meiosis

• haploid: a cell containing a single set of chromosomes (unpaired chromosomes) = n
• diploid: a cell containing pairs of homologous chromosomes, one from each parent 2n
• homologous chromatids: pairs of chromosomes in a diploid cell, one from each parent, of similar size and containing the same genes (which may have alternative alleles)
• somatic cell: all cells in a multicelluler organism other than the gametes = 2n
• gamete: a haploid cell required for sexual reproduction (sperm/pollen for males and egg/ovum for females) = n
• fertilisation: fusion of male and female gametes to form a zygote n+n = 2n
• zygote: a diploid cell formed as a result of the fusion of two haploid gamtes = 2n
• the significance of Meiosis:

  • sexual reproduction: parent + parent (gametes) -- meiosis --> fertilisation = zygote -- mitosis --> offspring
  • number of chromosomes halved: important as diploid number is restored during fertilisation, there are 4 cell divisions in meiosis resulting in 4 cells with half the number of chromosomes of somatic cells - both meiotic nuclear divisions have the same phases as mitosis ie: meiosis I (1st division) - prophase phase I, metaphase phase I, anaphase phase I, telophase phase I, Meiosis II (2nd division) - pro phase II, meta phase II, ana phase II, telo phase II
• there is no interphase II as there is no DNA replication before second meiotic cell division
• importance of meiosis (p2)

  • genetic variation: unlike mitosis, meiosis allows for genetic variation in resultant cells
• process of meiosis I (p1):

  interphase

  • a diploid cell starts the process
  • mitosis: DNA replicates in S phase

  prophase I

  • nuclear envelope degrades
  • centrioles migrate to opp. poles
  • spindle forms
  • longest stage of mieosis
  • chromosomes appear
  • homologous chromosomes to form bivalents
  • corssing over occurs
• process of meiosis I (p2):

  metaphase I

  • line up at equator
  • homologous pairs seperate
  • bivalents line up at equator
  • bivalents have 3 possible orientations
  • each bivalent orients itself independently of the others
  • maternal and paternal chromsomes of each bivalent can form at either pole
  • independent assortment occurs there are 2(23) possible combinations of orientation
• process of meiosis I (p3):

  Anaphase I:

  • spindle fibres shorten
  • reduction division occurs:
  • homologous chromosomes are seperated (2n->n)
  • only one copy of each pair of homologous chromosomes will be retained by each cell, albeit doubled up
  • resultant cells will be haploid
• process of meiosis II (p1): seperates sister chromatids

  telophase I and cytokinesis:

  • nuclear envelope reforms
  • spindle fibres disassemble
  • there is no (or short) interphase II
  • chromosomes disperse
  • chromatin reforms
  • no DNA replication occurs as there is no interphase II

  prophase II:

  • nuclear envelope degrades
  • centrioles migrate to opposite poles
  • spindle forms
  • chromosomes appear
• meiosis II (p2):

  metaphase II:

  • chromosomes line up at equator
  • sister chromatids are no longer identical
  • chromosomes orientate themselves independently of each other
  • chromatids can be in either orientation
  • independent assortment occurs
  • there are 2(23) possible combinations of orientation

  anaphase II:

  • spindle fibres shorten
  • non-individual sister chromatids are pulled to opposite poles
• telophase I and cytokinesis:

  • nuclear envelope reforms
  • spindle fibres disassemble
  • cytokinesis follows giving rise to 4 haploid cells
  • cells have 1/2 quantity of DNA of origincal - all cells are genetically different
  • chromsomes disperse
  • chromatin reforms
  • nucleolus reappears
• Prophase I:

  longest stage of meiosis:

  • chromosomes condense and appear
  • homologous chromosomes pair up to form bivalents
  • a bivalent or tetrad: homologs attached together by a thin network of protein

  ( no pairing of homologous chromosomes occurs in mitosis)

  each pair of homologous chromosomes has:

  • similar length
  • same position of centrosomes
  • same loci of genes

  adjoining non-sister chromatids of homologous pair are attached at points called chiasmata

  • at chiasmata, there is a breakage + rejoining of non-sister chromatids where genetic material is exchanged ie crossing over occurs
• prophase I (p2)

  • crossing over/genetic recombination: alleles are exchanged between the maternal and paternal chromosomes
  • chiasmata and crossing over can occur on several places on the homologous pair of chromosomes
  • genetic recombination is different fro every set of homologous chromosomes in every cell, thus contributing to genetic variation