A&P lab 1 lecture 5 meiosis
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A&P lab 1 lecture 5 meiosis

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A&P lab 1 lecture 5 meiosis A&P lab 1 lecture 5 meiosis Presentation Transcript

  • API LAB 5 - Meiosis
  • Meiosis Overview
    • Meiosis is the process by which sex cells (gametes) divide
    • Takes place during gametogenesis as egg and sperm cells are formed from germ cells of the ovaries and the testes
    • During meiosis, the chromosome number is reduced from 46 to 23 so as cells enter this process they are diploid and as they complete the process they are haploid.
      • In human cells, the diploid number (2n) is 46. Thus, the haploid number is 23.
  • Meiosis Overview Continued
    • At fertilization, two haploid cells unite to form a diploid zygote
    • Therefore, the zygote inherits 23 chromosomes from the paternal side and 23 from the maternal side.
  • Meiosis: The Phases
    • Meiosis involves two sequential cell divisions known as the meiotic or maturation divisions:
    • 1. M-I Phase : reduction division during which chromosome number is reduced from diploid to haploid
    • 2. M – II Phase : Precedes Mitosis
    • *** The sequence of events is similar to mitosis (prophase, metaphase, anaphase, telophase) ***
  • M-I: First Maturation Stage
    • Synapsis (pairing) of homologous chromosomes takes place
    • Complementary chromosomes align, side by side, during metaphase. This is a very precise alignment does not occur in mitosis.
    • Because each chromosome consists of two chromatids (2C), synapsis involves the formation of structures know as tetrads (4 chromatids or 4C)
    • During anaphase of M-I, one complete chromosome (2C) from each pair, migrates to form daughters.
    • - One tetrad is split into dyads (two chromatids)
    • - Each dyad represents one chromosome (2C) of the homologous pair
  • First Division of Meiosis – M - I
    • Prophase I – each chromosome duplicates (DNA replication). These are called sister chromatids.
      • Crossing over (mixing of traits by splicing and assembly of chromosome pieces) can occur at the later part of this stage.
    • Metaphase I – Homologous chromosomes align at the equatorial plate
      • One homologous chromosome comes from the individual’s father and one from the mother.
    • Anaphase I – Homologous chromosome pairs separate with sister chromatids remaining together
    • Telophase I – Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair
    • The resulting cells are haploid and contain 23, 2C chromosomes
  • The M – II Division of Meiosis
    • This division is similar to mitosis
    • The splitting of two chromatids (2C) of a chromosome (dyads) into single chromosomes (1C) occurs
    • This occurs because unlike mitosis and meiosis I no DNA synthesis or duplication occurs before meiosis II
    • As an end result, each normal human gamete contains 23, 1C chromosomes.
  • Second Division of Meiosis (MII) – Gamete Formation
    • Prophase II – DNA does not replicate
    • Metaphase II – Chromosomes align at the equatorial plate but there is no synapsis
    • Anaphase II – Centromeres divide and sister chromatids migrate separately to each pole
    • Telophase II – Cell division is complete. Four haploid daughter cells are formed
  • Results:
    • One parent cells produces four daughter cells
    • Daughter cells have half the number of chromosomes found in the original parent cell and are genetically different (crossing over)
    • Meiosis differs from mitosis because there are two cell divisions in meiosis, resulting in cells with a haploid number of chromosomes.
    • http://www.johnkyrk.com/meiosis.html
    • http://www.rothamsted.ac.uk/notebook/courses/guide/meiosis.htm
    • http://www.sumanasinc.com/webcontent/animations/content/meiosis.html
    • http://www.youtube.com/watch?v=uh7c8YbYGqo&NR=1