Meiosis is a two-step process of cell division. The first stage is referred to as meiosis I and the second stage is called meiosis II. Normal meiosis is illustrated in the diagram on the left. The diagram is looking at only one chromosome pair in order to simplify the illustration. However, it is important to remember that all 23 chromosome pairs are involved in this process simultaneously. Let's look at the diagram.
Say this is a pair of chromosome number 5. These are homologous chromosomes. We can call the yellow chromosome the maternal chromosome, if it was originally inherited from this person's mother. And we can call the blue chromosome the paternal chromosome, if it was originally inherited from this person's father.
In the first step each chromosome makes an identical copy of itself. The copy is attached at the centromere which make the "x-like" shape of the replicated chromosomes. The replicated copies are called sister chromatids.
During meiosis I, the first division, the homologous pairs separate into two new daughter cells.
During meiosis II, the second division, the replicated pair of sister chromatids separate into two new daughter cells.
The final result is four gametes or reproductive cells, each which contain 23 chromosomes.
Spermatogenesis is the making of sperm. In spermatogenesis, the result of meiosis is four sperm cells which can develop into mature sperm. Spermatogenesis is an ongoing process throughout life.
The making of eggs or ova is called oogenesis. Oogenesis is different from spermatogenesis. In oogenesis the result of meiosis is one mature egg or ovum and two polar bodies. Polar bodies are smaller cells which are the bi-products of the creation of the mature ovum. During meiosis I the primary oocytes undergoes a very unequal division. The larger cell, which receives most of the cells contents (cytoplasm and organelles), becomes the secondary oocyte, and the other smaller cell becomes the first polar body. In meiosis II, a second unequal cell division occurs in the secondary oocyte. The result is the ovum and the second polar body. The polar bodies die off. The ovum is much larger than a sperm cell and it contains the materials required for development. Human males produce 200,000,000 sperm per day, whereas females produce one egg (usually) each menstrual cycle.
Show me a diagram of the production of sperm and eggs.
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