Meiosis and Sexual Life Cycles

  1. Sexual Reproduction
    • Meiosis 
    • 2 parents
    • Offspring vary genetically from both parents and siblings
    • Sperm fertilizes egg to form zygote
  2. Asexual reproduction
    • Mitosis/Budding/Fission
    • 1 parent
    • Creates clones (offspring genetically IDENTICAL to each other and parent)
    • -Variations can arise through mutations 
    • No sperm or egg required
  3. What do human chromosomes inherit?
  4. How many chromosomes do humans have?
    • 46 total chromosomes (23 pairs)
    • -22 pairs of autosomes (homologous chromosomes)
    • -1 pair of sex chromosomes
  5. Diploid
    • 2n=46
    • Two copies of each chromosome (1 from mom and 1 from dad)
    • Somatic cells
  6. Haploid
    • n=23
    • One set of chromosomes
    • Gametes (sperm and eggs)
  7. Karyotype
    Visual representation of chromosomes in a cell
  8. Polyploidy
    • Having more than two paired sets of chromosomes (triploid, tetraploid)
    • Very common in plants (30-80%)
    • Chromosome count doesn't mean complexity
  9. Privet shrubs and humans each have a diploid number of 46 chromosomes per cell, why are the two species so dissimilar?
    The two species have different genes
  10. Why are triploid plants seedless?
    • They are seedless because they are sterile
    • There is an odd number of chromosomes so the gametes (seeds never form properly)
  11. Sister Chromatids
    Is one duplicated chromosome
  12. What is a homologous pair?
    • Two nonsister chromatids
    • A set from the mother and a set from the father
  13. Centromere
    the region of a chromosome to which the microtubules of the spindle attach, via the kinetochore, during cell division.
  14. What statement correctly describes homologous chromosomes?

    D) They carry the same genes
  15. What does meiosis do to chromosomal content?
    • It reduces chromosomal content from 2 n to n
    • Occurs in 2 steps (Meiosis I and Meiosis II)
    • Results in the formation of 4 haploid cells (may become sperm or eggs)
  16. Why is meiosis important for organisms who reproduce sexually?
    You need to ensure that the zygote has the appropriate chromosome count
  17. Interphase I
    The pair of homologous chromosomes in diploid parent cell duplicate to become a pair of homologous chromosomes consistent of sister chromatids
  18. Prophase I
    • Homologous chromosomes pair and exchange segments (crossing over)
    • Synapsis: pairing of homologs to form tetrad
  19. Metaphase I
    The pairs of homologous chromosomes (bivalents) line up at the metaphase plate and the spindle fibers attach to the chromosomes
  20. Anaphase I
    • The homologous pairs are separated. 
    • Sister chromatids remain attached
  21. What results at the end of Meiosis I
    • 2 haploid cells
    • Each chromosome still has two sister chromatids
  22. During anaphase 1, which of the following separate?
    Homologous chromosomes
  23. Telophase
    The chromosomes uncondenses and the nuclear envelope reforms
  24. Cytokinesis
    • When the cleavage furrow splits the cell
    • When the cytoplasm splits off and the cell membrane pinches off
    • Two haploid cells form; chromosomes are still double
  25. Prophase II
    • The 2 daughter cells, each with 23 chromosomes condense into X-shaped structures
    • The membrane around the nucleus disolves and meiotic spindle forms again
  26. Metaphase II
    The pair of (nonidentical) sister chromatids line up along the metaphase plate and the mitotic spindle fibers attach to each of the sister chromatids
  27. Anaphase II
    • The (nonidentical) sister chromatids are then pulled to opposite poles 
    • The separated chromatids are now individual chromosomes
  28. Telophase II
    • The chromosomes complete their move to the opposite poles of the cell
    • A membrane forms around each set of chromosomes to create two new cell nuclei
  29. Cytokinesis II
    • The cytoplasm splits creating 4 granddaughter cells
    • With 23 chromosomes
  30. Events unique to meiosis
    • Synapsis and crossing over
    • Line up of homologous pairs
    • Separation of homologs not chromatids
  31. Sources of genetic variation
    • Independent assortment
    • Crossing over
    • Random fertilization
  32. Independent assortment of chromosomes
    • Orientation of maternal and paternal homologs is random
    • Each homologous pair is sorted independently of the others
  33. How to figure out the number of combos for independent assortment
    • 2n
    • n= # of chromosomes
    • 223= 8.4 million possible gametes
  34. Crossing over
    • Produces recombinant chromosomes
    • -contains DNA derived from two different parents
    • 1-3 crossover events occur per homologous pair
  35. How and at what stage do chromosomes undergo independent assortment?
    Meiosis I at metaphase alignment
  36. Crossing over contributes genetic variability between...
    Homologous chromosomes
  37. Random fertilization
    • Any sperm (~84 million possibilities) can fertilize any egg (~8.4 million possibilities)
    • =70 trillion possible zygote combinations + variation due to crossing over
  38. The genotype of a human zygote will differ from that of both parents. Which of the following does not contribute to this variation?

    E) presence of dominant genes
  39. What happens when meiosis goes wrong?
    • Gametes end up with extra or missing chromosomes
    • -often results in spontaneous abortion of the fetus
    • Better tolerated in plants than animals
  40. What happens when nondisjunction occurs on homologous chromosomes in meiosis I
    • Products: n+1, n+1, n-1, n-1
    • number of chromosomes
  41. What happens when nondisjunction occurs on sister chromatids in meiosis II
    • Products: n+1, n-1, n, n
    • number of chromosomes
  42. Nondisjunction
    The failure of one or more pairs of homologous chromosomes or sister chromatids to separate normally during nuclear division, usually resulting in an abnormal distribution of chromosomes in the daughter nuclei.
  43. Aneuploidy
    • Abnormal chromosome number (i.e one extra or one missing)
    • Most are so disastrous that the embryo doesn't survive to term
  44. What are the most common autosomal abnormalities in live births
    Trisomy 21, 18 and 13
  45. Where is aneuploidy better tolerated?
    • Sex chromosomes
    • XXY, XYY, XXX, XO
    • XO- individual that only has 1 X so they are likely to be steril
  46. Trisomy 21
    • Down syndrome
    • 1/700 children born in the US
    • Symptoms range from mild to severe (characteristic facial features, developmental delays, short stature, steril)
    • Life expectancy has increased to around 55
  47. Link between maternal age and down syndrome
    • As a women's age increases so does her risk of having a child with down syndrome
    • This may be because of the way eggs develop
    • -Meiosis I begins when female is in the womb (arrests in prophase I and restarts when puberty is reached)
  48. About 80% of babies with down syndrome are born to women under age 35. Why?
    There are more women having more kids under 35
  49. Asexual reproduction...

    C) produces offspring genetically identical to the parent
  50. Sexual and asexual reproduction are alike in that they both...
    They both can occur in multicellular organizms
  51. How many copies of chromosome 14 do your muscle cells contain?
  52. Where do the 2 copies of chromosome 14 in your muscle cells come from?
    One from mother and one from father
  53. The paradox of sex
    • Asexual reproduction is more efficient
    • -no males (who cannot give birth) needed
    • -every individual capable of reproducing
  54. Why does sexual reproduction exist?
    • Purifying selection hypothesis
    • Changing-environment hypothesis
  55. Changing-environment hypothesis
    • Genetically identical offspring not likely to thrive if the environment changes (i.e new pathogen)
    • If off spring are genetically diverse, they are more likely to at least some will be resistant and can survive to pass on beneficial alleles
  56. Purifying selection hypothesis
    Natural selection against deleterious alleles (in asexual reproduction, all offspring would receive defective allele)
Card Set
Meiosis and Sexual Life Cycles