Reproduction

• Sexual reproduction

  Involves the development and fusion of haploid gametes
• Fertilisation


  Creates a unique diploid (zygote) cell which can develop into a new organism
• Spermatogenesis and oogenesis


  • Both involve mitosis, cell growth, two divisions of meiosis and differentiation
  • Result in different numbers of gametes with different amounts of cytoplasm
• Fertilization


  Can be internal or external
• Fertilization


  1. Involves the acrosome reaction, fusion of the plasma membrane of the egg and sperm and the cortical reaction

  2. Involves mechanisms that prevent polyspermy
• Implantation


  Of the blastocyst in the endometrium is essential for the continuation of pregnancy
• HCG


  Stimulates the ovary to secrete progesterone during early pregnancy
• Placenta


  Facilitates the exchange of materials between the mother and fetus
• Estrogen and progesterone


  Are secreted by the placenta once it has formed
• Birth


  Is mediated by positive feedback involving estrogen and oxytocin
• Mature egg


  • Haploid (n) contains 23 chromosomes to be passed from mother to child
  • Consists of a glycoprotein that protects the egg and prevents the entry of sperm
  • Not required – will break down
  • Contains nutrients to support the early development of fertilised egg
  • Makes the zona pellucida impenetrable to sperm (after fertilisation) to prevent polyspermy
  • Provides nutrients to support the early development of fertilised egg
  • Cortical granules
• Mature sperm


  • Haploid (n), contains 23 chromosomes to be passed from father to child
  • Contains enzymes which can digest the zona pellucida
  • Possesses helical mitochondria which provide the ATP (energy) for swimming (and other processes)
  • Contains protein fibres and microtubules to strengthen and allow the tail to move respectively
• The average 38-week pregnancy in humans can be positioned on a graph showing the correlation between animal size and the development of the young at birth for other mammals
• Assessing risks and benefits associated with scientific research: pollution from the female contraceptive pill
• Spermatogenesis


  Spermatogonia divide to produce spermatocytes
• Oogenesis


  1. Contains the primary oocyte surrounded by a single layer of supporting follicle cells

  2. Contains the secondary oocyte, ready for ovulation

  3. Outer layer of cells in the ovary
• Human ovulation captured on film
• Oogenesis


  • Production of ova (female gametes)
  • During fetal development large numbers of oogonia are formed by mitosis
  • Oogonia enlarge (growth) and undergo meiosis, but stop in prophase I (until puberty). They are now termed primary oocytes and are held in primary follicles
  • (at puberty) some follicles develop each month in response to FSH: the oocyte completes the first meiotic division, Division of the cytoplasm is unequal creating a polar body, the secondary oocyte continues into meiosis II and halts at prophase II, polar bodies eventually degenerate
  • Secondary oocytes develop along with the follicle. When the follicle is mature it rupture to release the secondary oocyte with a small number of cells (the mature egg) into the fallopian tube. The remaining follicle cells remain in the ovary to form the corpus luteum (which secretes progesterone)
  • The oocyte completes meiosis II (forming the ovum) if the cell is fertilized and another polar body
• Spermatogenesis


  Process involving mitosis, cell growth, two divisions of meiosis and differentiation
• Oogenesis


  Process involving mitosis, cell growth, two divisions of meiosis and differentiation
• Spermatogenesis and oogenesis result in different numbers of gametes with different amounts of cytoplasm
• Oogenesis


  1. Mitosis

  2. Cell growth

  3. Two divisions of meiosis

  4. Differentiation
• Spermatogenesis


  1. Mitosis

  2. Cell growth

  3. Two divisions of meiosis

  4. Differentiation
• Spermatogenesis and oogenesis both involve mitosis, cell growth, two divisions of meiosis and differentiation
• Processes in spermatogenesis and oogenesis result in different numbers of gametes with different amounts of cytoplasm
• Fertilization in animals can be internal or external
• Aquatic species with external fertilization

  • Fish
  • Amphibians
• External fertilization

  Eggs are released followed shortly by sperm. Susceptible to environmental variation, so animals often produce large quantities of eggs and sperm to compensate for losses.
• Internal fertilization


  Sperm is deposited into the female, in easy reach of the ova, during intercourse. Prevents dehydration of gametes or the developing embryo.
• Fertilization involves mechanisms that prevent polyspermy
• Fertilization

  1. Sperm pushes through follicular cells and binds to receptors in the zona pellucida

  2. Enzymes are released from the acrosome and digest the glycoprotein based zona pellucida

  3. Membranes of the sperm and the ova fuse, stimulating:

  • Exocytosis of cortical granules (vesicles) releasing proteases (enzymes) into the zona pellucida, causing it to 'harden' and become impenetrable to subsequent sperm, preventing polyspermy
  • An influx of Ca2+ into the ova which prompts the completion of meiosis II

  4. Nucleus of the sperm cell is deposited into the ova's cytoplasm and subsequently fuses with the ova's nucleus forming a diploid zygote (cell)
• Implantation of the blastocyst in the endometrium is essential for the continuation of pregnancy
• Blastocyst formation

  1. Morula (ball of cells) form after mitotic divisions

  2. Ball of cells continues to divide unequally, forming a fluid-filled cavity - this is now termed a blastocyst

  • Inner mass of cells (develops into the embryo)
  • Outer layer (develops into the placenta)
  • A fluid filled cavity
• HCG stimulates the ovary to secrete progesterone during early pregnancy
• During pregnancy

  Menstruation ceases

  No further mature eggs are released
• Estrogen and progesterone are secreted by the placenta once it has formed
• Corpus luteum

  Initially maintains HCG, then the placenta takes over the hormonal role
• Estrogen

  Maintains the lining of the uterus
• Progesterone

  Maintains the endometrium and prevents contractions
• The placenta facilitates the exchange of materials between the mother and fetus