1. Cell Division and Reproduction
Asexual and Sexual reproduction
By : D. Reis

2. Asexual Reproduction
A single organism produces offspring with identical genetic information

3. Sexual Reproduction
Haploid (n=23) sperm cell fertilizes a haploid (n=23) egg cell to produce a dipolid (2n=46) zygote.

4. Sexual Reproduction Cells of the human body have 46 chromosomes
When fertilization occurs a sperm and egg cell combine their chromosomes to make a zygote with 46 chromosomes.
How is this possible?

6. Meiosis
The division of cells to produce specialized sperm and egg cells 46 23 23 23 23 23 23

7. Phases of Meiosis Assume DNA replication occurs prior to meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I
Prophase II
Metaphase II
Anaphase II
Telophase II

8. Homologous Chromosomes
Paired chromosomes similar in shape, size, gene arrangement and gene information.
Each somatic cell contains 23 pairs of homologus chromosomes, or 46 in total

9. Stages of Meiosis I PROPHASE I – Nuclear membrane dissolves
Centrioles move to opposite poles
Formation of spindle fibers

10. Stages of Meiosis I PROPHASE I-
Homologus chromosomes pair up in a process called synapsis and crossing over occurs

11. Crossing Over/ Genetic Recombination
The exchange of genetic material between two homologus chromosomes leads to genetic variability.
Tetrad – a pair of homologus chromosomes composed of 4 chromatids.

12. Stages of Meiosis I METAPHASE I-
Homologus chromosomes line up at the equatorial plate.

13. Stages of Meiosis I
ANAPHASE I- Spindle fibers shorten, the homologus chromosome pairs move toward opposite poles. THE SISTER CHROMATIDS DO NOT PULL APART AS THEY DO IN MITOSIS.
23 duplicated chromosomes move to opposite poles.
TELOPHASE I- Two new genetically different haploid daughter cells are formed each containing 23 double stranded chromosomes
Formation of nuclear membrane

15. Stages of Meiosis II
PROPHASE II – Nuclear membrane dissolves, spindle fibers form and centrioles move to opposite poles.
METAPHASE II – Spindle fibers attach to the chromosomes and the chromosomes (2 sister chromatids) align along the equatorial plate.

16. Stages of Meiosis II
ANAPHASE II – Sister chromatids separate and move to opposite poles.
TELOPHASE II – Chromatids arrive at each pole and cell division begins.

17. Meiosis 2. Chromosomes are replicated and attached at the centromere
1. Homologus chromosomes: 23 paternal and 23 maternal
2. Chromosomes are replicated and attached at the centromere

18. Meiosis
3. Meiosis I reduction stage- Separation of homologus chromosomes
4. Meiosis II- separation of sister chromatids

19. Results of Meiosis
4 gametes/reproductive cells which contain 23 chromosomes
Haploid-each cell contains half the number of chromosomes from the parent cell
All 4 cells are genetically different

20. Results of Meiosis

21. Results of Meiosis Oogenesis- the production of egg cells in females
Spermatogenesis- the production of sperm cells in males

22. Mitosis and Meiosis Compared

23. Mitosis and Meiosis Compared
2 daughter cells are produced
Daughter cells are diploid
Daughter cells are the same as parent cells
Meiosis
4 daughter cells are produced
Daughter cells are haploid
Daughter cells are genetically different from parent cells