DNA Mitokondria dari Aspek Molekuler sampai Konsep Klinis
Channel: M. Rifki Al Ikhsan, MD
Date: 01/12/2020

Wanda, seorang mahasiswi Fakultas Kedokteran sedang mengikuti kuliah mengenai gametogenesis. 
Dosen menjelaskan bahwa sel gamet laki-laki dan perempuan memiliki struktur sel yang berbeda namun mengalami pembelahan sel yang sama yaitu pembelahan meiosis. 
Ketika Wanda mengikuti kuliah mengenai organel sel, dosen menjelaskan bahwa material genetik terdapat di nukleus dan juga di mitokondria. 
Namun material genetik mitokondria hanya diwariskan oleh ibu. 
Wanda menjadi bingung karena menurutnya informasi dari kedua kuliah tersebut bertentangan.
Wanda lalu bertanya kepada dosen, mengapa material genetik mitokondria hanya diwariskan oleh ibu padahal sel gamet laki-laki dan perempuan memiliki organel sel yang sama.
Gametogenesis
pembelahan sel  meiosis
2 pembelahan sel yang berturutan 
hasil akhir: 4 sel anak berkromosom ½ induk 
spermatogenesis
Hasil: 4 spermatozoa haploid, 
oogenesis 
dihasilkan 1 ovum haploid 
3 polar body yang haploid. 
Oogenesis: female gametogenesis. Note that only one pair of homologous chromosomes is shown (red,
maternal origin; blue, paternal origin). Synapsis is the process of pairing of homologous chromosomes. The point at which
the DNA molecule crosses over is called the chiasma and is where exchange of small segments of maternal and paternal
DNA occurs. Note that synapsis and crossing over occur only during meiosis I. The polar bodies are storage bodies for
DNA unnecessary for the further function of the cell and probably degenerate. There is no evidence that polar bodies
divide or undergo any other activity.

IV. OOGENESIS: FEMALE GAMETOGENESIS (FIGURE 1.2)
A. Primordial germ cells (46, 2N) from the wall of the yolk sac arrive in the ovary at week 6 and
differentiate into oogonia (46, 2N), which populate the ovary through mitotic division.
B. Oogonia enter meiosis I and undergo DNA replication to form primary oocytes (46, 4N). All primary
oocytes are formed by month 5 of fetal life. No oogonia are present at birth.
C. Primary oocytes remain dormant in prophase (dictyotene) of meiosis I from month 5 of fetal life
until puberty. After puberty, 5 to 15 primary oocytes begin maturation with each ovarian cycle,
with usually only 1 reaching full maturity in each cycle.
D. During the ovarian cycle and triggered by the luteinizing hormone (LH) surge, a primary oocyte
completes meiosis I to form two daughter cells: the secondary oocyte (23, 2N) and the first polar
body, which degenerates.
E. The secondary oocyte promptly begins meiosis II but is arrested in metaphase of meiosis II about
3 hours before ovulation. The secondary oocyte remains arrested in metaphase of meiosis II until
fertilization occurs.
F. At fertilization, the secondary oocyte completes meiosis II to form a mature oocyte (23, 1N) and a
second polar body.
G. Approximate number of oocytes
1. Primary oocytes: At month 5 of fetal life, 7 million primary oocytes are present. At birth,
2 million are present (5 million have degenerated). At puberty, 40,000 are present (1.96 million
more have degenerated).
2. Secondary oocytes: Twelve secondary oocytes are ovulated per year, up to 480 over the entire
reproductive life of the woman (40 years × 12 secondary oocytes per year = 480). This number
(480) is obviously overly simplified since it is reduced in women who take birth control
pills (which prevent ovulation), in women who become pregnant (ovulation stops during
pregnancy), and in women who may have anovulatory cycles.

Spermatogenesis: male gametogenesis. Note that only one pair of homologous chromosomes is shown (red,
maternal origin; blue, paternal origin). Synapsis is the process of pairing of homologous chromosomes. The point at which
the DNA molecule crosses over is called the chiasma and is where exchange of small segments of maternal and paternal
DNA occurs. Note that synapsis and crossing over occur only during meiosis I.
V. SPERMATOGENESIS: MALE GAMETOGENESIS (FIGURE 1.3)
Spermatogenesis is classically divided into three phases:
A. Spermatocytogenesis
1. Primordial germ cells (46, 2N) from the wall of the yolk sac arrive in the testes at week 6 and
remain dormant until puberty. At puberty, primordial germ cells differentiate into type A
spermatogonia (46, 2N).
2. Type A spermatogonia undergo mitosis to provide a continuous supply of stem cells
throughout the reproductive life of the male. Some type A spermatogonia differentiate into
type B spermatogonia (46, 2N).
B. Meiosis
1. Type B spermatogonia enter meiosis I and undergo DNA replication to form primary
spermatocytes (46, 4N).
2. Primary spermatocytes complete meiosis I to form secondary spermatocytes (23, 2N).
3. Secondary spermatocytes complete meiosis II to form four spermatids (23, 1N).
C. Spermiogenesis
1. Spermatids undergo a postmeiotic series of morphological changes to form sperm (23, 1N).