1. Cell Reproduction
Part 2

2. Meiosis
Nuclear division that occurs only in reproductive cells of an organism.
A variation of mitosis, in which daughter cells receive half of the usual number of chromosomes of the parent.

3. Meiosis (cont’d)
Only one chromosome of each homologous pair goes into the daughter cell.
Results in a haploid number (1n).

4. Meiosis (cont’d) Involves 2 successive cell divisions: Meiosis I
Meiosis II
Produces 4 cells.

5. Meiosis (cont’d)
Each division has stages similar to those of mitotic division, but with certain important differences.

6. Interphase
Cells still undergo G1, S, and G2 phases and begin meiosis with a duplicate set of chromosomes.

7. Meiosis I
4 phases: Prophase I, Metaphase I, Anaphase I, and Telophase I

8. Prophase I DNA coils into chromosomes. Spindle fibers appear.
Nucleus and nucleolus disappear.

9. Prophase I (cont’d)
Every chromosome lines up next to its homologue and joins at the centromere.
This pairing is called synapsis.

10. Prophase I (cont’d)
Since each chromosome has replicated before this, this combination has 4 chromatid strands.
This group of chromatid strands is call a tetrad.

11. Prophase I (cont’d)
During synapsis, the chromatids twist around each other and sometimes exchange segments.
Crossing over
Results in genetic recombination – new mixture of genetic material.

12. Crossing Over

13. Metaphase I
Tetrads of homologous chromosomes become attached to the spindle fibers at their centromere.

14. Metaphase I (cont’d) Move to the central plane.
Orientation of the homologous pairs is random with respect to the poles.

15. Independent Assortment

16. Anaphase I
Each homologous chromosome (2 chromatids attached at a centromere) moves to the opposite pole.
This is called disjunction.

17. Anaphase I (cont’d)
The random separation of homologous chromosomes is called independent assortment.

18. Anaphase I (cont’d)
Results in the random separation of maternal and paternal chromosomes.
Results in genetic recombination.

19. Anaphase I (cont’d) This is the phase where mistakes can happen.
Nondisjunction
When chromosomes do not separate properly.

20. Telophase I Chromosomes reach the opposite ends of the cell.
New nuclear membranes form.
Cytokinesis begins, dividing the cell into 2 cells.

21. Interphase
A brief interphase occurs with no DNA replication.

23. Meiosis II Follows the same course as mitotic division.
4 phases: Prophase II, Metaphase II, Anaphase II, and Telophase II

24. Prophase II
Spindle fibers form.
Chromosomes recoil.

25. Metaphase II
Chromosomes move to the central plane.

26. Anaphase II Centromere divide.
Chromatids separate and move to the opposite poles of the cell.

27. Telophase II Chromosomes uncoil. Cytokinesis occurs. 4 cells formed.
Each contain half (haploid number) of the original number of chromosomes (diploid number).

28. Telophase II (cont’d)
Nuclear membrane forms around the chromosomes in each of the 4 new cells.

30. Formation of Gametes
In animals, meiosis produces haploid reproductive cells called gametes.
This occurs only in the testes and ovaries.

31. Spermatogenesis Occurs in the testes.
Produces 4 male gametes called sperm cells or spermatozoa.
Meiosis forms 4 haploid cells called spermatids which then develop into sperm cells.

33. Oogenesis
Occurs in the ovaries.
Produces mature egg cells or ova.

34. Oogenesis (cont’d) In cytokinesis, the cytoplasm is unequally divided.
One egg is formed.
3 polar bodies are formed, which degenerate.

36. Asexual Reproduction Production of offspring by one parent.
New organisms are usually produced by mitotic division of the parent cell.
Offspring are genetically identical to the parent.

37. Asexual Reproduction (cont’d)
Unicellular organisms: new organisms are produced by binary fission or mitosis.
Multicellular organisms: new organisms produced by budding of portions of parent.

38. Sexual Reproduction
Production of offspring through meiosis and the union of sperm and egg.
Offspring are genetically different from the parents.
Genes were combined in new ways in meiosis.