Meiosis

• What is meiosis?


  • Process by which germ cells divide to form gametes
  • Involves nuclear division that halves number of chromosomes
  • Evolved for sexual reproduction
• How many chromosomes in a human cell?


  46 (23 pairs)

  22 pairs of autosomes (exist in matching pairs- 1 from each parent)

  1 pair of sex chromosomes (XX or XY)
• What is a zygote?


  • Single cell formed at fertilisation by union of female & male gamete
  • All cells of a person arise from this
• Where does fertilisation take place?


  Uterine tube
• Advantages of sexual reproduction:

  Genetic diversity- best survival for population

  Genetically unique offspring so improves fitness of population
• What type of cells are gametes?


  Haploid (23 chromosomes)
• Where are gametes produced?


  • Testis (male)
  • Ovary & uterine tube (female)
• Where does meiosis occur?


  Germ cells (sex cells of ovary & testis)
• Errors in meiosis:


  • Non-disjunction leads to disorders in chromosome number (chromosomes replicated don't segregate correctly in anaphase)
  • Structural rearrangements of chromosomes (unequal translocation)
• Examples of non-disjunction:


  • Trisomy chromosome 21 (Down Syndrome)- 3 copies of ch21
  • Turner syndrome XO- only 1 X chromosome in females (no ovaries so infertile)
• What happens before meiosis?


  DNA replicates
• What happens in meiosis 1 (reduction division)?


  • Prophase- chromosomes condense, homologues align, come together & exchange homologous segments 'crossing over' Recombinant chromosomes begin to separate (longest phase of meiosis)
  • Metaphase- Nuclear membrane disappears, chromosomes align in middle
  • Anaphase- separation
  • Telophase- Cell splits into 2 haploid daughter cells (with sister chromatids)
• What happens in meiosis 2 (equatorial division)?


  • Normal prophase, metaphase, anaphase & telophase
  • Each daughter cell divides into 2
  • 4 genetically unique daughter cells produced
• Consequences of meiosis:


  • Each chromosome (esp. large ones) will typically have 3 or more segments of alternating parental origin
  • Little chance 2 gametes are exactly same/have identical genome
• Meiosis in human male:


  • Meiosis 1 begins at puberty (60-65 days)- makes sperm
  • First sperm in ejaculate= Spermache
• What is the Testis controlled by?


  • Pituitary gland
• Which hormone helps make testosterone?

  LH (Luteinizing hormone)
• How does FSH act upon the seminiferous tubule?


  Starts process of differentiation of spermatogonia into sperm
• What is spermatogenesis?


  Gamete formation in male within seminiferous tubule epithelium

  Sperm formation

  Involves mitotic proliferation of spermatogonia, meiotic division & spermiogenesis (remodelling to form sperm)
• Spermiation:


  Release sperm (swims away in tubular fluid)
• Spermiogenesis:


  Elongate sperm

  Formation of acrosome, flagellum & midpiece & loss of excess cytoplasm
• What do myoid cells allow?

  Contraction
• What lines the inside of the seminiferous tubule?


  • Spermatogonia stem cells
  • Some continue to make sperm
  • Type B spermatogonium (committed stem cell) divides by mitosis to form primary spermatocyte (diploid cell that enters meiosis)
• What does meiosis 1 in the male form?


  • Secondary spermatocyte (haploid)
  • enters meiosis 2 to form round spermatids (haploid) & remodel to form gametes (spermatozoa)
• What are Sertoli cells?


  Autosomal support cells with cytoplasm that engulf germ cells differentiating.

  Form tight junctions with neighbouring cells & engulfs whole process- sperm is attached to cytoplasm of sertoli & then detached during spermiation
• Meiosis in human female:


  • All pre-eggs formed before birth
  • Pre-egg in dormant state: arrested in prophase 1 of meiosis 1
  • Programmed cell death in ovary (atresia) (begins in foetus) results in continual decline of number of pre-eggs (egg reserve/fertility potential)
• When do women have the highest number of pre-eggs (oocytes)?


  Maximal around 30 weeks pregnancy (midpoint of pregnancy)
• What are oogonia?


  Stem cells in ovary
• What does a primordial follicle look like?


  1 layer of cells around a primary oocyte (small)
• What is the number of primordial follicles in the ovary equivalent to?


  Fertility potential

  (more follicles, higher potential)
• What do ovarian growth factors spontaneously trigger?


  Primordial follicle to start growing to form primary follicle (larger)

  (oocyte grows)
• When does the ovarian cycle begin?


  Puberty

  can develop primary follicle into secondary follicle

  12 secondary follicles develop each month (1 destined to ovulate)
• What are oestrogen, FSH & LH used to help with?


  To mature follicle & oocyte within it
• What is the antrum?


  Space in follicle filled with oestrogen- helps nurture rapid growth
• Which follicle will ovulate?


  Follicle that grows fastest & dominates ovary & is where most oestrogen is
• When does meiosis 1 start in the human female?


  Foetus (suspended in prophase 1)

  Completed at ovulation more than 12 years later (dominant follicle responds to LH surge that restarts meiosis 1)
• When does meiosis 2 start in human females?


  Completed at fertilisation

  Begins during ovulation (restarted by LH surge- in some women even 50 years after meiosis 1 was suspended)

  Produces 3 polar bodies & 1 maturing oocyte (gamete)

  Cells from first division unequal- 1 cell= ovum (bigger)-maturing oocyte
• What is a polar body?


  Remnant containing unwanted DNA content (smaller cell)
• What are the names of the 2 gametes?


  • Oocytes
  • Spermatozoa
• What does the seminiferous tubule look like?


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