Meiosis & Sexual Reproduction

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 *

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

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

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

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

14. 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

15. 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

16. 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

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

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

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

20. 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

21. 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

22. Mitosis vs. Meiosis

23. 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

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

25. 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

26. 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

27. 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

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

29. Procedure: Amniocentesis and Karyotyping

30. 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)