1. 1. 6a: Meiosis Describe cells during the cell cycle as eitherhaploid or diploid Describe the stages of meiosis Identify male and female gametes Describe the process of crossing over andindependent assortment as means ofcreating genetic diversity
2. 2. Some background... You are a multi cellular organism thatstarted as a single cell... how? How many chromosomes did your first cellhave? How was that first cell made? How many cells must those cells have had? What process made those cells?
3. 3. Some background... You are a multi cellular organism that started as a singlecell... how? Mitosis! How many chromosomes did your first cell have? 46 How was that first cell made? Sperm fertilized an egg How many cells must those cells have had? Half of 46 = 23 What process made those cells? Meiosis!
4. 4. Vocabulary Gamete: A sex cell (sperm for male & egg infemales) Fertilization: The process in which an eggcell fuses with a sperm cell and male andfemale genetic material combines Zygote: A fertilized egg. Contains two sets ofchromosomes and will divide by mitosis todevelop into an embryo
5. 5. Chromosomes A somatic (body) cell contains two sets ofchromosomes. One from mom and one from dad. These chromosomes exist in pairs calledhomologous chromosomes. Homologous Chromosomes: Contain genes forthe same traits in the same locations, althoughthey may contain different variations of thosegenes. (eg. Both contain eye colour gene, one for blue and theother for brown.)
6. 6. Homologous Chromosomes
7. 7. Chromosome Number Diploid: Cell containing two sets of chromosomes Represented as 2n (in humans 2n = 46) All body cells are diploid Haploid: Cell has only one set of chromosomes Contains only 1 chromosome from each homologous pair. eg. Sex chromosomes (1 X or 1 Y) Represented as n (in humans n = 23) Only gametes are haploid
8. 8. Meiosis Process that produces gametes (egg orsperm) with half the number ofchromosomes (haploid) In meiosis there is one round of DNAreplication (during interphase) and two celldivisions Produces 4 genetically unique haploid cells
9. 9. Life Cycle(mitosis and meiosis)
10. 10. MeiosisA: DNA replicationB: Meiosis IC: Meiosis II
11. 11. Stages of Meiosis Meiosis is divided into two parts each withprophase, metaphase, anaphase, telophase Meiosis I: Pairs of homologous chromosomes line up atcenter and separate (different from mitosis wheresister chromatids separate ) Produces two daughter cells that are haploid Meiosis II: Sister chromatids are separated (like mitosis) Produces four daughter cells that are haploid
12. 12. Meiosis I
13. 13. Meiosis II
14. 14. Result of Meiosis Meiosis produces four genetically uniquedaughter cells Each cell contains one chromatid (daughterchromosome) from each homologous pair Cells are haploid (humans = 23chromosomes)
15. 15. Genetic Variation in Meiosis Every gamete is unique from all othergametes produced because of twoprocesses that occur during meiosis I1. Crossing Over2. Independent Assortment
16. 16. Crossing Over Occurs between each homologous pair Pairs of non-sister homologous chromatidscross over and exchange DNA Multiple cross overs can occur leading toinfinite possibilities
17. 17. Independent Assortment Homologous chromosomes line uprandomly at metaphase I, so the mixture ofchromosomes that end up in each daughtercell after separation varies
18. 18. Gamete Formation Meiosis is the same in male and females, aftermeiosis further development occur to specializea cell as either egg or sperm Male: Meiosis occurs in testes and produces 4sperm cells Produced from puberty on (millions every day) Female: Meiosis occurs in ovaries and produces1 egg and 3 smaller polar body cells Produced before birth (about 1-2 million eggspresent at birth)
19. 19. Gamete Formation