Meiosis

1. Meiosis

2. What is Meiosis? • Meiosis produces daughter cells that have one half the number of chromosomes as the parent cell. • 2N ® N Meiosis enables organisms to reproduce sexually. Gametes (sperm and eggs) are haploid. Meiosis involves two divisions producing a total of four daughter cells.

3. What is Meiosis? • Mitosis is normal cell division, which goes on throughout life in all parts of the body. • Meiosis is the special cell division that creates the sperm and eggs, the gametes. We will discuss meiosis separately.

4. Meiosis • Cell division to form the gametes, sperm (male gamete) and egg (female gamete). • Characteristic of eukaryotes only: not in prokaryotes. • Normal cells are diploid: 2 copies of every gene. • Diploidy is useful because 2 copies of every gene means that there a backup copy if one gets mutated. Mutations are very frequent in the cells of large organisms. We wouldn’t survive with just one copy of each gene. • Gametes are haploid: 1 copy of every gene • Need to choose 1 copy of each gene randomly. • Why have sexual reproduction? Shuffling of alleles between parents and offspring leads to new combinations. Bad combinations die without reproducing; good combinations survive and reproduce more offspring.

5. Summary of the Phases of Meiosis • Start with a diploid cell, with 2 copies of each chromosome, one form each parent. The two copies are called homologues. • Chromosomes each with 2 chromatids attached at the centromere.

6. Summary of the Phases of Meiosis • Use 2 cell divisions: • Meiosis 1. First separate the homologues • Meiosis 2. Then separate the 2 chromatids.

7. Summary of the Phases of Meiosis • The stages of meiosis have the same names as in mitosis: prophase, metaphase, anaphase, telophase. Each of the 2 cell divisions has all of these stages. • Meiosis 1 is unusual and needs a bit of study, but meiosis 2 is just like mitosis

8. meiosis 1:prophase 1, metaphase 1, anaphase 1, and telophase 1 • meiosis 2:prophase 2, metaphase 2, anaphase 2, and telophase 2 • A cell undergoing meiosis will divide two times; the first division is meiosis 1 and the second is meiosis 2. The phases have the same names as those of mitosis. A number indicates the division number (1st or 2nd):

9. Meiosis 1 In the first meiotic division, the number of cells is doubled but the number of chromosomes is not. This results in 1/2 as many chromosomes per cell. • Two important events in meiosis 1: crossing over in prophase, and the pairing of homologues in metaphase.

10. Meiosis 1 • Prophase I • Homologous chromosomes become paired. • Crossing-over occurs between homologous chromosomes.

11. Meiosis 1 Metaphase I • Homologous pairs become aligned in the center of the cell.

12. Meiosis 1 Anaphase I • Homologous chromosomes separate. • Homologous Chromosomes • Diploid organisms have two copies of each chromosome (except the sex chromosomes). Each pair of chromosomes is homologous. For example, the two #7 chromosomes are homologous. The homologue to the #3 chromosome would be the other #3 chromosome.

13. Meiosis 1 Telophase I • This stage is absent in some species • The nuclear membrane reappears.The chromosomes uncoil.The spindle apparatus breaks down.The cell divides into two.

14. Meiosis 1 Interkinesis • Interkinesis is similar to interphase except DNA synthesis does not occur.

15. Result of Meiosis 1 • Go from 1 cell to 2 cells. Each daughter cell contains 1 copy of each chromosome: they are haploid, with the chromosomes still having 2 chromatids each. • For humans: start with one cell containing 46 chromosomes (23 pairs) to 2 cells containing 23 chromosomes. • As a result of crossing over, each chromosome is the mixture of the original homologues.

16. Meiosis 2 • Meiosis 2 is just like mitosis • No replication of DNA between meiosis 1 and meiosis 2. • Chromosomes line up individually on the equator of the spindle at metaphase. • At anaphase the centromeres divide, splitting the 2 chromatids. • The one-chromatid chromosomes are pulled to opposite poles.

17. More Meiosis 2

18. Summary of Meiosis • 2 cell divisions. • Start with 2 copies of each chromosome (homologues), each with 2 chromatids. • In meiosis 1, crossing over in prophase mixes alleles between the homologues. • In metaphase of meiosis 1, homologues pair up, and in anaphase the homologues are separated into 2 cells. • Meiosis 2 is just like mitosis. The centromeres divide in anaphase, giving rise to a total of 4 cells, each with 1 copy of each chromosome, and each chromosome with only 1 chromatid.

19. Life Cycles • Meiosis converts a diploid cell into haploid cells. Fertilization combines the 2 haploid gamete cells (sperm and egg) back into a diploid cell. • Eukaryotes alternate between diploid and haploid stages. This is called the life cycle of the organism. • In plants, the haploid cells grow into multicellular organisms. Unicellular eukaryotes and many of the more primitive plants (such as mosses and ferns) spend most of their time as haploids. The diploid stage in these organisms is quite short. • In higher plants (the conifers and flowering plants), the haploid stage is very short and small. Pollen grains, for instance, have a total of 3 haploid cells in them, and the ovules contain 8 haploid cells. The rest of the plant body is diploid.

20. Gamete Formation in Animals • Differences between male and female gametes. • In males, all 4 products of meiosis develop into sperm cells. They lose most of their cytoplasm, remodel their cell shape, and grow a long flagellum (tail). • In females, the cell divisions of meiosis are asymmetric: most of the cytoplasm goes into 1 of the 4 meiotic products, which becomes the egg. Eggs also develop large amounts of yolk proteins, which are used to feed the developing embryo. The other 3 meiotic cells are small “polar bodies”, which degenerate.