1. Lesson Overview 11.4 Meiosis

2. Chromosome Number
Homologous chromosomes – for each chromosome in normal body cells, there is one that comes from the mother and one that comes from the father
Ex.

3. Diploid vs. Haploid Cells
DIPLOID CELLS (somatic, or normal body cells)
A cell that contains both sets of homologous chromosomes is diploid, meaning “two sets.”
Represented by the symbol 2n

4. Diploid vs. Haploid Cells
HAPLOID CELLS (gametes, or sex cells)
Some cells contain only a single set of chromosomes, and therefore a single set of genes; such cells are haploid, meaning “one set.”
Gametes of sexually reproducing organisms are HAPLOID
can be written as n

5. Phases of Meiosis
Meiosis: process in which the number of chromosomes per cell is cut in half
Meiosis usually involves two distinct divisions, meiosis I and meiosis II.
By the end of meiosis II, the ONE diploid cell becomes FOUR haploid cells

6. Meiosis I Before meiosis I, the cell goes through INTERPHASE
Each replicated chromosome consists of two identical chromatids joined at the center

7. PROPHASE I
Chromosomes pair up forming a tetrad, which contains four chromatids
They undergo a process called crossing-over
Crossing-over is important because it is a source of genetic variation

8. METAPHASE I / ANAPHASE I
Paired homologous chromosomes line up across the center of the cell (metaphase)
Homologous chromosome pair pulled toward opposite ends of the cell (anaphase)

9. TELOPHASE I / CYTOKINESIS
Nuclear membrane forms around each cluster of chromosomes (telophase)
Two new cells formed (cytokinesis)

11. MEIOSIS I – IMPORTANT EVENTS
1. In prophase I of meiosis, crossing over
occurs
2. During metaphase I of meiosis independent
assortment occurs

12. Meiosis II
The two cells produced by meiosis I now enter a second meiotic division (MEIOSIS II)
NEITHER CELL GOES THROUGH INTERPHASE (no replicating of DNA)

13. MEIOSIS II Basically same as phases as mitosis
By the end of meiosis II, the diploid cell becomes four haploid cells (GAMETES)
Male gamete = SPERM
Female gamete = EGG

14. Gametes to Zygotes
Fertilization—the fusion of male and female gametes—generates new combinations of alleles in a zygote

19. Comparing Meiosis and Mitosis