6a: Meiosis
- 1. 6a: Meiosis ● Describe cells during the cell cycle as either haploid or diploid ● Describe the stages of meiosis ● Identify male and female gametes ● Describe the process of crossing over and independent assortment as means of creating genetic diversity
- 2. Some background... ● You are a multi cellular organism that started as a single cell... how? ● How many chromosomes did your first cell have? ● How was that first cell made? ● How many cells must those cells have had? ● What process made those cells?
- 3. Some background... ● You are a multi cellular organism that started as a single cell... 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. Vocabulary ● Gamete: A sex cell (sperm for male & egg in females) ● Fertilization: The process in which an egg cell fuses with a sperm cell and male and female genetic material combines ● Zygote: A fertilized egg. Contains two sets of chromosomes and will divide by mitosis to develop into an embryo
- 5. Chromosomes ● A somatic (body) cell contains two sets of chromosomes. One from mom and one from dad. ● These chromosomes exist in pairs called homologous chromosomes. ● Homologous Chromosomes: Contain genes for the same traits in the same locations, although they may contain different variations of those genes. – (eg. Both contain eye colour gene, one for blue and the other for brown.)
- 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. Meiosis ● Process that produces gametes (egg or sperm) with half the number of chromosomes (haploid) ● In meiosis there is one round of DNA replication (during interphase) and two cell divisions ● Produces 4 genetically unique haploid cells
- 9. Life Cycle (mitosis and meiosis)
- 10. Meiosis A: DNA replication B: Meiosis I C: Meiosis II
- 11. Stages of Meiosis ● Meiosis is divided into two parts each with prophase, metaphase, anaphase, telophase ● Meiosis I: – Pairs of homologous chromosomes line up at center and separate (different from mitosis where sister 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. Meiosis I
- 13. Meiosis II
- 14. Result of Meiosis ● Meiosis produces four genetically unique daughter cells ● Each cell contains one chromatid (daughter chromosome) from each homologous pair ● Cells are haploid (humans = 23 chromosomes)
- 15. Genetic Variation in Meiosis ● Every gamete is unique from all other gametes produced because of two processes that occur during meiosis I 1. Crossing Over 2. Independent Assortment
- 16. Crossing Over ● Occurs between each homologous pair ● Pairs of non-sister homologous chromatids “cross over” and exchange DNA ● Multiple cross overs can occur leading to infinite possibilities
- 17. Independent Assortment ● Homologous chromosomes line up randomly at metaphase I, so the mixture of chromosomes that end up in each daughter cell after separation varies
- 18. Gamete Formation ● Meiosis is the same in male and females, after meiosis further development occur to specialize a cell as either egg or sperm ● Male: Meiosis occurs in testes and produces 4 sperm cells – Produced from puberty on (millions every day) ● Female: Meiosis occurs in ovaries and produces 1 egg and 3 smaller polar body cells – Produced before birth (about 1-2 million eggs present at birth)
- 19. Gamete Formation