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Meiosis & Sexual Life Cycle

Meiosis & Sexual Life Cycle. Chapter 13. AP Essential Knowledge. Essential knowledge 3.A.2: In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization.

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Meiosis & Sexual Life Cycle

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  1. Meiosis & Sexual Life Cycle Chapter 13

  2. AP Essential Knowledge Essential knowledge 3.A.2: In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization. b. Mitosis passes a complete genome from the parent cell to daughter cells. 4. Mitosis is a continuous process with observable structural features along the mitotic process. • Students must know the order of the processes (replication, alignment, separation).

  3. AP Essential Knowledge • c. Meiosis, a reduction division, followed by fertilization ensures genetic diversity in sexually reproducing organisms. • Gametes have a haploid set of chromosomes • Homologous chromosomes are paired and then separated ensuring haploid set of chromosomes • During meiosis (Prophase I), homologous chromatids exchange genetic material via a process called “crossing over” which increases genetic variation in the resultant gametes.

  4. Homologous Chromosomes • 2 chromosomes that have same length, centromere position, and staining pattern • Autosomes • Non-sex chromosomes • Chromosomes that do not determine gender • Sex Chromosomes • Chromosomes that determine gender

  5. Chromosomes • Human somatic cell = 44 autosomes + 2 sex chromosomes • Human gamete = 22 autosomes + 1 sex chromosome • Sex Chromosomes • Can be XX or XY • XX = Homologous chromosomes • XY = Not homologous chromosomes • Egg must contain X, sperm may contain X & Y • Hence, males determine the gender of offspring

  6. Unnecessary Censorship

  7. Meiosis • 2 Stages of Meiosis • Meiosis I & Meiosis II • Much of Meiosis resembles Mitosis • Chromosomes are replicated only once • Before Meiosis I • 4 daughter cells are produced

  8. Meiosis: An Overview Assume that an organism has: 1 Homologous Pair = 2 Chromosomes (Diploid cell – 2n) STEP 1: Each of the chromosomes is replicated in Interphase STEP 2: Chromosome pairs of copies separate in Meiosis I (Haploid cell – n) BUT 2 copies of each one STEP 3: Each of the copies (sister chromatids) in a cell separates creating 4 haploid cells (Haploid cell with only 1 copy)

  9. What is different in Prophase I? What is different in Anaphase I?

  10. Meiosis I • Prophase I • Longest phase • Homologous pairs align • Crossing-Over may occur • Synapsis – pairing of homologous pairs tied tightly together • Tetrads form (4 chromosomes = 2 pairs) • Each tetrad has 1 or more chiasmata • Criss-crossed regions where crossing over has occurred

  11. Meiosis I (Page 2) • Metaphase I • Tetrads are aligned at the metaphase plate • Each chromosome pair faces a pole • Anaphase I • Homologous chromosomes (composed of 2 copies of each chromosome called chromatids) are pulled apart

  12. What is different between Meiosis I & II? This division is sometimes called the Mitotic division, why?

  13. Meiosis vs. Mitosis • Tetrads align in Prophase I, • Chromosomes align in Prophase mitosis • Chromosomes position @ metaphase plate (Mitosis) • Tetrads position @ metaphase plate (Meiosis) • Homologues separate in Meiosis I • Sister chromatids separate in Meiosis II & Mitosis • Crossing over = Meiosis NOT mitosis

  14. Mitosis Meiosis • DNA replicates in interphase • 1 division • No synapsis • 2 Diploid cells • Genetically identical cells • Responsible for: -- Zygote growth into multicellular organism • DNA only replicates in Pre-meiotic interphase • 2 divisions • Synapsis occurs during prophase I forming tetrads • Crossing over occurs now • 4 haploid cells • Genetically different cells • Responsible for: -- Gamete production -- Genetic variation

  15. Genetic Diversity • The reason for meiosis + sexual reproduction • Mutations are the original source of genetic diversity • 3 main sources of Genetic Diversity 1. Independent Assortment of Chromosomes 2. Crossing Over 3. Random Fertilization

  16.  Each daughter cell has a 50% chance of getting maternal chromosome (or its copy)  Similarly, 50% chance of getting paternal chromosome (or its copy)  Independent assortment - each chromosome is positioned independently of the other chromosomes

  17.  When homologous pairs are formed in Prophase I, a recombinant chromosome can be formed • -- A chromosome that has DNA from 2 different parents  2 chromosome segments trade places (cross over) producing chromosomes with new combos of maternal & paternal genes  1-3 times per chromosome in humans  Increases genetic variation

  18. Random Fertilization • Egg + sperm cells are genetically different from parent cells • Their combination (fertilization) increases variation even more

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