1. Meiosis
Section

2. Learning Targets: I can… Describe the basic process of meiosis
Compare and contrast mitosis and meiosis

3. Chromosomes
Hair color, eye color, height, etc. are all characteristics that are passed on from a parent to child
Each of these characteristics are called traits
Instructions for each trait are located on chromosomes found in the nucleus of cells

4. Chromosomes
DNA on chromosomes are arranged in segments that control the production of proteins
The DNA segments are called genes
Each chromosome consists of hundreds of genes
Each gene plays an important role in determining the characteristics and functions of the cell

5. Homologous Chromosomes
Human body cells have 46 chromosomes
Each parent contributes 23 chromosomes, resulting in 23 pairs of chromosomes
Each pair (one chromosome from each parent) are called homologous chromosomes
Each homologous chromosome will be the same size/shape and the centromere will be in the same position

6. Homologous Chromosomes
Each homologous chromosome carries genes that control the same inherited traits
Ex: the gene for earlobe type will be located at the same position on both homologous chromosomes
Though both genes will code for earlobe type, they will not necessarily code for the same earlobe type

7. Homologous Chromosomes

8. Haploid Cells
In order to maintain the same chromosome number from generation to generation, an organism produces gametes
Gametes: sex cells that have half the number of chromosomes
Each human gamete contains 23 chromosomes
Gametes are haploid cells
Haploid: cells with a single (n) set of chromosomes

9. Diploid Cells
When one haploid gamete combines with another haploid gamete fertilization occurs
The cell created will have 2n chromosomes (n from mother and n from father)
This is called a diploid cell
In humans diploid cells have 46 chromosomes or 23 homologous chromosomes

10. Meiosis Gametes are formed during a process called Meiosis
Meiosis is a type of cell division that reduces the number of chromosomes
Occurs in the reproductive structures of organisms that reproduce sexually
Meiosis involves 2 consecutive cell divisions called meiosis I and meiosis II

11. Meiosis I Steps of Meiosis I
Interphase: Undergo interphase like mitosis
Cells carry out various metabolic processes and replicate DNA

12. Meiosis I Prophase I Replicated chromosomes become visible
Consist of 2 sister chromatids
Homologous chromosomes begin to form pairs as they condense in a process called synapsis
Homologous chromosomes are held tightly together

13. Meiosis I Prophase I cont…
Crossing Over: process during which chromosomal segments are exchanged between a pair of homologous chromosomes
Centrioles move to the cell’s opposite poles, spindle fibers form and bind to the sister chromatids at the centromere

14. Meiosis I
Prophase I

15. Crossing Over

16. Meiosis I
Metaphase I
Pairs of homologous chromosomes line up at the equator
Spindle fibers attach to each homologous chromosome

17. Meiosis I Anaphase I Homologous chromosomes separate
Each member of the pair is guided by spindle fibers and moves toward opposite poles
Unlike mitosis, when homologous chromosomes split in anaphase, each still has 2 sister chromatids

18. Meiosis I
Anaphase I

19. Meiosis I
Telophase I
Homologous chromosomes reach the cell’s opposite poles
Each pole only contains one member of the original pair of homologous chromosomes
Note that each chromosome may not be identical because of crossover that occurred during synapsis
Cytokinesis also happens at this point

20. Meiosis II Steps of Meiosis II Prophase II
Spindle apparatus forms in each new cell from Meiosis I
Chromosomes condense
Spindle fibers attach to each chromosome

21. Meiosis II Metaphase II Anaphase II
Centromeres of chromosomes line up randomly at the equator of each cell
Anaphase II
Sister chromatids are pulled apart at the centromere by the spindle fibers
Sister chromatids move toward opposite poles

22. Meiosis II Telophase II At the end of Meiosis II, cytokinesis occurs
Sister chromatids reach opposite poles
Nuclear membrane and nuclei reform
At the end of Meiosis II, cytokinesis occurs
Results in 4 haploid cells

23. Meiosis

24. Importance of Meiosis Meiosis provides variation among offspring
This occurs when homologous chromosomes line up at the equator during metaphase I randomly
Since crossing over may have occurred, the chromosomes are not identical, therefore each of the 4 haploid cells may not be identical

25. Asexual v. Sexual Reproduction
Asexual Reproduction – organisms inherit all of its chromosomes from a single parent
Therefore, the new individual is genetically identical to its parent
Some organisms use both asexual and sexual reproduction
Considered less advance because it has been shown that beneficial genes multiply faster with sexual reproduction