Meiosis and Fertilization

Section 1: Meiosis and Fertilization

I. Meiosis

When Mendel revealed the laws of heredity, cytological research also made significant progress. German zoologist August Weismann (A. Weismann, 1834-1914), who was contemporary with Mendel, theoretically predicted that during the maturation of sperm and egg cells, there must be a special process that reduces the number of chromosomes by half; during fertilization, when the sperm and egg cells fuse, the number of chromosomes is restored to normal. This brilliant prediction was confirmed by other scientists through microscopic observation. The special process that Weismann predicted is actually a special type of mitosis called meiosis. How does meiosis occur? Let's explore this in conjunction with the formation of sperm and egg cells in mammals.

Formation of Sperm

Meiosis in higher plants and animals occurs in the sexual reproductive organs. In humans and other mammals, sperm is formed in the testes, which contain many coiled seminiferous tubules (Figure 2-1). These tubules are filled with numerous spermatogonia, which are the primitive male germ cells that proliferate through mitosis, each containing the same number of chromosomes as somatic cells. When male animals reach sexual maturity, some of the spermatogonia in the testes begin to undergo meiosis. Scientists have discovered that before meiosis, each spermatogonium's chromosomes replicate once, and during meiosis, the cell divides twice consecutively, eventually forming four spermatids. These two divisions are called Meiosis I (also known as the first meiotic division) and Meiosis II (also known as the second meiotic division). The spermatids then undergo transformation to become mature male gametes—sperm (Figure 2-2).

During the interphase before meiosis, some spermatogonia increase in size and their chromosomes replicate, becoming primary spermatocytes. Each replicated chromosome consists of two identical sister chromatids, connected by a single centromere. At this point, the chromosomes appear as chromatin threads.

Shortly after Meiosis I begins, the previously dispersed chromosomes in the primary spermatocytes shorten, thicken, and pair up. The paired chromosomes, which are generally the same in shape and size, with one originating from the father and the other from the mother, are called homologous chromosomes. During meiosis, the phenomenon of homologous chromosomes pairing up is called synapsis. Since each chromosome consists of two sister chromatids, each pair of homologous chromosomes forms a tetrad. Non-sister chromatids within the tetrad often intertwine and exchange corresponding segments.

Subsequently, each pair of homologous chromosomes aligns on either side of the cell's equatorial plate, with the centromeres attached to spindle fibers. Shortly thereafter, under the pull of the spindle fibers, the paired homologous chromosomes separate and move toward opposite poles of the cell. As a result, each pole of the cell receives one chromosome from each pair of homologous chromosomes. After the two groups of chromosomes reach the poles of the cell, the primary spermatocyte divides into two secondary spermatocytes.

During this division, the halving of chromosome numbers occurs in Meiosis I because homologous chromosomes separate and enter two daughter cells, resulting in each secondary spermatocyte having only half the number of chromosomes as the original primary spermatocyte.

There is typically no interphase or a very short one between Meiosis I and Meiosis II, and the chromosomes do not replicate again. During Meiosis II, the centromeres of each chromosome split, and the two sister chromatids separate and become individual chromosomes. Under the pull of spindle fibers, these chromosomes move toward opposite poles of the cell and are incorporated into two daughter cells as the cell divides. In this way, the two secondary spermatocytes formed during meiosis undergo Meiosis II, resulting in four spermatids. Compared to the primary spermatocytes, each spermatid contains half the number of chromosomes.

After meiosis, the spermatids undergo a complex transformation to become sperm. Sperm are tadpole-shaped, with a head and a tail. The head contains the nucleus, while the long tail can move.

Formation of Egg Cells

In humans and other mammals, egg cells are formed in the ovaries, located in the abdominal cavity. The ovaries contain numerous follicles at different stages of development, with one cell at the center of the follicle destined to develop into an egg cell (Figure 2-4).

The formation of egg cells is fundamentally similar to that of sperm. First, during the interphase before meiosis, oogonia enlarge, and their chromosomes replicate, transforming into primary oocytes. Then, primary oocytes undergo Meiosis I and Meiosis II to form egg cells. The main difference between the formation of egg cells and sperm is that during Meiosis I, primary oocytes undergo unequal division, forming two cells of different sizes: a larger secondary oocyte and a smaller polar body. The secondary oocyte also undergoes unequal division during Meiosis II, resulting in a larger egg cell and a smaller polar body. The polar body formed during Meiosis I further divides into two polar bodies. Thus, one primary oocyte undergoes meiosis to produce one egg cell and three polar bodies (Figure 2-5). Both egg cells and polar bodies contain half the number of chromosomes. Shortly afterward, the three polar bodies degenerate and disappear, so one oogonium undergoing meiosis ultimately forms only one egg cell. Unlike sperm formation, egg cell formation does not require transformation.

In summary, meiosis is a type of cell division that reduces the number of chromosomes by half, occurring in organisms that reproduce sexually during the formation of mature gametes. Before meiosis, chromosomes replicate once, and the cell undergoes two consecutive divisions during meiosis. The result of meiosis is that the number of chromosomes in mature gametes is halved compared to the original germ cells (Figure 2-6).

II. Fertilization

The sperm and egg cells formed through meiosis must combine to form a zygote, which can then develop into a new individual. A deeper understanding of gametes can help clarify the essence and significance of fertilization.

Diversity in Chromosome Combinations in Gametes

All spermatogonia within a father’s body have the same chromosome composition, as do all oogonia within a mother’s body. However, “a mother gives birth to nine children, each one different.” Can the same spermatogonia (or oogonia) produce different gametes?

In the somatic cells of organisms, the number of chromosomes is often quite large. For example, human somatic cells contain 23 pairs of chromosomes. So, how many types of gametes could a human produce when forming sperm or egg cells? If we also consider the exchange of segments between non-sister chromatids, the types of gametes produced through meiosis would be even more diverse. Think about what this might mean for the formation of biological diversity.

Fertilization

Fertilization is the process by which egg cells and sperm recognize each other and fuse to form a zygote. During fertilization, usually, only the head of the sperm enters the egg cell (Figure 2-7), leaving the tail outside. Simultaneously, the egg cell’s membrane undergoes complex physiological changes to prevent other sperm from entering. Shortly after the sperm head enters the egg cell, the sperm nucleus fuses with the egg cell nucleus, causing their chromosomes to unite. This restores the number of chromosomes in the zygote to the same number found in somatic cells, ensuring the stability of the species’ chromosome number, with half of the chromosomes coming from the sperm (father) and the other half from the egg cell (mother).

Before fertilization, the egg cell’s respiration and material synthesis processes are relatively slow. The fertilization process activates the egg cell, making it highly active. Subsequently, the zygote rapidly undergoes cell division and differentiation, marking the beginning of a new life, in which genetic material interacts with the environment to drive development.

Meiosis and fertilization ensure that the chromosome number of an organism remains constant across generations, maintaining the stability of genetic inheritance. Additionally, through sexual reproduction, a new generation inherits genetic material from both parents, whereas asexual reproduction only inherits from one parent. During sexual reproduction, the diverse chromosome combinations in the gametes formed through meiosis, coupled with the randomness of egg and sperm fusion during fertilization, ensure that offspring from the same parents exhibit diversity. This diversity helps organisms adapt to changing natural environments and facilitates evolution through natural selection, demonstrating the advantages of sexual reproduction. Therefore, meiosis and fertilization are crucial for biological inheritance and variation.