Cell Division and Reproduction

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 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 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 Mitosis and Meiosis Compared