Gametogenesis - Definition, Stages and Types - NEET PG Anatomy

Learn what is Gametogenesis, Mitosis vs Meiosis, Oogenesis in this detailed article which will upgrade your NEET-PG exam preparation. 

What is Gametogenesis?

The process of producing gametes from haploid progenitor cells is called gametogenesis. Different differentiation programmes are used in animals and higher plants to produce the two physically distinct types of gametes (male and female). Animals produce a tissue called the germ line that is specifically responsible for developing gametes.

Zygote has maternal and paternal nuclei. The genetic make-up of a zygote is referred to as [2n, 2N]. Zygote further divides and forms → Blastocyst. Blastocyst has an Inner Cell Mass (ICM). ICM has an epiblast. This epiblast forms Primordial Germ Cells (PGCs). PGCs are the 1^st sex cells of the body

The formation of gametocytes can be summarized as below

PGCs

↓

Gonocytes

↓

Gametogonia

↓

Primary & Secondary Gametocytes

• PGCs form male or female gametes (depending upon the presence or absence of Y chromosome)

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MITOSIS vs MEIOSIS

Mitosis

It is also known as Equational division (because the chromosome number remains the same). Before any cell division, DNA duplication occurs. DNA duplication occurs in the ‘S phase’ or the ‘Synthesis phase’ of the Interphase. This means that the cell changes from a [2n, 2N] to [2n, 4N]. The cell remains diploid [2n] but the DNA amount is doubled [4N]. The sister chromatids are available.

• Phases of Mitosis: 

Prophase

↓

Metaphase

↓

Anaphase

↓

Telophase

• Finally, 2 daughter cells are formed. The daughter cells have the same genetic make-up, that is [2n, 2N].

Meiosis 

It is also known as → Reduction division (because the chromosome number is reduced to half). DNA duplication occurs in the ‘S phase’ or the ‘Synthesis phase’ of the Interphase.

• Meiosis has 2 parts:
  • Meiosis I (Prophase I, Metaphase I, Anaphase I, Telophase I)
  • Meiosis II (Prophase II, Metaphase II, Anaphase II, Telophase II)
• Prophase I → further divided into 4 phases
  • Leptotene
  • Zygotene
  • Pachytene
  • Diplotene
• Tetrad stage (4 sister chromatids) and chiasma formation occurs in the Pachytene stage. Chiasma formation helps in exchange of genetic material between the maternal and paternal chromosome. This is genetic recombination. Genetic Recombination due to chiasma formation is a characteristic of meiosis.
• By the end of Meiosis I, the maternal and paternal chromosomes separate. Therefore, by the end of Meiosis I → 2 Primary gametocytes [n, 2N] are formed.  Primary gametocytes can be a primary spermatocyte or a primary oocyte, depending on the presence or absence of Y chromosome.
• Meiosis II → is just like mitosis because there is no change in the chromosome number. In Meiosis II → only the sister chromatids are separated. Therefore, by the end of Meiosis II → Gametes [n, N] are formed

Now that you’ve reached till here, complement your learning further with this detailed video lecture on Gametogenesis by Dr Prassan Vij:

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OOGENESIS

Primordial germ cell (PGC) differentiates to form the oogonium and further the primary oocyte via the process of mitosis. Primary gametocyte is the largest of all the cells in the process of gametogenesis.  Primary oocyte undergoes meiosis.

• Primary oocyte enters Meiosis I as [2n, 4N]. Meiosis I can only be completed after an LH surge. Primary oocyte remains arrested in ‘Diplotene stage’ of Prophase I until puberty. This is because only after puberty, can there be an LH surge, after which the Primary oocyte can divide into a Secondary oocyte.
• By the end of Meiosis I → Secondary oocyte [n, 2N] and 1^st Polar Body [n, 2N] are formed. Secondary oocyte undergoes the process of ovulation. Secondary oocyte completes Meiosis II only if it is fertilized by a sperm. Secondary oocyte completes Meiosis II after fertilization and forms a mature egg and also forms a 2^nd Polar body.

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MENSTRUAL CYCLE

Normally, menstrual cycle duration is 28 days (average). Day 1 → here, refers to the 1^st day of the last menstrual period (LMP). If it is a 28-days cycle, then ovulation will occur on Day 14 (mid-cycle) of the menstrual cycle.

• LH Surge: Rise in the concentration of LH (Luteinizing Hormone) before ovulation.LH surge starts about 36 hours before ovulation.
• LH Peak: When maximum concentration of LH is reached it is known as LH peak. LH peak occurs about 12 hours before ovulation.1^st polar body is released when LH peak occurs (i.e., about 12 hours before ovulation).
• 2^nd polar body is released after fertilization. Fertilization should occur within 24 hours of ovulation for the zygote to form. This is because, the secondary oocyte degenerates within 24 hours of ovulation. Therefore, fertile period for a female is within 24 hours of ovulation.
• Sperms only have the capacity to fertilize the ovum for about 2 days or 48 hours after ejaculation. Therefore, if sperms are ejaculated on/after day 12 to day 14, fertilization can occur, and the female can conceive. If the sperms are ejaculated before day 12 or after 14, fertilization is not possible.
• Hence, most fertile period is 3 days (Day 12, 13 and 14) around the day of ovulation.

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