1. Meiosis

2. 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

3. Chromosomes and Chromosome Number
Sexual Reproduction and Genetics
Chromosomes and Chromosome Number
Same length
Same centromere position
Carry genes that control the same inherited traits

4. Haploid and Diploid Cells
Sexual Reproduction and Genetics
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.

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

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

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

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

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

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

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

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

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

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

15. The sister chromatids are
Sexual Reproduction and Genetics
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

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

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

18. Meiosis Provides Variation
Sexual Reproduction and Genetics
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.

19. Meiosis
the process of cellular division where the number of the chromosomes is halved.
produces four daughter cells
all daughter cells are haploid
chromosomes are shuffled in the process, so that each daughter cell has a unique combination
used to create gametes (sperm and egg)

21. Beginning Stages Prophase 1 homologous chromosomes pair up (tetrad)
crossing-over occurs
Metaphase & Anaphase
tetrads line up along the equator, homologs separate

22. Crossing Over
Crossing Over - chromosomes exchange sections (increases variability)

23. Independent Assortment
In addition to Crossing-Over, the process of meiosis ensures that chromosomes are randomly assorted. The following images show three separate possibility for a single cell that has undergone meiosis. Look at all the different combinations.

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

25. Sexual Reproduction
Involves the fusion of gametes, creates unique offspring
Eggs are much larger than sperm
Image of egg and sperm

26. Sexual Reproduction

27. Asexual Reproduction
--Does not involve the fusion of gametes --Offspring are identical to parents --Budding, Fragmentation (Regeneration), Fission