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Meiosis and Sexual Lifecycles. Chapter 13. Objectives. Understand the meaning of “SEX” Understand the process of meiosis Understand the need to undergo meiosis as sexual organisms. Need For Sex. Sex: results in the recombination of genetic information

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Presentation Transcript
  • Understand the meaning of “SEX”
  • Understand the process of meiosis
  • Understand the need to undergo meiosis as sexual organisms
need for sex
Need For Sex
  • Sex: results in the recombination of genetic information
  • Organisms with sexual lifecycles have a greater range of genetic variation that may result in a greater ability to adapt to an environment
  • Meiosis is key to enhancing this variation
sex terms
Sex Terms
  • Diploid: having 2 of each chromosome type
  • Haploid: having a single representative of each chromosome type
  • Homologous chromosomes (homologues): chromosomes from different parents that are of the same type (contain similar information)
  • Sister Chromatids: 2 “identical” strands of DNA connected by a centromere that contains a kinetochore. Makes up each member of a homologous pair

produces 2 daughter cells

daughters are clones of the parent

daughter cells are diploid

growth, replacement, repair


produces up to 4 daughter cells

daughters are distinct from parents

daughter cells are haploid

generation of gametes (sex cells): egg & sperm

meiosis the process
Meiosis (the process)
  • Meiosis: nuclear division with a reduction in chromosome number by half
  • Two parts to the process
    • Meiosis I: separation of homologous pairs
    • Meiosis II: separation of sister chromatids (just as in mitosis)
meiosis i
Meiosis I
  • Prophase I: same as mitosis
  • Metaphase I: both members of a homologous pair arrange themselves as a tetrad (synapsis) along the metaphase plate; crossover (chiasma) may occur
  • Anaphase I: Kinetochore remains intact and homologous pairs are separated as spindle fibers shorten
meiosis ii
Meiosis II
  • Telophase I: same as mitosis (includes cytokinesis)
  • Meiosis II is the same process as mitosis except starting from Telophase I instead of Interphase
    • no doubling of DNA has occurred
  • Result is up to four daughter cells
genetic variation
Genetic Variation
  • Variation in the DNA of a zygote can be induced by three different methods occurring the during sexual cycle
    • Random fertilization: 8+ million possible combinations of sperm and egg = 70,368,744,000,000 possible combinations
    • Independent Assortment
    • Crossing over
independent assortment
Independent Assortment
  • There are about 8 million possible combinations of chromosomes based on random assortment of the different types in the human genome (8,388,744).
  • This occurs during Anaphase I of meiosis as homologous pairs are separated
  • Genes that do not reside on the same chromosome may sort independent of one another
crossing over
Crossing Over
  • During tetrad formation (Prophase I) chromosomes may cross (chiasma).
  • Sometimes genetic information may be exchanged between homologues further shuffling the genetic information of the eventual gamete


different zygotes with a single crossover)