The human race is teeming with genetic variation. The spectrum of hair colors, eye colors and body shapes is a testament to this. But what accounts for these remarkable differences in appearance? Meiosis, the process of cellular division, is the hallmark of genetic variation. During meiosis, both male and female gametes, or germ cells, are formed. And when gametes, of the opposite sex, join together, they yield zygotes, which form embryos, and ultimately, fetuses and newborn babies. However, this phenomenon would cease to exist without spermatogenesis and oogenesis. A unique type of meiosis forms sperm (spermatogenesis) and egg cells (oogenesis), respectively.
Spermatogenesis
The process of spermatogenesis occurs through meiosis. The male testes, or testicles, consist of small ducts/channels which contain diploid cells, or, the precursors to sperm. The purpose of spermatogenesis to convert the diploid cells into four haploid sperm cells. While diploid cells contain two sets of genetic material (chromosomes), haploid cells contain only one set.
Diploid cells are quadrupled during the process of meiosis, in which cells are divided for reproductive purposes. Once 2 daughter cells are formed, they exchange genetic material amongst their sister pairs of DNA, known as chromatids. This step is highly significant in the context of human genetic variability.
During the second phase of meiosis, four cells are produced, all of which contain 23 chromosomes a piece. Eventually, these 4 haploid cells transform into sperm cells. Once a male reaches the state of puberty, he will produce millions of sperm every day.
Ontogenesis
Oogenesis is relatively similar to spermatogenesis. However, this process occurs in the female reproductive system, rather than the male reproductive system. During the process of oogenesis, one begins with a primary oocyte, or, a female germ cell, which resides in the female ovaries. During the first phase of meiosis, a diploid cell is transformed into haploid cells.
While gamete production between men and women is similar, there are many notable differences between the two. For instance, while men generate four sperm cells, women produce only one ovum or egg cell. When the female germ cell divides during the process of meiosis, it produces an ovum, in addition to three smaller structures, known as polar bodies; these polar bodies are eventually destroyed. The process of oogenesis occurs once every month, and it commences at puberty and ceases at menopause. When the ovum is not implanted by a sperm cell, the endometrial lining sheds, causing what is known as the menstrual cycle.
