Question

• Explain how sperm and eggs are formed

Answer 1

Answer

Sperms are the male gametes and eggs (ovum) are female gametes.

Sperm formation :

• The process of sperm formation is known as spermatogenesis.
• Undifferentiated spermatogonia also known as type A spermatogonia (germ cells) are lined at the base of seminiferous tubules in testes. They are dipolid cells having 46 chromosomes.
• At puberty, these type A cells, undergoes mitosis to produce type B spermatogonia.
• Type B spermatogonia undergoes mitosis to produce diploid primary spermatocytes.
• Primary spermatocytes undergoes meiosis - I (cell division that only occurs in gametes). It leads to formation of two haploid secondary spermatocytes having 23 chromosomes.  
• Meosis - II of secondary spermatocytes produce 4 haploid spermatids.
• Spematids are immature gametes.
• They undergo spermiogenesis to produce 4 fully functional hapolid sperm cells.
• Sperm cells are stored in epididymis and travels through vas deferens and urethra during ejaculation.

Egg formation :

• The process of of forming egg (ovum) is known as oogenesis.
• Primordial germ cell gets differentiated into oogonium at it arrives in the ovaries during the fetal life.
• Oogonium undergoes mitosis to produce primary oocytes. They are dipolid cells having 46 chromosomes.
• Primary oocyte begins meiosis -I before birth but are arrest at prophase-I.
• When puberty is attained, this primary oocyte completes meiosis-I to produce one secondary oocyte and one non-functional polar body. Both of these cells are haploid having 23 chromosomes.
• Secondary oocyte is arrested at metaphase-II of meiosis -II when it is released from mature follicle during ovulation.
• Meiosis - II is only completed when a sperm fertilizes seocndary oocyte.
• If fertilization occurs, secondary oocyte completes meiosis -II forming a mature ovum and a secondary polar body.