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Meiosis & Sexual Reproduction

Learn about the process of meiosis and sexual reproduction, including the production of gametes, crossing over, and its role in introducing genetic variation. Understand the advantages of sexual reproduction and the changes in chromosome number that can occur. Discover common genetic disorders related to chromosomal abnormalities.

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Meiosis & Sexual Reproduction

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  1. Meiosis & Sexual Reproduction Modified from: Kim Foglia, Explore Biology

  2. How about the rest of us? • What if a complex multicellular organism (like us) wants to reproduce? • joining of egg + sperm • Do we make egg & sperm by mitosis? + 46 92 46 egg sperm zygote

  3. 46 46 23 23 46 23 23 How do we make sperm & eggs? • reduce 46 chromosomes  23 chromosomes • half the number of chromosomes egg meiosis fertilization sperm

  4. Meiosis: production of gametes • Alternating processes,alternating stages • chromosome number must be reduced • diploid  haploid • 2n  n • humans: 46  23 • meiosis reduces chromosome number • fertilization restores chromosome number • haploid  diploid • n  2n

  5. Homologous chromosomes • Paired chromosomes • both chromosomes of a pair carry genes • control same inherited characters • homologous = sameinformation diploid2n homologouschromosomes double strandedhomologous chromosomes

  6. Double divisionof meiosis DNA replication 1st division of meiosis separateshomologous pairs 2nd division of meiosis separatessister chromatids

  7. Steps of meiosis • Meiosis 1 • interphase • prophase 1 • metaphase 1 • anaphase 1 • telophase 1 • Meiosis 2 • prophase 2 • metaphase 2 • anaphase 2 • telophase 2 1st division of meiosis separateshomologous pairs (2n  1n) 2nd division of meiosis separatessister chromatids (1n  1n) * just like mitosis *

  8. Crossing over • During Prophase 1 • homologous pairs swappieces of chromosome • sister chromatids intertwine • crossing over tetrad synapsis

  9. What are theadvantages ofsexual reproduction? Crossing over • 3 steps • cross over • breakage of DNA • re-fusing of DNA • New combinations of traits

  10. Genetic variation • Meiosis & crossing over introduce great genetic variationto population • drives evolution

  11. And more variation… • Crossing over • creates completely new combinations of traits in next generation

  12. Sources of genetic variability • Genetic variability in sexual reproduction • independent assortment • homologous chromosomes in Meiosis 1 • crossing over • between homologous chromosomes in prophase 1 • random fertilization • random ovum fertilized by a random sperm metaphase1

  13. Cell A Cell B Cell C Cell D Cell A has 4 chromosomes and Cell D has 2 chromosomes. What process created Cell D? • Mitosis • Asexual Reproduction • Fusion • Meiosis 10

  14. Cell A is considered _______ while Cell D is considered ________. • Haploid; Diploid • Tetrad; Diploid • Diploid; Haploid • Homologous; Haploid Cell A Cell B Cell C Cell D 10

  15. Cell A is considered _______ while Cell D is considered ________. • somatic cell; gamete • gamete; somatic cell • stem cell; somatic cell • stem cell; gamete 30

  16. How can we best describe the diagram below? • Two sister chromatids • Homologous chromosomes • One replicated chromosome • Haploid chromsomes 10

  17. What process is best demonstrated by the diagram below? • Anaphase I • Segregation of alleles • Crossing over • Independent assortment 10

  18. Which of the following leads to genetic variation during meiosis? • All of the above (A, B, C, D) • A & B • A & D • A, B, & D • A & C • Crossing Over • DNA replication • Independent assortment • Mitotic spindle formation 10

  19. Mitosis 1 division daughter cells genetically identical to parent cell produces 2 cells 2n  2n produces cells for growth & repair no crossing over Meiosis 2 divisions daughter cells genetically different from parent produces 4 cells 2n  1n produces gametes crossing over Mitosis vs. Meiosis

  20. Mitosis vs. Meiosis

  21. Changes in Chromosomes Number • Euploidy = correct # of chromosomes • Aneuploidy = a change in chromosomes number due to non-disjunction during meiosis • Monosomy- only 1 copy of an individual chromosome • Trisomy- 3 copies of an individual chromosome

  22. Non-disjuction Primary nondisjuction = Meiosis I Secondary nondisjuction = Meiosis II

  23. Trisomy 21 : Down Syndrome • Delayed mental and social skills • Decreased muscle tone at birth • Asymmetrical or odd-shaped skull • Small skull • Small mouth with protruding tongue • Broad short hands • Increased risk of developing Leukemia and Alzheimer’s later in life

  24. Trisomy 18 : Edward’s Syndrome • Most children die in the first year of life, some have lived 10 years • Growth deficiency • Feeding difficulties • Breathing difficulties • Developmental delays • Mental Retardation • Overlapped, flexed fingers • Webbing of the second and third toes • Clubfeet • Structural heart defects at birth

  25. Trisomy 13 : Patau Syndrome • Mental retardation, severe • Seizures • Small head • Scalp defects • Cleft lip and/or palate • Eyes close set (hypotelorism) –may fuse • Extra digits (polydactyl) • Hernias • Undescended testicle • Children die in the first year of life

  26. Karyotype • A visual display of the chromosomes arranged by size, shape, and banding pattern • Used to identify aneuploid conditions

  27. Procedure: Amniocentesis and Karyotyping

  28. Changes in Sex Chromosome # • Turners Syndrome (XO) – missing Barr Body • Kleinfelter’s Syndrome (XXY) • Swyer Syndrome (XY female) • La Chapelle Syndrome (XX male) • Poly-X Females (XXX, XXXX) • Jacob’s Syndrome (XYY males) -SRY gene (located on short arm of Y chromosome) -hormone= testis-determining factor -Barr Body – Inactive X chromosome (XX)

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