bialogy chapter 6

• Haploid


  Single set of chromosomes. Gametes are said to be "haploid" or 1N.
• Diploid


  Diploid cells contain two complete sets of chromosomes (2n).
• Components of sexual reproduction

  • Gametes: Sex cells (Sperm and egg/ova cells)
  • Fertilization: Union of sperm and egg
  • Zygote: A fertilized egg
• Crossing Over


  The exchange of genetic material between homologous chromosomes during prophase I of meiosis.
• Crossing Over


  • During prophase I, each pair of chromatids lines up next to its homologue. This pairing of homologous chromosomes produces tetrads. A tetrad consists of four chromatids. It is possible for the chromatids within a homologous pair to twist around one another. Portions of the chromatids may break off and attach to adjacent chromatids.
• Crossing over increases genetic variation among offspring.
• Egg and sperm cells are gametes, which are involved in sexual reproduction.
• Egg and sperm cells need to have half the number of chromosomes (haploid) to ensure that when they combine during fertilization, the resulting zygote has the correct diploid number of chromosomes.
• Tetrads


  Formed during prophase I of meiosis when homologous chromosomes pair up. Each tetrad consists of four chromatids, two from each homologous chromosome pair.
• Homologous pairs are the same size and shape and have genes in the same location. Each chromosome may have different versions of those genes, as shown by the different shades of each color. Homologous chromosomes carry the same genes, but they may have different expressions of the genes.
• Advantages of Asexual Reproduction

  • It is faster.
  • Large numbers of offspring are produced.
  • The parent does not have to find a mate.
• Disadvantages of Asexual Reproduction

  • No genetic variation.
• Advantages of Sexual Reproduction

  • All the offspring are different. (genetic variation)
• Disadvantages of Sexual Reproduction

  • The parent must find a mate.
  • Very few offspring are produced.
  • It takes longer.
• In a body cell, let's say 2n = 8, which means the total number of chromosomes is 8. Therefore, n (haploid number) would be half of 8, which is 4.
• Reduction division

  The number of chromosomes per cell is reduced by half through the separation of homologous chromosomes
• Meiosis produces four haploid cells that are different.
• In males, meiosis results in four sperm cells.
• In females, four cells are produced, but only one will become a functional egg cell. All of the cytoplasm and all of the organelles are put into one egg cell. The other three cells will never be functional.
• Prophase I

  Chromosomes shorten and thicken. Each chromosome pairs with its corresponding homologous chromosomes to form a tetrad (four). There are four chromatids in a tetrad. This pairing of homologous chromosomes does not occur in mitosis.
• Metaphase I

  Tetrads line up at the center of the cell
• Anaphase I

  The Tetrads break apart and the pairs move to opposite sides of the cell. Sister chromatids remain attached at their centromeres.
• Telophase I and Cytokinesis

  The cell separates into two cells. Meiosis I results in two haploid (1N) daughter cells. Each daughter cell has half the number of chromosomes as the original cell. Sister chromatids remain attached.
• There is no replication of chromosomes by either cell before entering meiosis II.
• Prophase II


  The pairs of sister chromatids start toward the center.
• Metaphase II


  Pairs of sister chromatids line up at the center of the cell.
• Anaphase II


  The sister chromatids are moved to opposite sides of the cell.
• Telophase II and Cytokinesis

  Meiosis II results in four haploid daughter cells.
• Homologous chromosomes

  The two copies of each chromosome, one coming from the female parent and one coming from the male parent
• Meiosis


  1. Reduction division

  2. Separation of homologous chromosomes

  3. Chromosome number per cell reduced by half
• Purpose of meiosis


  To reduce the chromosome number in the egg and sperm by half
• Meiosis


  One diploid cell divides to produce four haploid gametes