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Review of Terms Used in Meiosis

Review of Terms Used in Meiosis. Meiosis nuclear cell division in sexually reproducing organisms Sexual reproduction involves gamete and zygote formation Chromosomes and homologous chromosomes Chromatids and sister chromatids Genes (DNA) and alleles, locus Diploid and haploid

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Review of Terms Used in Meiosis

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  1. Review of Terms Used in Meiosis • Meiosis nuclear cell division in sexually reproducing organisms • Sexual reproduction involves gamete and zygote formation • Chromosomes and homologous chromosomes • Chromatids and sister chromatids • Genes (DNA) and alleles, locus • Diploid and haploid • Autosomes and sex chromosomes • karyotype

  2. Meiosis-Main Source of Genetic Variability • Formation of gametes (eggs and sperm) • Sexual reproduction (Why?) • Fertilization-union of sperm nuclei with egg nuclei=diploid zygote (fertilized egg) • Produces new cells with different DNA than parent cell • The gametes formed are haploid, they contain half as much DNA as the parent cell

  3. Brief Overview of Meiosis • Chromosome movement and division of cells is just like mitosis • Diploid germ cell starts off in interphase, DNA duplicated in S phase • 2 sets of divisions (2 PMATs) called Meiosis I and II • No DNA duplication after Meiosis I • Homologous chromosomes separate in Meiosis I • Sister chromatids separate during Meiosis II

  4. Genetic Variation • Crossing Over-exchange of genetic material between nonsisterchromatids of homologous chromosomes  prophase • Independent assortment (random alignment) metaphase, 223 combos • Fertilization male and female gamete fuse, (223)2, • Plus 1 cross over: (423)2 different zygotes = 5 and 27 zeros

  5. Meiosis I: reduction • DNA duplicated in Interphase (S) • Prophase I-chromosomes condense, centrioles duplicated and migrate to opposite poles, microtubules grab on to chromosomes, nuclear envelope disappears, crossing over occurs • Metaphase I-microtubules align chromosomes at equator; chromosome organization is called Independent Assortment, the 2 kinetochores of the duplicated chromosome are attached to the same kinetochore spindle fiber • Anaphase I- spindle apparatus (microtubules contracting) separates homologous chromosomes; each chromosome still has 2 chromotids

  6. Meiosis I: reduction • Telophase I-no formation of nuclear envelope, cell separates by cytokinesis • Interkinesis  similar to interphase, no DNA replication

  7. Meiosis II: division • Prophase II-centrioles duplicate, move to opposite pole, microtubules grab chromosomes • Metaphase II-chromosomes align at metaphase plate, like mitosis except….? • Anaphase II-spindle apparatus pulls sister chromatids apart

  8. Meiosis II • Telophase II-nuclear envelope forms, cytokinesis results in 4 distinctly unique new cells • Animals  gametes • Plants  spores • Plants have an Alternation of Generations: multicellular haploid and diploid forms for plants

  9. Mitosis vs Meiosis

  10. Mitosis vs Meiosis • Meiosis= 2 chromosomal divisions; Mitosis= 1 • Meiosis=4 daughter nuclei; Mitosis=2 • Following meiosis, 4 daughter cells are haploid (1/2 chromosome # of parent cell; following mitosis the daughter cells have same chromosome # • Following meiosis daughter cells are genetically distinct form each other and parent cell; following mitosis daughter cells are genetically identical to each other and parent cell.

  11. Where are Gametes Produced? • In Humans: • Sperm  testis, eggs  ovaries • In Plants: • Sperm  anthers, eggs  ovaries • Life cycle of plants and animals consists of both mitosis and meiosis • Sperm + egg  zygote  Adult  gametes

  12. Spermatogenesis • Formation of sperm through meiosis • Spermatogonium  primary spermatocyte  secondary spermatocyte  spermatids sperm • Produces 4 genetically distinct gametes • Sperm production begins with male puberty and continues

  13. Oogenesis • Formation of eggs through meiosis • Oogonium primary oocyte  secondary oocyte + 1 polar body  ovum + 3 polar bodies • Polar bodies are smaller than ovum and are the result of unequal cytoplasmic division • Egg production in utero (fetus has many primary oocytes), mature at puberty • Secondary oocyte arrests at metaphase 2, leaves ovary, enters oviduct • Entry of sperm triggers completion of meiosis II • Why must an egg be large?

  14. Mitosis vs. Meiosis • Mitosis • somatic cells • growth and repair • diploid  diploid • 1 cell  2 cells • same DNA as parent cell • 1 set of divisions (1 PMAT) • Meiosis • germ (sex) cells • gamete formation • diploid  haploid • 1 cell  4 cells • DNA different than parent cell • 2 sets of divisions (2 PMATs)

  15. Changes in Chromosome Numbers • Nondisjunction-one or more pairs of chromosomes fail to separate during anaphase of mitosis or meiosis • Aneuploidy-one extra or one less chromosome • Polyploidy-having 3 or more chromosomes • If nondisjunction occurs in Anaphase II of meiosis, how many chromosomes will each gamete have?

  16. Changes in Autosome Number: Down’s Syndrome • Nondisjunction in reproductive cells that give rise to gametes • Extra chromosome #21-trisomy visible in karyotype • mildsevere mental retardation, heart defects, abnormal skeletal development • 1 out of 1100 births • Increased risk as age of mother at conception increases

  17. Changes in Sex Chromosome Number • Nondisjunction during gamete formation • Turner’s Syndrome-XO, female, short, sterile, nonfunctional ovaries, 1 out of 10,000 newborn girls • Klinefelter Syndrome-XXY, male, taller, sterile, abnormal testis, mild retardation, can be asymptomatic, 1 out of 5001000 newborn boys • Jacob’s Syndrome-XYY, male nondisjunction in sperm, taller, mild retardation, phenotypically normal, asymptomatic, 1 out of 1000 newborn boys • Poly-X females-usually no mental retardation, 1 out of 1500 • Barr bodies- inactive X chromosomes • SRY gene – male determining factor

  18. Changes in Chromosome Structure • Change in physical structure of chromosome • Rare spontaneous occurrence • Can lead to genetic disorder or abnormality • Can be caused by viruses, chemicals, radiation

  19. Types of Chromosome Structure Changes • Duplication-parts of chromosome repeated many times • Inversion-alteration of position and sequence of genes in a chromosome • Translocation-part of one chromosome is transferred to a non homologous chromosome • Deletion-loss of part of a chromosome

  20. Deletion Syndromes • Williams Syndrome - chromosome 7 loses part of its end  “pixie” looking, turned up noses, wide mouths, small chin, large ears • Cri-du-Chat Syndrome- “cry of the cat” loss of part of chromosome #5 results in abnormal larynx and mental retardation

  21. Translocation Syndromes • Alagille syndrome  transfer between 2 and 20, distinctive face, abnormalities of the eyes and internal organs • Chronic Myelogeneous Leukemia, transfer between #9 and #22, “Philadelphia Chromosome” • Burkitt lymphoma  translocation of chromosome #8 and 14, tumors of the jaw

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