1. Meiosis
Sexual Reproduction

2. Meiosis

3. Chromosomes and Chromosome Number
Sexual Reproduction and Genetics
Meiosis
Chromosomes and Chromosome Number
Human body cells have 46 chromosomes
Each parent contributes 23 chromosomes
Homologous chromosomes—one of two paired chromosomes, one from each parent

4. Homologous Chromosomes
Sexual Reproduction and Genetics
Meiosis
Homologous Chromosomes
Same length
Same centromere position
Carry genes that control the same inherited traits

5. Gametes Gametes are sex cells Female gametes Animals & Plants= eggs
Sexual Reproduction and Genetics
Meiosis
Gametes
Gametes are sex cells
Female gametes
Animals & Plants= eggs
Male gametes
Animals= sperm
Plants= pollen

6. Haploid and Diploid Cells
Sexual Reproduction and Genetics
Meiosis
Haploid and Diploid Cells
An organism produces gametes to maintain the same number of chromosomes from generation to generation.
Human gametes contain 23 chromosomes.
A cell with n chromosomes is called a haploid cell.
A cell that contains 2n chromosomes is called a diploid cell.
All human body cells have 46 chromosomes

7. The sexual life cycle in animals involves meiosis.
Sexual Reproduction and Genetics
Meiosis
Meiosis I
The sexual life cycle in animals involves meiosis.
Meiosis produces gametes (sex cells).
When gametes
combine in fertilization, the number of
chromosomes is restored.

8. Sexual Reproduction and Genetics
Meiosis
Stages of Meiosis I
Reduces the chromosome number by half through the separation of homologous chromosomes
Involves two consecutive cell divisions called meiosis I and meiosis II

9. Chromosomes replicate.
Sexual Reproduction and Genetics
Meiosis
Meiosis I
Interphase I
Chromosomes replicate.
Chromatin condenses.
Interphase

10. Pairing of homologous chromosomes occurs.
Sexual Reproduction and Genetics
Meiosis
Meiosis I
Prophase I
Pairing of homologous chromosomes occurs.
Each chromosome consists of two chromatids.
Prophase I
The nuclear envelope breaks down.
Spindle fibers form.

11. Sexual Reproduction and Genetics
Meiosis
Meiosis I
Prophase I
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad
Tetrad
Contains 4 chromatids

12. Crossing over produces exchange of genetic information.
Sexual Reproduction and Genetics
Meiosis
Meiosis I
Prophase I
Crossing over produces exchange of genetic information.
Crossing over—chromosomal segments are exchanged between a pair of homologous chromosomes.

13. Chromosome centromeres attach to spindle fibers.
Sexual Reproduction and Genetics
Meiosis
Meiosis I
Metaphase I
Chromosome centromeres attach to spindle fibers.
Metaphase I
Homologous chromosomes line up at the equator.

14. Homologous chromosomes separate and move
Sexual Reproduction and Genetics
Meiosis
Meiosis I
Anaphase I
Homologous chromosomes separate and move
to opposite poles of the cell.
Anaphase I

15. The spindles break down.
Sexual Reproduction and Genetics
Meiosis
Meiosis I
Telophase I
The spindles break down.
Telophase I
Chromosomes uncoil and form two nuclei.
The cell divides.

16. A second set of phases begins
Sexual Reproduction and Genetics
Meiosis
Meiosis II
Prophase II
A second set of phases begins
as the spindle apparatus forms and the
chromosomes condense.
Prophase II

17. A haploid number of chromosomes
Sexual Reproduction and Genetics
Meiosis
Meiosis II
Metaphase II
A haploid number of chromosomes
line up at the equator.
Metaphase II

18. The sister chromatids are
Sexual Reproduction and Genetics
Meiosis
Meiosis II
Anaphase II
The sister chromatids are
pulled apart at the centromere by spindle
fibers and move toward the opposite poles
of the cell.
Anaphase II

19. The chromosomes reach the poles, and
Sexual Reproduction and Genetics
Meiosis
Meiosis II
Telophase II
The chromosomes reach the poles, and
the nuclear membrane and nuclei reform.
Telophase II

20. Sexual Reproduction and Genetics
Meiosis
Meiosis II
Cytokinesis results in four haploid cells, each with n number of chromosomes.
Cytokinesis

22. Meiosis

23. The Importance of Meiosis
Sexual Reproduction and Genetics
Meiosis
The Importance of Meiosis
Meiosis consists of two sets of divisions
Produces four haploid daughter cells that are not identical
Results in genetic variation

24. Meiosis Provides Variation
Sexual Reproduction and Genetics
Meiosis
Meiosis Provides Variation
Depending on how the chromosomes line up at the equator, four gametes with four different combinations of chromosomes can result.
Genetic variation also is produced during crossing over and during fertilization, when gametes randomly combine.

25. Sexual Reproduction v. Asexual Reproduction
Sexual Reproduction and Genetics
Meiosis
Sexual Reproduction v. Asexual Reproduction
Asexual reproduction = Mitosis
The organism inherits all of its chromosomes from a single parent.
The new individual is genetically identical to its parent.
Sexual reproduction = Meiosis
Beneficial genes multiply faster over time.
Genetic Variation

26. Meiosis Square Dance

27. Difference between Mitosis vs. Meiosis