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Meiosis and genetic variation

Meiosis and genetic variation. Honors Biology Unit 5 2013-2014. Genome. Genome : Complete complement of an organism’s DNA. Includes genes (control traits) and non-coding DNA organized in chromosomes . Genome > Chromosome > DNA > Genes. Genes.

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Meiosis and genetic variation

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  1. Meiosis and genetic variation Honors Biology Unit 5 2013-2014

  2. Genome • Genome: Complete complement of an organism’s DNA. • Includes genes (control traits) and non-coding DNA organized in chromosomes. Genome > Chromosome > DNA > Genes

  3. Genes • Eukaryotic DNA is organized in chromosomes. • Genes have specific places on chromosomes.

  4. Heredity • Heredity – way of transferring genetic information to offspring • Chromosome theory of heredity: chromosomes carry genes. • Gene – “unit of heredity”.

  5. Reproduction • Asexual: self-reproducing, produce clones (offspring genetically identical to parent) • Many single-celled organisms reproduce by splitting, budding, parthenogenesis. • Some multicellular organisms can reproduce asexually.

  6. Sexual reproduction • Fusion of two gametes to produce a single zygote. • With exception of self-fertilizing organisms (e.g. some plants), zygote has gametes from two different parents.

  7. In humans … • Total = 46 chromosomes (23 pairs) 44 autosomes + 2 sex chromosomes

  8. Two kinds of Chromosomes • Autosomes: • Code for most genes in your body (not sex determining chromosomes) • In humans chromosome #1-22

  9. Two kinds of Chromosomes • Sex Chromosomes: • chromosomes that determine what gender you will be. • #23 in humans • Females are: XX • Males are XY

  10. Important Vocabulary • Gametes: sperm/egg • haploid (n) • Haploid= Contains a single set of chromosomes (23) • Zygote:fertilized egg • now diploid (2n) • Diploid= Contains a two sets of chromosomes (23x2=46) • Somatic cell: any cell other than gametes, most of the cells in the body. – All are diploid (2n) Meiosis KM

  11. Sex Chromosomes • Mammals use a chromosomal method of determining sex: XX is female and XY is male. • Birds use a ZW system: ZZ is male and ZW is female. • the evolutionary origin of mammalian and bird sex chromosomes is different • Some reptiles use developmental temperature to determine sex: depends on the species, but hot is male and cold is female in some.

  12. Homologues • Chromosomes exist in homologous pairsin diploid cells. • homologous pairs: chromosomes that code for the same genes but possibly have different alleles (get one from each parent) Exception: Sex chromosomes (X, Y). All autosomes are in homologous pairs.

  13. Homologous chromosomes after replication • Now each homologous chromosome has a sister chromatid • Still considered diploid (2n) cell because still just two copies of each chromosome (even though they now have a twin) Meiosis KM

  14. Meiosis KM

  15. Meiosis KM

  16. Chromosome numbers All are even numbers – diploid (2n) sets of homologous chromosomes! Ploidy = number of copies of each chromosome. Diploidy

  17. Meiosis: What is it? • Meiosis: cell division process by which the number of chromosomes per cell is cut in half • Why does it occur: Meiosis is used to produce the haploid(n) gametes(sperm and eggs)

  18. Meiosis: Key differences from mitosis • Meiosis reduces the number of chromosomes by half. • Daughter cellsdiffer from parent, and each other. • Meiosis involves two divisions, Mitosis only one.

  19. Meiosis KM

  20. Meiosis KM

  21. Meiosis 1 First division of meiosis • Prophase 1: Duplicated homologous chromosomes match up forming tetrads. Crossing-overoccurs at the chiasmata. • Metaphase 1: Tetrads align at the equator of the cell. • Anaphase 1: Homologous pairs separate with sister chromatids remaining together. • Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

  22. Crossing over • produces recombinantchromosomes, mixing the genes of the mother and father, recombining them. • Occurs during Prophase 1

  23. Meiosis KM

  24. Harlequin chromosomes

  25. Meiosis II – Similar to mitosis Second division of meiosis: Gamete formation • Prophase 2: DNA does not replicate. • Metaphase 2: Chromosomes line up at the equatorial. • Anaphase 2: Sister chromatids move separately to opposite ends of the cell. • Telophase 2: Cell division is complete. Four haploid(n) daughter cells are produced.

  26. Mitosis vs. meiosis Meiosis KM

  27. Meiosis KM

  28. Meiosis creates genetic variation • During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n) • Meiosis results in genetic variationby: • Shuffling of maternal and paternal chromosomes and crossing over. • No daughter cells formed during meiosis are genetically identical to either mother or father • During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring.

  29. Independent assortment Meiosis KM

  30. Independent assortment Number of combinations: 2n e.g. if there are 2 chromosomes in haploid 2n = 4; n = 2 2n = 22 = 4 possible combinations

  31. In humans e.g. 23 chromosomes in haploid 2n = 46; n = 23 2n = 223 = ~ 8 million possible combinations!

  32. Random fertilization At least 8 million combinations from Mom, and another 8 million from Dad … >64 trillion combinations for a diploid zygote!!! Meiosis KM

  33. Gamete Formation in Animals Male gamete formation - Spermatogenesis • In males, all 4 products of meiosis develop into sperm cells. They lose most of their cytoplasm, remodel their cell shape, and grow a long flagellum (tail).

  34. Gamete Formation in Animals Female gamete formation - Oogenesis • In females, most of the cytoplasm goes into 1 of the 4 meiotic products, which becomes the egg. • The other 3 meiotic cells are small “polar bodies”, which degenerate.

  35. Why Sexual Reproduction … • More genetic diversity: more potential for survival of species when environmental conditions change. • Shuffling of genes in meiosis • Crossing-over in meiosis • Fertilization: combines genes from 2 separate individuals • DNA back-up and repair. • Asexual organisms don't have back-up copies of genes, sexual organisms have 2 sets of chromosomes and one can act as a back-up if the other is damaged. (some organisms have more) • Sexual mechanisms, especially recombination, are used to repair damaged DNA - the undamaged chromosome acts as a template and eventually both chromosomes end up with the correct gene.

  36. Review Questions 1. How does metaphase of mitosis differ from metaphase I of meiosis? 2. What happens as homologous chromosomes pair up during prophase I of meiosis? 3. What is the sole purpose of meiosis? 4. What specific activities, involving DNA, occur during interphase prior to both mitosis and meiosis? 5. Compare mitosis and meiosis on the following points: a. number of daughter cells produced. b. the amount of DNA in the daughter v. parent cell c. mechanism for introducing genetic variation. 6.What is a zygote and how is it formed?

  37. Warm-up • Summarize the process of meiosis in 5 sentences or less (include the following in your summary) • What is the sole purpose of Meiosis? • What does it start with and what does it end with (include males and females) • Use and underline vocab words gamete, diploid, haploid, crossing over, and homologous pairs, and sister chromatids

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