1. Chapter 10: Sexual Reproduction and Genetics
Meiosis

2. Objectives Main Idea: Meiosis produces haploid gametes.
Essential Questions:
How does reduction in chromosome number occur during meiosis?
What are the stages of meiosis?
What is the importance of meiosis in providing genetic variation?

3. Introduction
There are no two individuals that are identical to each other.
Genetic variation – a variety of traits in organisms

4. Chromosomes and Chromosome Number
Trait – Characteristics (hair color, height, or eye color)
Encoded in your genetic code, DNA
Genes – DNA segment that control the production of a protein.

5. Homologous chromosomes

6. Homologous Chromosomes
Humans have 46 chromosomes.
Each parent contributes 23 chromosomes, resulting in 23 pairs of chromosomes.
Homologous Chromosomes – Chromosomes that make up a pair. One chromosome from each parent.

7. Haploid and Diploid Cells
Gametes – sex cells that have half the number of chromosomes
Haploid Cell – A cell with n number of chromosomes.
Fertilization – The process by which one haploid gamete combines with another haploid gamete.
Diploid Cell – A cell with 2n number of chromosomes.

8. Meiosis I & II

9. Overview
Meiosis – A type of cell division that reduces the number of chromosomes. (reduction division)
Meiosis will create cells that have n number of chromosomes.
Meiosis starts with diploid cells which then divide and create haploid cells.

10. Meiosis I
Interphase:
Chromosomes replicate
Chromatin condenses

11. Meiosis I
Prophase I:
Pairing of homologous chromosomes occurs, each chromosome consists of two chromatids.
Centrioles move to opposite poles, spindle fibers form and bind to sister chromatids at centromere.
The nuclear envelope breaks down.
Synapsis forms between the pairs of homologous chromosomes.
Chromsomes are physically bound
together.

12. Meiosis I
Prophase I:
Crossing over produces exchange of genetic information. (Figure 10.4)

13. Prophase I

14. Meiosis I

15. Metaphase: Meiosis I vs Mitosis
Metaphase in Meiosis I
Metaphase in Mitosis
Mitosis: sister chromatids line up at the cell’s equator
Meiosis: homologous chromosomes line up at the cell’s equator

16. Meiosis I

17. Anaphase I
Chromosome number is reduced from 2n to n when the homologous chromosomes separate.
(Watch the Amoeba sister video on the website on demystifying chromosome reduction.)

18. Telophase I

19. Meiosis II Meiosis is only halfway completed at the end of meiosis I.
Meiosis II will continue to divide the chromosomes to produce haploid cells.

20. Prophase II Chromosomes condense. Spindles from in each new cell.
Spindle fibers attach to chromosomes.

21. Metaphase II
Centromeres of chromosomes line up randomly at the equator of each cell.
Difference between Mitosis and Meiosis:
Mitosis: a diploid number of chromosomes line up at the equator
Meiosis: a haploid number of chromosomes line up at the equator

22. Anaphase II Centromeres split.
Sister chromatids separate and move to opposite poles.

23. Telophase II Chromosomes reach the poles.
Four nuclei form around chromosomes.
Spindles break down.
Cells divide.

24. Product Four cells have formed.
Each nucleus contains a haploid number of chromosomes.

25. Animation Click on the link to see an animation of Meiosis: