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MEIOSIS

MEIOSIS. Sperm & Eggs & Variation..OH MY!. Sexual Reproduction requires Cells Made by Meiosis. What if a new individual was formed through mitosis?. Meiosis – Key Terms. g amete genome haploid homologues intron karyotype nondisjunction placenta r eduction division

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MEIOSIS

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  1. MEIOSIS Sperm & Eggs & Variation..OH MY!

  2. Sexual Reproduction requires Cells Made by Meiosis • What if a new individual was formed through mitosis?

  3. Meiosis – Key Terms • gamete • genome • haploid • homologues • intron • karyotype • nondisjunction • placenta • reduction division • sexual reproduction • zygote • allele • amniocentesis • asexual reproduction • autosome • binary fission • chorionic villi sampling • crossing over • diploid • exon • fertilization

  4. Genome • Genome: Complete complement of an organism’s DNA. • includes genes (control traits) and non-coding DNA organized in chromosomes

  5. Genes • Eukaryotic DNA is organized in chromosomes • genes have specific places on chromosomes • exon: portion of a gene that is translated into protein (more in unit 8) • intron: non-coding segment of DNA, often found within an exon; removed before transcription

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

  7. Reproduction • Asexual • splitting • budding • parthenogenesis (egg develops w/o fertilization) • occurs naturally in some invertebrate animal species (e.g., water fleas, aphids, nematodes, some bees, some scorpion species, and parasitic wasps) and a few vertebrates (e.g., some fish, amphibians, reptiles,and very rarely birds)

  8. Sexual reproduction • Fusion of two gametes to produce a single zygote. • Introduces greater genetic variation, allows genetic recombination. • With exception of self-fertilizing organisms (e.g. some plants), zygote has gametes from two different parents.

  9. Chromosomes • Karyotype: • ordered display of an individual’s chromosomes • collection of chromosomes from mitotic cells • staining can reveal visible band patterns, gross anomalies • Make a Karyotype

  10. KARYOTYPING • Obtain some cells from the individual • Culture them in a test tube with nutrients • Treat cells w/chemical that stops them exactly midway through cell division (chromosomes are coiled thickly and more visible than usual) • Cells are placed on a microscope slide and a stain is added that binds to the chromosomes, making them visible • Chromosomes are arranged by size and shape and displayed on a monitor or in a photograph

  11. Karyotyping

  12. HomologOUs CHROMOSOMES • Chromosomes exist in homologous pairs in diploid (2n) cells. • One chromosome of each homologous pair comes from the mother (called a maternal chromosome) and one comes from the father (paternal chromsosome). • Homologous chromosomes are similiar but not identical. Each carries the same genes in the same order, but the alleles (alternative form of a gene) for each trait may not be the same.  • Exception: sex chromosomes (X, Y)

  13. HomologOUs CHROMOSOMES

  14. In humans … • 23 chromosomes donated by each parent (total = 46 or 23 pairs). • Gametes (sperm/ova): • contain 22 autosomes and 1 sex chromosome • haploid (haploid number “n” = 23 in humans) • Fertilization results in zygote with 2 sets of chromosomes - now diploid (2n). • Most cells in the body produced by mitosis. • Only gametes are produced by meiosis.

  15. Chromosome numbers All are even numbers – diploid (2n) sets of homologous chromosomes.

  16. Meiosis – key differences from mitosis • Meiosis reduces the number of chromosomes by half. • Daughter cells differ from parent, and each other. • Meiosis involves two divisions, Mitosis only one. • Meiosis I involves: • synapsis • homologous chromosomes pair up • chiasmataform (crossing over of non-sister chromatids) • metaphase I: homologous pairs line up at metaphase plate • anaphase I: sister chromatids do NOT separate • overall, separation of homologous pairs of chromosomes, rather than sister chromatids of individual chromosome

  17. Animation

  18. Meiosis I • Prophase 1 • each chromosome duplicates and remains closely associated (sister chromatids) • crossing-over can occur during the latter part of this stage • Metaphase 1 • homologous chromosomes align at the equatorial plate

  19. Meiosis I • 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

  20. Meiosis II Second division of meiosis: Gamete formation • Prophase 2 • DNA does not replicate • Metaphase 2 • chromosomes align at the equatorial plate

  21. MEIOSIS II • Anaphase 2 • centromeres divide • sister chromatids migrate separately to each pole • Telophase 2 • cell division is complete • 4 haploid daughter cells

  22. Mitosis vs. meiosis

  23. Meiosis creates genetic variation • During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n) • Meiosis results in genetic variation by 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

  24. Independent assortment

  25. Independent assortment Number of combinations: 2n e.g. 2 chromosomes in haploid 2n = 4; n = 2 2n = 22 = 4 possible combinations

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

  27. Crossing over Chiasmata – sites of crossing over, occur in synapsis. Exchange of genetic material between non-sister chromatids. Crossing over produces recombinantchromosomes.

  28. HOW SEX IS DETERMINED IN HUMANS • Females don’t have a Y chromosome in any of their cells, yet they are able to develop and live normal, healthy lives.  • For this reason, we know that nothing on the Y chromosome is absolutely necessary.

  29. SEX DETERMINATION IN OTHER SPECIES

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

  31. Meiosis & sexual life cycles • Life cycle = sequence of stages in organisms reproductive history; conception to reproduction • Somatic cells = any cell other than gametes, most of the cells in the body • Gametes produced by meiosis

  32. Meiosis & sexual life cycles Generalized animal life cycle

  33. NONDISJUNCTION Unequal distribution of chromosomes during meiosis Resulting gametes zero or two copies of a chromosome instead of a single copy

  34. Down's Syndrome

  35. TOO MANY OR TOO FEW CHROMOSOMES

  36. Sex is costly! • Large amounts of energy required to find a mate and do the mating: specialized structures and behavior required • Intimate contact provides route for infection by parasites (AIDS, syphillis, etc.) • Genetic costs: in sex, we pass on only half of genes to offspring. • Males are an expensive luxury- in most species they contribute little to rearing offspring.

  37. But … • 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

  38. MITOSIS VS MEIOSIS REVIEW • In what cellular processes is mitosis involved? In what cellular processes is meiosis involved? • In what type of cells does mitosis occur? In what type of cells does meiosis occur? • How many times does DNA replicate in mitosis? How many times does DNA replicate in meiosis? • How many cellular divisions occur in mitosis? How many cellular divisions occur in meiosis?

  39. MITOSIS VS MEIOSIS REVIEW • How many daughter cells are formed by mitosis? How many daughter cells are formed by meiosis? • What is the chromosome number in daughter cells formed by mitosis from diploid parent cells? What is the chromosome number in daughter cells formed by meiosis from diploid parent cells? • In mitosis, are daughter cells identical to or different from parent cells? In meiosis, are daughter cells identical or different from parent cells? • In mitosis, when do synapsis and crossing over occur? In meiosis, when do synapsis and crossing over occur?

  40. CREDITS • cchs.churchill.k12.nv.us/marshk/Notes/meiosis.ppt • Genetic Science Learning Center, University of Utah, http://learn.genetics.utah.edu • http://www.phschool.com/science/biology_place/labbench/lab3/concepts2.html

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