1. MITOSIS REVIEW

2. Sexual Cell Reproduction
MEIOSIS and GENETIC RECOMBINATION

3. Meiosis
Meiosis is the process of cell division where sex cells are produced
Cells formed from meiosis differ from regular daughter cells in that they are gametes, containing half the genetic information of the original mother cell

5. Since each gamete contains half the original number of chromosomes, when fertilized, the zygote contains a full set of chromosomes and is now diploid
The zygote will then develop into an organism by the process of mitosis

6. Advantages of Sexual Reproduction
Due to the recombination of DNA that occurs during sexual reproduction, greater variation occurs in organisms that reproduce sexually
Each offspring contains half the genetic information from each parent
In humans, 23 chromosomes from the mother pair with 23 chromosomes from the father

7. Homologous Chromosomes
This pairing creates homologous chromosomes and have genes that code for the same characteristics
For example, each chromosome in the pair have genes for eye colour, however, one gene will code for characteristic actually displayed in the organism
Each characteristic is determined by how the two chromosomes in the pair interact

8. Reduction (M I) Division (M II)
Stages of Meiosis
Since the cells produced by meiosis are haploid,
two cell divisions must occur instead of one
Otherwise, upon fertilization, the genetic information would be double what it is supposed to be
Thus, four haploid cells are produced when meiosis occurs
During the first division, called meiosis I, the homologous chromosomes move to different poles of the mother cell, thus reducing the number of chromosomes to half the original
During Meiosis II, the chromatids separate
Reduction (M I) Division (M II)

9. Meiosis I – Prophase I Nuclear membrane dissolves
Centrioles move to poles
Spindle fibres form

10. Meiosis I – Prophase I
Replicated xsomes come together in homologous pairs
Because each pair consists of 4 chromatids, this pairing is often called a tetrad
This process is referred to as synapsis
As the tetrads come close together, often times they intertwine, and switch pieces of genetic information
This is known as crossing over

12. Meiosis I – Metaphase I
Xsomes line up at equatorial plate in homologous pairs
(in their tetrad)
*** Difference from MITOSIS – do not line up single file***
Spindle fibres attach to centromeres
INDEPENDENT ASSORTMENT:
xsomes line up in random order
along the equatorial plate

13. Meiosis I - Anaphase I
During anaphase I, the homologous chromosomes move towards opposite poles
This process is called segregation, and reduces the number of chromosomes to half of the original number
One member of each homologous pair is found in each of the new cells

14. WHY??!? Meiosis I – Telophase I Cytoplasm divides (cytokinesis)
Nuclear membrane forms
***Different from mitosis, the two new cells formed are not identical***
WHY??!?
The two new cells then prepare for meiosis II

15. Meiosis II
The two new cells will undergo meiosis II at relatively the same time
The entire process repeats from prophase to telophase, and during the second division, the chromatids will separate
**NOTE: no replication of chromosomes between MI and MII

16. MEIOSIS II
Prophase II - the nuclear membrane dissolves and spindle fibers form
Metaphase II - the chromosomes align themselves on the equatorial plate
The chromatids are still pinned together with the centromere
Anaphase II - sister chromatids are pulled apart and moved to opposite poles of the cell
Telophase II – two cells separate and there are now four, non- identical cells called gametes

18. Mitosis VS Meiosis

19.
Mitosis VS Meiosis

20. GENETIC RECOMBINATION
What processes create NEW combinations of genetic material in meiosis?
Crossing Over
Prophase I – homologous xsomes exchange genetic material (between NON SISTER chromatids)
RESULT: xsomes contain some maternal AND some paternal info (genes)
Independent Assortment
Metaphase I – M/P xsomes line up at equator in ANY arrangement
RESULT: gametes have different combinations of parental xsomes

21. STOP CAT!!