3. Meiosis
The process in which the nucleus of a cell completes two successive divisions that produce four nuclei, each with a chromosome number that has been reduced by half.
Known as reduction division
A diploid cell is reduced to four haploid cells through two divisions

4. Genetic Variety
Meiosis is a process to convert a diploid cell to a haploid gamete, and cause a change in the genetic information to increase diversity in the offspring.
Formation of sex cells (gametes)
Female- produces the egg
Male- produce the sperm

5. new individual is formed by a combination of two haploid sex cells (gametes). 223 possibilities
The different types of gametes produced is 2n, where n = the haploid number

6. Haploid (n)-- one set of chromosomes
Diploid (2n)-- two sets of chromosomes
Gamete-haploid cell that fuses with another during fertilization (the egg and sperm)

10. Meiosis I The first four of eight total phases of meiosis
These phases have the same names as mitosis, but the events of each phase differ significantly.

11. Prophase I
Chromosomes start to coil up and so become shorter and thicker
Homologous chromosomes pair up
Crossing over occurs
Centrioles move to the poles in animal cells
Nucleoli break down
At the end of prophase I the nuclear membrane breaks down

12. Metaphase I Chromosomes continue to shorten and thicken
Spindle microtubles attach to the centromeres
Homologous chromosomes line up on the equator
Chiasmata slide towards the ends of the chromosomes, causing the shape of the pairs to change
At the end of metaphase I the chromosomes start to move

13. Anaphase I
The two chromosomes of each pair move to opposite poles. This halves the chromosome number. Each chromosome consists of two chromatids. Because of crossing over the two chromatids are not identical.
At the end of anaphase I the chromosomes reach the poles

14. Telophase I
Nuclear membranes form around the groups of chromosomes at each pole
The cell divides to form two haploid cells
The chromosomes uncoil partially
At the end of telophase I the two cells either enter a brief period of interphase or immediately proceed to the second division of meiosis. The DNA is NOT replicated

15. Meiosis II
Meiosis II is identical to mitosis except that the chromosomes DO NOT replicate before they divide.
The final product of Meiosis II are four haploid cells (or the egg and sperm)

16. Prophase II Chromosomes become shorter and thicken again by coiling
Centrioles move to the poles in animal cells
At the end of prophase II the nuclear membrane breaks down

17. Metaphase II Spindle microtubules attach to the centromeres
Chromosomes line up on the equator
At the end of metaphase II the centromeres divide

18. Anaphase II
The two chromatids of each chromosome move to opposite poles
At the end of anaphase II the chromatids reach the poles

19. Telophase II
Nuclear membranes form around the groups of chromatids at each pole. Each chromatid is now considered to be a chromosome
The two cells each divide to form four cells total
The chromosomes uncoil
Nucleoli appear
In most organisms the cells formed at the end of telophase II develop into gametes

21. Crossing-over-the exchange of genetic information by non-sister chromatids of paired chromosomes at the start of meiosis (prophase I)
Chiasma (ki’az-um) point where homologous chromosomes are held together (forms the X)
Homologous chromosome-the two copies of each chromosome (diploid) that have the same genes, in the same sequence, but not necessarily the same alleles

22. Recombination
The reassortment of genes or characteristics into different combinations from those of the parents

24. Non-disjunction
The failure of chromosomes to separate during either meiosis I or meiosis II
Results in gametes with either one too many or too few chromosomes
Gametes with one too few usually die quickly
Gametes with one too many sometimes survive

40. Meiosis Video