Each human body cell has two sets of chromosomes, with each set containing 23 chromosomes, totaling 46 chromosomes. These are considered diploid (2n). Chromosomes can exist as a single condensed strand of DNA or, after DNA replication occurs, as two strands of condensed DNA that are joined at a single point called a centromere (with each being n, c). Each side of these replicated chromosomes is called a chromatid. Chromosomes have two chromatids only after DNA replication occurs prior to cell division. If a cell is not undergoing cell division, each chromosome has a single chromatid.
Every chromosome has a homologous chromosome. Homologous chromosomes contain the same genes but may have different forms of the genes. One set of genes came from the mother, and one set came from the father. Cells with both sets of chromosomes are diploid (2n), and cells with one set of chromosomes are haploid (n). In order for sexual reproduction to produce an offspring with the same number of chromosomes as the parents, special haploid cells (gametes) are created in males (sperm) and females (eggs). A sperm and egg (two haploid cells, each carrying 23 chromosomes) combine during fertilization to produce a zygote. A zygote is a diploid cell formed from the fusion of a male and female gamete.
Meiosis is a type of cell division that results in the formation of gametes. A gamete is a mature haploid cell carrying genetic material. Male gametes are sperm, and female gametes are eggs. The specific type of meiosis that creates sperm is called spermatogenesis, while females undergo oogenesis to produce eggs. During meiosis, two consecutive cell divisions occur after a single round of DNA replication. Meiosis products include four daughter cells that each have half as many chromosomes as typical body cells.
Spermatogenesis involves a sequence of events that takes place in the seminiferous tubules of the testes:
- Step 1: Spermatogonia are cells in the seminiferous tubules that undergo meiosis. These diploid stem cells can divide during puberty to produce sperm. Once puberty begins, spermatogonia undergo mitotic divisions to produce two types of spermatogonia. One type of spermatogonia remains undifferentiated (Type A) and functions as a reservoir for future cell division and sperm production. The other type of spermatogonia (Type B) differentiates into primary spermatocytes, each of which will produce four sperm.
- Step 2: Primary spermatocytes are diploid (2n, 2c), having 46 chromosomes. The primary spermatocyte replicates its DNA so that each chromosome contains two joined chromatids, or two strands of condensed DNA. This primary spermatocyte is diploid, with each chromosome containing two chromatids (2n, 4c). After DNA replication, the cell enters meiosis.
- Step 3: During meiosis I, each primary spermatocyte divides to produce two secondary spermatocytes. During this division, homologous chromosomes separate into secondary spermatocytes. These cells are haploid but each chromosome still contains two chromatids, meaning each chromosome has a replicated copy (n, 2c). The two secondary spermatocytes each undergo a second round of meiotic division (meiosis II). The sister chromatids are separated during this cell division, and the result is four haploid cells with one chromatid (n, c). Each of these four cells is called a spermatid.
- Step 4: The haploid spermatids develop into mature sperm via the process of spermiogenesis. Cell division does not occur during this final process of spermatogenesis. Rather, each spermatid undergoes a physical transformation to become a sperm cell. They elongate, develop an acrosome, lose some cytoplasm, develop a flagellum, and multiply their mitochondria.