11.4 - Meiosis

• Meiosis is a kind of cell division that forms egg and sperm cells.
• Chromosome Number

  • All organisms have different numbers of chromosomes in their cells
  • Human body cells have 46 chromosomes: 23 from the male parents and a matching 23 chromosomes from the female parent
  • These two sets of chromosomes are called homologous pairs
  • Human body cells have 23 homologous pairs
  • karyotype - photo of 23 homologous pairs
• Diploid Cells

  • A cell that contains both chromosomes in a pair (two sets) is diploid
  • The number of chromosomes in a diploid cell is represented by the symbol 2N
  • For Drosphila, the diploid number is 8, so 2N = 8
  • Humans 2N = 46
  • Diploid cells are "somatic cells" or cells that make up the body
  • skin, nervous, etc.
• Haploid Cells

  • Gametes are reproductive cells - egg & sperm
  • Gametes are haploid - they only contain a single set of chromosomes
  • Gametes need to be haploid so that when two gametes unite in fertilization, the zygote will have two sets of chromosomes (a set from each parent), and be diploid.
  • The haploid number for an organism is represented by the symbol N
  • For Drosophilia, the haploid number is N, which can be written as N=4
  • Humans N=23
• Two Phases of Meiosis

  • Meiosis is a process of reduction division in which the number of chromosomes in a cell is cut in half - this division takes place in ovaries and testes of living organisms
• Meiosis involves two divisions of a diploid cell, meiosis I & meiosis II, which result in four haploid cells that will become sex cells at the end of both divisions.
• Meiosis I is called a reduction division - when the two sets of chromosomes split and become haploid
• Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell - one of each homologous pair. However, each chromos is still replicated.
• The two haploid cells produced by meiosis I now enter a second division to separate the chromatid.
• Interphase: cells undergo a round of DNA replication forming duplicated chromosomes (like in mitosis)
• Prophase I: each chromosome pairs with its corresponding homologous chromosome to form a tetrad

  • when they form tetrads in meiosis I, they exchange portions of chromatids in a process called crossing-over
  • crossing-over produces new gene combinations
• Metaphase I: homologous pairs line-up NEXT to each other in the middle of the cell
• Goal of Meiosis II: each of the haploid cells chromosomes are still replicated, so they must divide again to separate the chromatids into separate cells at the end of Meiosis II
• Result = four haploid cells that each have one single set of chromosomes which will go on to form mature egg or sperm.
• Two phases of Meiosis

  A) prophase I

  B) interphase

  C) metphase I

  D) anaphase I

  E) telephase I

  F) haploid

  G) prophase II

  H) metaphase II

  I) anaphase II

  J) telephase II

  
• Gamete Formation

  • In male animals, meiosis results in four equal-sized gametes called sperm
  • In many female animals, only one egg results from meiosis - the other three cells, called polar bodies, are not involved in reproduction
• Comparing Mitosis and Meiosis

  • Mitosis results in the product of two genetically identical diploid cells
  • Meiosis produces four genetically different haploid cells
• Comparing Mitosis and Meiosis

  • Meiosis
  • cells produced have half the number of chromosome as the parent cell (crossing over)
  • these cells are haploid and are genetically different from each other & parent cell
  • meiosis is how sexually-reproducing organisms produce gametes (egg & sperm)
  • two divisions
  • occurs in ovaries & testes
  • four cells formed
• Comparing Mitosis and Meiosis

  • Mitosis
  • cells produced have the same number of chromosomes as the original cell
  • cells produced are diploid & identical to parent
  • mitosis allows an organism to grow & replace cells due to damage
  • one division
  • occurs in all body cell tissues (somatic)
  • two cells formed