Lecture #5 Meiosis and Gamete Formation Unit: Cellular Reproduction
The Birds & The Bees Humans and most other animals reproduce sexually. • This means that two cells fuse to make a new ‘baby’ organism. • These specialized cells are called gametes, or sex cells. • Sperm and egg cells are human gametes
Question: Where do sperm & egg come from? Answer: Meiosis! Who does it? • All sexually reproducing organisms. Where does it happen? • Males- in the testes • Females- in the ovaries
Why does Meiosis happen? Answer It is the only way some organisms (like humans!) can reproduce. Meiosis allows sex cells to join and form a complete set of chromosomes necessary to make a new organism.
Why is a process other than mitosis needed inorder to make sperm and egg? • Mitosis results in two daughter cells that are diploid. • Egg and sperm need to be haploid. The chromosome number needs to be cut in half in order for fertilization to occur!
Meiosis Meiosis is a special kind of cell division that halves the number of chromosomes in gametes. This ensures that the correct number of chromosomes are passed on to offspring.
There are two parts to meiosis (Meiosis I & Meiosis II) Meiosis is sometimes referred to as ‘reduction- division’. Meiosis I (‘reduction’) – starts with 1 diploid cell but the end result is 2 cells that have had their chromosome # reduced by half (2 haploid cells). Meiosis II (‘division’) – end result is 4 haploid cells
Meiosis 1 (reduction-division’) Prophase I Homologous chromosomes pair and exchange Segments (crossing over) Meiosis KM
Crossing-Over Definition- process during meiosis in which homologous chromosomes cross and exchange pieces. • This leads to great genetic diversity of the egg and sperm that are produced.
Metaphase I: spindle moves pairs of Homologous chromosomes to the equator of the cell Different than in Mitosis where they lined up Anaphase I: Homologous pairs separate with sister chromatids remaining together. * Different than Mitosis! *
Telophase I: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair. • This is the stage where the diploid number is reduced to the haploid number!
Meiosis II (‘reduction-division’) ** The chromosomes do NOT replicate between meiosis I and meiosis II ** ** The steps of Meiosis II are just like what happens in mitosis (PMAT) ** The end result is very different however, due the fact that the cells starting it out are haploid!
End Result of Meiosis II Four haploid cells that are genetically unique!
Take 2 minutes to summarize what you have learned about the events of Meiosis. Be sure to mention how many cells start off the process and how many cells are present at the end. How is the cell at the start of meiosis different than the cells at the end? How is it similar?
Question Is Meiosis different in males and in females? Answer: YES; it ends differently.
Oogenesis (Meiosis in Females) Oogenesis in ovaries 46 Egg Polar Bodies (not functional) 23
Meiosis in Females Oogenesis - the process by which gametes are produced in female animals The cytoplasm divides unequally during meiosis I and meiosis II in females. • One of new cells produced receives almost all of the cytoplasm and will give rise to (meiosis I) or become (meiosis II) the egg (ovum). • The other cells are very small (polar bodies) and will NOT survive.
Why does the egg need all of that cytoplasm? Answer Because the surviving cell has a large share of cytoplasm, it has a rich storehouse of nutrients to nourish the young organism that develops if the ovum is fertilized. In summary: Meiosis in females starts with one diploid egg, and ends with one unique haploid egg.
Spermatogenesis (Meiosis in Males) Spermatogenesis in testes 46 SPERM 23 23 23 23
Meiosis in Males Spermatogenesis - the process by which sperm are produced in male animals. • After meiosis II, the four gametes change in form and develop a tail to become male gametes called sperm. In summary: Meiosis in males starts with one diploid cell and ends with 4 unique haploid cells
Take 2 minutes to summarize what you have learned regarding the similarities and differences in meiosis in males and females