1. Meiosis A reduction division

2. Meiosis Multi-celled organisms Multi-celled organisms produce gametes produce gametes (cells) through the process (cells) through the process of meiosis. of meiosis. In humans and many other organisms, these cells are called ova(eggs) and sperm. In humans and many other organisms, these cells are called ova(eggs) and sperm. When an ovum and a sperm combine, a new organism is formed with a full set of chromosomes, half of which came from the father, and half of which came from the mother. When an ovum and a sperm combine, a new organism is formed with a full set of chromosomes, half of which came from the father, and half of which came from the mother.

3. So what is a chromosome anyway?

4. Meiosis is a type of cell division where one cell divides to form new cells, each of which have half the number of chromosomes as the original (ex – Humans have ? normally?). Meiosis is a type of cell division where one cell divides to form new cells, each of which have half the number of chromosomes as the original (ex – Humans have ? normally?). This condition is called haploid. This condition is called haploid. Cells with the original number of chromosomes are called diploid. Cells with the original number of chromosomes are called diploid. Another way to say this is that each new daughter cell gets half of the parent cell’s DNA. Another way to say this is that each new daughter cell gets half of the parent cell’s DNA.

5. Humans have 23 pairs of chromosomes, for a total of 46. Humans have 23 pairs of chromosomes, for a total of 46. In each pair, one chromosome comes from mom and one comes from dad. In each pair, one chromosome comes from mom and one comes from dad. Both members of the chromosome pair are the same size, shape, and carry the same genetic information. Both members of the chromosome pair are the same size, shape, and carry the same genetic information.

6. There are two main stages of meiosis: Meiosis I and Meiosis II. There are two main stages of meiosis: Meiosis I and Meiosis II. Each of these is divided into four stages: Prophase, Metaphase, Anaphase and Telophase Each of these is divided into four stages: Prophase, Metaphase, Anaphase and Telophase Before the cell starts dividing, it has to complete Interphase – What happens during this very important time? Before the cell starts dividing, it has to complete Interphase – What happens during this very important time?

7. Meiosis We refer to the 23 pairs as homologous pairs. Why? We refer to the 23 pairs as homologous pairs. Why? During the earliest stage of meiosis, the members of each pair will temporarily attach together to form a structure called a tetrad. Why are there four instead of two? During the earliest stage of meiosis, the members of each pair will temporarily attach together to form a structure called a tetrad. Why are there four instead of two? homologous pair tetrad

8. Prophase I Very similar to Prophase of mitosis. Very similar to Prophase of mitosis. What happens during Prophase? What happens during Prophase? Chromatin does what? Chromatin does what? N guys do what? N guys do what? Spindle fibers? Spindle fibers? Chromosomes (tetrads) do what? Chromosomes (tetrads) do what? Centrioles do what? Centrioles do what? There are a few extras to add! There are a few extras to add! The tetrads actually exchange some of their DNA in a process called crossing-over (Swapping spit – Disgusting!!) The tetrads actually exchange some of their DNA in a process called crossing-over (Swapping spit – Disgusting!!)

9. Metaphase I _______, not individual chromosomes, line up on the equator of the cell. _______, not individual chromosomes, line up on the equator of the cell. _______ fibers attach to the centromeres of chromosomes. _______ fibers attach to the centromeres of chromosomes.Tetrads Spindle

10. Anaphase I Spindle fibers contract and pull _____ apart. Spindle fibers contract and pull _____ apart. Entire double stranded chromosome pulled to opposite ends of the cell. Entire double stranded chromosome pulled to opposite ends of the cell. Chromosome number is now half of original (haploid). Chromosome number is now half of original (haploid). tetrads

11. Telophase I Cell plate (plant) or cleavage furrow (animal) divides the _________. Cell plate (plant) or cleavage furrow (animal) divides the _________. Cell usually goes directly into Prophase II, but if not, chromosomes relax, nuclei reform, and spindle fibers break down. Cell usually goes directly into Prophase II, but if not, chromosomes relax, nuclei reform, and spindle fibers break down. Each daughter cell has _____ of the number of chromosomes as the original. cytoplasm half Chromosomes not to scale

12. Summary of Meiosis II The mechanics of Meiosis II are _______ to Mitosis. The mechanics of Meiosis II are _______ to Mitosis. Each haploid daughter cell produced by Meiosis I divides again, so that four haploid cells are produced altogether. Each haploid daughter cell produced by Meiosis I divides again, so that four haploid cells are produced altogether. In _____________, all four cells survive to produce viable sperm. In _____________, all four cells survive to produce viable sperm. In _________, only one cell survives to produce a viable ovum. In _________, only one cell survives to produce a viable ovum. spermatogenesis oogenesis identical

13. Overview Tetrads form (Pro I) Tetrads form (Pro I) Crossing over can occur (Pro I) Crossing over can occur (Pro I) Tetrads line up and are separated (Meta I) Tetrads line up and are separated (Meta I) Daughter cells are haploid Daughter cells are haploid Produces gametes (ova and sperm) Produces gametes (ova and sperm)

14. Compare Critical differences between Mitosis and Meiosis: Critical differences between Mitosis and Meiosis: Tetrads form (Pro I) Tetrads form (Pro I) Crossing over can occur (Pro I) Crossing over can occur (Pro I) Tetrads line up and are separated (Meta I) Tetrads line up and are separated (Meta I) Daughter cells are haploid Daughter cells are haploid Only occurs in germ cells Only occurs in germ cells Produces gametes (ova and sperm) Produces gametes (ova and sperm)

15. Mitosis vs. Meiosis I Left: Metaphase of MITOSIS: no pairing!!!! Right: Metaphase I of Meiosis: homologous chromosomes Pair UP!