bio 8

• Meiosis


  A special type of cell division that produces haploid gametes (n) from diploid cells (2n)

  
• Meiosis


  • Happens in the gonads (testes and ovaries)
  • Used to make haploid cells from diploid cells
  • Upon fertilization, 2 haploid gametes (1 sperm + 1 egg) fuse to form a diploid cell (zygote)
  • Zygote divides and grows into an embryo, fetus, and finally an adult human

  
• Homologous chromosomes


  The two chromosomes of each pair that are the same size and have the same centromere position, carrying the same genes in the same order (1 maternal and 1 paternal)

  
• Diploid cells


  Cells with 2 sets of chromosomes (2n)
• Haploid cells


  Cells with 1 set of chromosomes (n)
• Human somatic cells (skin, liver, kidney) have 44 autosomes (22 pair) and 2 sex chromosomes (1 pair XX or XY)

  
• Human gametes (sperm, egg) have 22 autosomes and 1 sex chromosome (sperm: X or Y, egg: X)

  
• Meiosis I


  1. Prophase I: Homologous chromosomes pair up and form tetrads, crossing over occurs

  2. Metaphase I: Tetrads line up at metaphase plate, homologous chromosomes line up independently

  3. Anaphase I: Homologous chromosomes separate and move to opposite poles

  4. Telophase I: Nuclear envelope reforms, chromosomes decondense, cytokinesis occurs to form 2 haploid cells

  
• Meiosis II


  1. Prophase II: Chromosomes condense, spindle forms

  2. Metaphase II: Chromosomes line up at metaphase plate

  3. Anaphase II: Sister chromatids separate and move to opposite poles

  4. Telophase II: Nuclear envelope reforms, chromosomes decondense, cytokinesis occurs to form 4 haploid cells

  
• Crossing over


  Exchange of genetic material between non-sister chromatids of homologous chromosomes, producing new allele combinations

  
• Mitosis


  Used for growth, repair, and development, involves 1 cell division
• Meiosis


  Used for reproduction, involves 2 cell divisions

  
• Meiosis produces haploid gametes (n) not diploid cells (2n)

  
• Fertilization: 2 haploid gametes (1 sperm + 1 egg) fuse to form a diploid cell (zygote)

  
• Heredity: Offspring acquire alleles from parents
• Somes

  • 8 chromatids
  • 2n
• CYTOKINESIS I


  1. End of CYTOKINESIS I

  2. 2 chromosomes

  3. 4 chromatids

  4. n

  
• Meiosis II: separating sister chromatids

  1. PROPHASE II

  2. METAPHASE II

  3. ANAPHASE II

  4. TELOPHASE II

  
• Very similar to mitosis:
• PROPHASE II


  1. 2 chromosomes

  2. 4 chromatids

  3. n

  
• METAPHASE II


  1. 2 chromosomes

  2. 4 chromatids

  3. n

  
• ANAPHASE II


  1. 4 chromosomes

  2. n

  
• TELOPHASE II


  1. 4 chromosomes

  2. n
• CYTOKINESIS II


  1. end of CYTOKINESIS II

  2. 2 chromosomes

  3. n

  
• Possible causes of Down Syndrome

  • One parent transmits 3 copies of chromosome 21, the other parent fails to transmit any copies
  • Failure to separate homologous chromosome 21 during anaphase 1 of meiosis resulting in one of the final gametes have 2 copies of chromosome 21
  • Failure to separate the sister chromosomes during anaphase II of meiosis and therefore one of the parental gametes has 2 copies of chromosome 21
  • During crossing over of prophase 1, all of the genetic information is transferred from one sister chromatid to the other
  • Both (c) and (d) are correct

  
• Only one DNA duplication event happens. In interphase before meiosis I. Not between meiosis I and II

  
• Differences between Mitosis and Meiosis

  • DNA replication (when?)
  • Number of divisions
  • Pairing of homologous chromosomes
  • Number of resulting daughter cells
  • Genetic composition of daughter cells
  • Role in animal body

  
• Mechanisms of genetic variation

  • Independent assortment
  • Crossing over
  • Random fertilization

  
• Independent assortment (aka Mendel's second law)


  • Random orientation of tetrads at the metaphase plate (metaphase I)
  • Meiosis I daughter cells have a 50/50 chance of receiving the paternal or maternal chromosome for each chromosome
  • Possible combinations = 2^n

  
• For a buffalo with 60 chromosomes, the possible combinations are 2^60
• Crossing over


  • Crossing over during prophase I creates recombinant chromosomes
  • Recombinant chromosomes have a mix of maternal and paternal copies of genes (alleles)

  
• Crossing over mechanism


  • Special proteins cut the homologous sister chromatids at the same location (one paternal and one maternal)
  • The chromatids are joined together and in the process the cuts on the non-sister chromatids are joined together

  
• Random fertilization: Any sperm can fuse with any egg to produce a zygote
• Possible combinations = 8 million sperm X 8 million eggs = ~70 trillion