Gametogenesis (7.2)

Question 1

Distinguish between somatic and germ line cells

Answer 1

Somatic cells: Any cell which forms the body of an organism. Divide by mitosis. Are identical to the parent cell.

Germ line cells: Any cell which gives rise to sex cells/gametes. Divide by meiosis. Are not identical to the parent cell

Question 2

Outline the cell cycle and mitosis

Answer 2

Phases:

G1: Growth of the cell and replication of cellular organelles

S: Synthesis of genetic material

G2: Further growth and proliferation

G1 + S + G2 = Interphase

M: Sees the division of the cell to produce 2 daughter cells, which are identical to the parent cell.

Cells undergo prophase, metaphase, anaphase and telophase - making up mitosis. Followed by cytokinesis.

Cells may enter G0, where they are terminally differentiated (no division)

Question 3

Outline/draw mitosis and meiosis

Answer 3

*Crossover only occurs in meiosis (prophase I specifically)*

Question 4

Describe the process of gametogenesis: Oogenesis

Answer 4

• Begins before birth. The oogonium undergo mitosis, generate 2 million cells.
• Meiosis 1 begins but then arrests at prophase I.
• This generates the primary oocyte.
• Meisosis 1 continues many years later, upon ovulation.
• Meiosis 1 completes are meiosis 2 begins.
• Meiosis 2 arrests at metaphase 2.
• This generates the secondary oocyte.
• Meiosis 1 continues upon fertilisation of the egg.
• This yields 1 mature ovum and 3 polar bodies (which either die or are reabsorbed)

Question 5

Describe the process of gametogenesis: Spermatogenesis

Answer 5

• Males begin to produce sperm upon reaching puberty. With the process occuring in the testes, in the semi-iniferous tubules (embedded in the tubule?).
• Spermatogonium undergo mitosis producing 2 identical cells.
• 1 of the cells, A1 spermatogonia, go on to replenish the pool of spermatogonia
• The other cell, B type spermatogonia, will eventually become a mature sperm cell
• The type B spermatogonia replicate through mitosis to produce primary spermatocytes
• The onset of puberty prompts division of the primary spermatocytes via meiosis
• The primary spermatocytes undergo meiosis 1, producing secondary spermatocytes
• The secondary spermatocytes undergo meiosis 2, producing spermatids
• The spermatids undergo spermatogenesis(differentiation), as they travel through the seminferous tubules, giving rise to mature spermatozoa.
• The sperm then travel to the rete testes where the sperm is concentrated through the removal of excess fluid. They then travel to the epididymis.

Question 6

Describe the basis of spermiogenesis

Answer 6

Spermiogenesis: The differentiation of spermatids allowing the generation of mature spermatozoa

• Takes approximately 70 days
• Acrosomal vesicles form - contain enzymes required to reach and penetrate the egg
• Microtubules are organised in the developing flagellum
• Histones are replaced by protamines - allowing for denser packaging of genomic material
• Excess cytoplasm is pinched off - forming a residual body
• Mitochondria accumuluate in the midpiece
• LATE STAGES OF SPERMATOGENESIS: The sperm remains attached to sertoli cells
  
  • ABP concentrates androgens allowing for spermatogenesis to proceed
  • Sertoli cells provide nutrients to support spermiogenesis
  • The sertoli cells maintain the blood-testes barrier: prevents release of antigens which may prompt immune reponse
  • Anti-mullerian hormone is secreted in the developing testes - suppresses female ducts

Question 7

Describe hormonal control of gametogenesis in males and females

Answer 7

Females:

• LH surge, from oestrogen positive feedback, prompts ovulation. In turn ovulation prompts the continuation of meiosis 1 (from prophase 1).

Males:

• Testosterone, concentrated by ABP, allows for the maturation of sperm
• Increased pulsatile release of GnRH, seen at the onset of puberty, allows for sex hormone synthesis and subsequent spermatogenesis

Question 8

Briefly describe the events leading up to fertilisation

Answer 8

1. The oocyte is suspended in meiosis 2, at metaphase 2
2. The acrosomal enzymes carried by the sperm begin to breakdown the ECM holding the corona radiata together. This requires multiple sperm.
3. 1 sperm contacts the oocyte body.
4. This allows for the continuation of meiosis 2, generating a mature ovum and the ejection of a 2nd polar body.
5. Polyspermy is blocked by the depolarisation of the oocyte membrane.