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MEIOSIS

MEIOSIS. Heredity The transmission of traits from one generation to the next Organisms pass on their traits to offspring through reproduction. Offspring acquire genes from their parent’s chromosomes. Types of Reproduction. Asexual Produces clones Single parent

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MEIOSIS

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  1. MEIOSIS

  2. Heredity • The transmission of traits from one generation to the next • Organisms pass on their traits to offspring through reproduction. • Offspring acquire genes from their parent’s chromosomes.

  3. Types of Reproduction • Asexual • Produces clones • Single parent • Little variation in population - only through mutations • Fast and energy efficient • Ex. budding, binary fission • Sexual • Meiosis produces gametes (sex cells) • 2 parents: male/female • Lots of variation/diversity • Slower and energy consumptive • Ex. humans, trees

  4. Chromosome Number • Depending on the function of a cell, the number of and • types of chromosomes it has may differ. • - somatic cells: non sex cells, contain pairs of homologous chromosomes (1 from each parent), 2n • - homologous chromosomes: two copies of each autosome, similar in size, shape, and genetic material • - diploid number(2n): full set of homologous chromosomes • ex: somatic(2n) germ(n) • humans 46 23 • corn 10 5 • goldfish 96 48 • fruit flies 8 4

  5. Homologous Chromosomes in Somatic Cells

  6. Chromosome Number • - gametes/ germ cells: sex cells (egg, sperm), n • - haploid number: half set of chromosomes “one set” (n) • half the diploid number • - zygote: fertilized egg cell resulting from joining of sex cells (2n) • ex: fruit flies 2n = 8 diploid • n = 4 haploid • WHY IS IT IMPORTANT THAT GERM CELLS HAVE THE HAPLOID #

  7. Chromosome Number • Sex Determination • In humans there are a total of 46 chromosomes (23 pairs) • - autosomes: homologous chromosomes not involved in gender determination • humans: 22 pairs • - sex chromosomes: determine gender • one pair : XX or XY • female male • varies among organisms • ex: grasshoppers: no Y • birds, moths, butterflies: male XX female X

  8. Karyotype • Photo of metaphase chromosomes of an individual • Arranged from largest to smallest pair

  9. Karyotypes: used to determine genetic abnormalities • Identify the disorders

  10. Karyotype of Melanoma Cancer Cell

  11. Karyotype of Breast Cancer Cell

  12. Human Life Cycle • Reproductive history of an organism from conception to • production of a zygote • fertilization: joining of 2 gametes (n) to form a zygote (offspring- 2n)

  13. Meiosis • Reduction Division • Cells divide twice • Result: • 4 non- identical haploid daughter cells

  14. Meiosis • 2 Stages • Meiosis I: • Homologous chromosomes are separated • into separate cells • Meiosis II: • Chromatids are segregated into separate cells

  15. Meiosis I Overview

  16. BEFORE MEIOSIS Complete interphase with S phase (replication of DNA occurs) PROPHASE I DNA condenses and becomes chromosomes. Spindle forms, nucleolus and nucleus disappears. Homologous chromosomes move toward each other Meiosis I

  17. 4. Synapsis occurs: parallel alignment of sister chromotids (NOW CALLED A TETRAD) 5. Crossing over occurs Non sister chromatids break apart and exchange alleles and re-attach. Result: Recombination (NEW COMBOS OF GENES)

  18. ANAPHASE I Homologous chromosomes separate and one of each pair is pulled to opposite poles. **chromatids still joined by centromere** • METAPHASE I • Tetrads line up on the metaphase plate andremain together.

  19. TELOPHASE I Cytokinesis occurs. Two daughter cells formed. (do NOT have same genetic info due to crossing over) END RESULT Twodaughter cells with haploid # (N) **homologous chromosomes have separated, but sister chromatids remain together • cleav • furrow

  20. Meiosis II Overview

  21. PROPHASE II 1. chromosomes condense. (brief interphase with NO DNA replication) nuclear membrane disappears. 3. chromosomes start to move to equator. MEIOSIS IIsecond division2 (n) cells go into meiosis II.Chromatids are segregated into different cells.

  22. ANAPHASE II Centromeres divide and each sister chromatid moves to opposite poles • METAPAHSE II • Chromosomes with sister • chromatids joined by • centromere line up at • metaphase plate

  23. TELOPHASE II 1. spindle dissolves & nuclear membrane reforms around chromosomes in each daughter cell 2. cleavage furrow or cell plate forms and cytokinesis occurs END RESULT Four cells with haploid # (n)**chromatids separate** Meiosis animation

  24. Events Unique to Meiosis • Prophase I: Synapsis and crossing over • Metaphase I: pairs of homologous chromosomes line up on metaphase plate • Anaphase I: homologous pairs separate  sister chromatids still attached at centromere

  25. Mitosis 1 diploid cell makes 2 diploid daughter cells daughter cells genetically identical to mother cell occurs in somatic cells unique features meiosis Meiosis 1 diploid cell makes 4 haploid cells resulting cells are genetically different from diploid cells and each other occurs in gametes Mitosis vs meiosis comparison animation Differences Mitosis and Meiosis

  26. Mitosis vs Meiosis

  27. Sources of Genetic Variation • Crossing Over • Exchange of genetic material • Recombinant • chromosomes

  28. Sources of Genetic Variation • Independent Assortment of Chromosomes • Random orientation of homologous pairs • in Metaphase I

  29. Sources of Genetic Variation • Random Fertilization • random joining of two gametes with new combinations • Outcomes: 223 x 223 = 64 trillion outcomes

  30. Spermatogenesis: Production of sperm in testes - starts with 1 diploid germ cell - result: 4 mature sperm cells Oogenesis: Production of ova (eggs) in ovaries - result: 1 ovum, 3 polar bodies - female is born with set number of eggs which are frozen at prophase I - puberty hormones cause eggs to complete meiosis and then pauses again - at fertilization: egg undergoes meiosis II Spermatogenesis Oogenesis GametogenesisMeiosis is primary event in formation of gametes.

  31. Haploid: simplest - Haploid cells occupy major portion of life cycle - Zygote is only diploid cell, undergoes meiosis as soon as formed, again become haploid protists, fungi, algae Eukaryotic Sexual Life Cycles

  32. Diploid: adults are diploid and inherit chromosomes from parents - adult individuals are diploid with each individual getting chromosomes from each parent (thru fertilization) - major portion of life is spent as diploid - diploid repro. cell undergoes meiosis to form haploid gametes (only haploid cell) most animals Eukaryotic Sexual Life Cycles

  33. 3. Alternation of generations: life cycle regularly alternates between a haploid phase and diploid phase plants and some algae - Sporophytes: (2n) produces haploid spores by meiosis - Spore: haploid reproductive cell (gametophyte) - Gametophyte: haploid phase gametes give rise to diploid phase by mitosis Cycles differ only in which phases become diploid. Eukaryotic Sexual Life Cycles

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