1. Meiosis
Cell Division Part 2

2. Limitations of Mitosis
Change in DNA only occurs as a result of mutations
Genetic Variation is required for
Natural Selection
Adaption
Evolution

3. Cell Division in 2 Parts
Meiosis: two-stage cell division in which the chromosome number of the parental cell is reduced by half. Meiosis is the process by which gametes are formed. Meiosis occurs in the gonads. Gametes: sex cells that have a haploid chromosome number

4. Meiosis
Haploid: refers to the number of chromosomes in a gamete Diploid: refers to twice the number of chromosomes in a gamete. Every cell of the body, with the exception of sex cells, contains a diploid chromosome number. In humans: Haploid Chromosome # = 23 Diploid Chromosome # = 46

5. Unlike mitosis, meiosis produces 4 non-identical haploid cells.
Their purpose is for reproduction only.
Also meiosis has the same stages as mitosis, however they occur twice in order to reduce the chromosome number from diploid to haploid (reduction division).

7. Meiosis Offspring carry genetic information from each of the parents
Each of the 23 chromosomes you receive from your father is matched by 23 chromosomes from your mother.
Homologous chromosomes: paired chromosomes similar in shape, size, gene arrangement, and gene information

8. Meiosis
During fertilization, a haploid (n = 23) sperm cell unites with a haploid (n = 23) egg cell to produce a diploid (2n = 46) zygote.
Zygote: a cell resulting from the union of a male and female sex cell, until it divides and then is called an embryo

9. Stages of Meiosis
Overall: 2 nuclear divisions that produce 4 haploid cells Meiosis I  Reduction Division – diploid reduced to haploid Prophase I Metaphase I Anaphase I Telophase I Meiosis II  Separation of two chromatids Prophase II Metaphase II Anaphase II Telophase II

10. Prophase I Nuclear membrane dissolves
Chromosomes are attached to their copy by a centromere.
Centrioles appear and move to the poles.
All this is similar to mitosis.
Tetrad: a pair of homologous chromosomes, each with two chromatids
Synapsis: the pairing of homologous
chromosomes

11. Crossing Over
Crossing over: the exchange of genetic material between two homologous chromosomes
 recombination shuffles the allele content between homologous chromosomes (increasing genetic diversity)

12. Metaphase I
Once aligned, the tetrads are held in place by spindle fibres released from the centrioles at the poles.
The spindle fibres attach to the centromeres of the sister chromatids.

13. Anaphase I
The spindle fibres contract and pull apart the tetrad such that one pair of sister chromatids goes to one pole and the other pair goes to the other pole.
Notice that the centromere did not split apart during this phase. It still holds the chromosome copies (sister chromatids) together.

14. Telophase I and Cytokenesis
A nuclear membrane reforms around each new nucleus and the cytoplasm is divided by cytokinesis creating 2 non-identical cells.
These two cells immediately proceed into the next round of meiosis as there is no second round of interphase.

15. Meiosis I
DNA replication has occurred before cell division

16. Main points so far How is genetic variation possible during meiosis I?
Crossing Over
Independent Assortment
What was the purpose of meiosis I?
Hapoid Chromosome #

17. Meiosis II
The stages of meiosis II are all exactly identical to the happenings from mitosis.
The only difference is that they are occurring in two cells at the same time.
The end product is four, non-identical, haploid cells.
The process of meiosis occurs somewhat differently in males and females.

18. Meiosis II Pairs of chromatids separate and move to opposite poles
There is NO replication of chromosomes before meiosis II
each cell has the half the # of chromosomes as the parent cell

19. Meiosis II What was the purpose of meiosis II?
To produce four daughter cells each with one half the number of chromosomes of the original parent cell
Are the daughter cells genetically identical to the parent? Why or why not?
No due to crossing over and independent assortment in Meiosis I

20. Mitosis vs Meiosis