploid daughter cells (n) that can mature to become gametes (sperm or egg).

5. Meiosis Goal: reduce genetic material by half
Why? n (mom) + n (dad) = 2n (offspring with variation)
from mom
from dad
child
too
much!
meiosis reduces
genetic content
Just right!

6. Eye color
Mom #1
Dad #1
Green
Blue
These chromosomes have the same genes on them but each allele could be different. Examples: Brown hair versus Blond hair, Green eyes versus Brown Eyes

7. Meiosis: cell division in two parts
Gamete formed
Sister
chromatids
separate
Haploid
Diploid
Meiosis I
(reduction
Division 2n to n)
Meiosis II
Homologous
chromosomes
separate
Sister
chromatids
separate 2n n n
Result: one copy of each chromosome in a gamete.

8. Interphase occurs before meiosis I.

9. Meiosis I : the reduction division
Prophase I
(early)
(diploid)
(late)
Metaphase I
Anaphase I
Telophase I
Nucleus
Spindle
fibers
Nuclear
envelope

10. Meiosis : Prophase I
Early prophase
Homologous chromosomes pair
Non-sister Chromatids cross.
Crossing over occurs.
Creates variation
Late prophase
Chromosomes condense.
Spindle fibers forms.
Nuclear envelope fragments.
Centrioles move to opposite poles
Notice the similarities and differences with mitosis

11. Mom Dad Tetrad This occurs in prophase 1
Chismata
Tetrad
Synapsis: when homologous chromosomes pair up closely
Mom and dad’s genes mix (creating variation)

12. Metaphase I
Homologous Chromosome pairs align along the equator of the cell. (metaphase plate)
Cell is still 2n
Independent assortment occurs during metaphase I giving even more variation.
Notice the similarities and differences with mitosis
In mitosis 46 individual chromosomes align, however in meiosis 23 pairs of chromosomes align

13. Independent assortment states: homologous chromosomes randomly align on the metaphase plate during meiosis I
Mom is blue
Dad is red
1 mom on one side and 1 dad on the same side
All mom on one side and all dad on one side
2n = 4
n = 2

14. Anaphase I Homologous Chromosomes separate and move to opposite poles.
Sister chromatids remain
Attached at their centromeres
Cells are haploid (n)
Notice the similarities and differences with mitosis
In mitosis sister chromatids separate, however in meiosis homologous chromosomes separate

15. Telophase I Nuclear envelopes reassemble.
Spindle apparatus disappears.
Chromosomes DO NOT uncoil.
Cytokinesis divides cell into two.
Cells are haploid (n)
Notice the similarities and differences with mitosis

16. Meiosis II Gene X Sister chromatids carry identical genetic
Meiosis II produces gametes with
one copy of each chromosome and thus one copy of each gene.
Sister chromatids carry
identical genetic
information.
Notice the similarities and differences with mitosis
There is no interphase between meiosis I and meiosis II, Why?

17. Meiosis II : the equational division
Prophase II
(haploid)
Metaphase II
Anaphase II
Telophase II
Four
Non-identical
haploid
daughter cells
Meiosis II is very similar to mitosis

18. There is NO Interphase between Meiosis I and II

19. Prophase II Nuclear envelope fragments.
Spindle fibers (apparatus) forms.
Centrioles move to opposite poles
Cells are haploid (n)
Notice the similarities and differences with mitosis

20. Chromosomes align along equator of cell.
Metaphase II
Chromosomes align along equator of cell.
Notice the similarities and differences with mitosis

21. Anaphase II Sister chromatids separate and move to opposite poles.
Notice the sister chromatids are not genetically identical, Why?
Only 23 chromosomes (n) move to each opposite pole
Notice the similarities and differences with mitosis

22. Telophase II Nuclear envelope assembles. Chromosomes unravel.
Spindle apparatus disappears.
Cytokinesis divides cell into two.
Notice the similarities and differences with mitosis

23. Results of meiosis Four genetically different haploid cells
Variation
Crossing over (prophase I)
Independent assortment (metaphase I)
Could be 4 Sperm
Could be an egg
Could be a spore if you are a plant
One copy of each chromosome
n=23 (for a human)

24. “Putting It All Together” – Meiosis, Fertilization and Sexual Reproduction produces Variation
Dad
2n=46
n=23
YOU
(a little like your mom
a little like your dad)
n=23
Mom

25. Meiosis allows the creation of unique individuals
What Meiosis is About
Meiosis allows the creation of unique individuals
through sexual reproduction. It provides the variation that allows natural selection to work on in order to allow evolution to occur

26. Comparing Mitosis to Meiosis
Type of reproduction
Asexual
Sexual

28. Somatic cells (diploid) Interphase
Germ cell (diploid)
Interphase G1 S G2
M phase
Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I
Cytokinesis
2 haploid daughter cells move on to Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
4 haploid gametes used or reabsorbed. Do NOT return to Interphase.
Somatic cells (diploid)
Interphase G1 S G2
M phase
Mitosis
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
2 diploid somatic daughter cells return to Interphase (G1)

29. Mistakes in Meiosis: Nondisjunction occurs when homologous chromosomes or sister chromatids do not separate producing sperm or egg that does not have the normal number of chromosomes (23)
Notice that none of the gametes are normal
Notice that two of the gametes are normal and two are not