Sexual Reproduction

Sexual Reproduction

Most animals and plants reproduce sexually. In humans, gametes of opposite sex unite to form a cell that, dividing repeatedly by mitosis, eventually gives rise to an adult body with some 100 trillion cells. The gametes that form the initial cell are the products of a special form of cell division called meiosis. Meiosis is far more intricate than mitosis, and the details behind it are not as well understood. The basic process,however, is clear. Also clear are the profound consequences of sexual reproduction: It plays a key role in generating the tremendous genetic diversity that is the raw material of evolution.

The essence of sexual reproduction is the merging of the genetic contribution of two cells from different individuals. This mode of reproduction results in evolutionary advantages that biologists have long recognized. However, we are only recently making progress on understanding the underlying mechanism that produces the elaborate behavior of chromosomes that occurs during meiosis, the process that underlies sexual reproduction. To begin, we briefly consider the history of meiosis and its relationship to sexual reproduction.

Fertilization in Sexual Reproduction 

Only a few years after Walther Flemming’s discovery of chromosomes in 1879, Belgian cytologist Edouard van Beneden was surprised to find different numbers of chromosomes in different types of cells in the roundworm Ascaris. Specifically, he observed that the gametes (eggs and sperm) each contained two chromosomes, but all of the nonreproductive cells, or somatic cells, of embryos and mature individuals each contained four.

From his observations, van Beneden proposed in 1883 that an egg and a sperm, each containing half the complement of chromosomes found in other cells, fuse to produce a single cell called a zygote. The zygote, like all of the cells ultimately derived from it, contains two copies of each chromosome. The fusion of gametes to form a new cell is called fertilization, or syngamy.

It was clear even to early investigators that gamete formation must involve some mechanism that reduces the number of chromosomes to half the number found in other cells. If it did not, the chromosome number would double with each fertilization, and after only a few generations the number of chromosomes in each cell would become impossibly large. For example, in just 10 generations, the 46 chromosomes present in human cells would increase to over 47,000 (46 × 2^10).

The number of chromosomes does not explode in this way because of a special reduction division, meiosis. Meiosis occurs during gamete formation, producing haploid cells—cells with half the normal number of chromosomes. The subsequent fusion of two of these cells to form a diploid cell—a cell with twice as many chromosomes as haploid cells—ensures a consistent chromosome number from one generation to the next. This reduction division process, the subject of this chapter, lies at the heart of sexual reproduction.

Fusion of Two Haploid Cells (Sperm and Egg)

Meiosis and fertilization together constitute a cycle of reproduction. Below Figure illustrates how two haploid cells, a sperm cell containing three chromosomes contributed by the father and an egg cell containing three chromosomes contributed by the mother, fuse to form a diploid zygote with six chromosomes.

Figure:Diploid cells carry chromosomes from two parents. A diploid cell contains two versions of each chromosome, a maternal homologue contributed by the haploid egg of the mother, and a paternal homologue contributed by the haploid sperm of the father.

Reproduction that involves this alternation of meiosis and fertilization is called sexual reproduction. Its outstanding characteristic is that offspring inherit chromosomes from two parents, as you saw in above figure. You, for example, inherited 23 chromosomes from your mother (maternal homologue) and 23 from your father (paternal homologue).
The life cycles of all sexually reproducing organisms follow a pattern of alternation between diploid and haploid chromosome numbers, but there is variation in the pattern’s timing.

Many types of algae, for example, spend the majority of their life cycle in a haploid state. Most plants and some algae alternate
between a multicellular haploid phase and a multicellular diploid phase. In most animals, by contrast, the diploid state dominates. The zygote first undergoes mitosis to produce diploid cells. Then, later in the life cycle, some of these diploid cells undergo meiosis to produce haploid gametes (see figure)

Figure:The sexual life cycle in animals. In animals, the zygote undergoes mitotic divisions and gives rise to all the cells of the adult body. Germ-line cells are set aside early in development and undergo meiosis to form the haploid gametes (eggs or sperm). The rest of the body cells are called somatic cells.

Somatic and Germ Cells

In animals, the single diploid zygote undergoes mitosis to give rise to somatic cells that form all of the cells in the adult body. 
The cells that will eventually undergo meiosis to produce gametes are set aside from somatic cells early in the course of development. These cells are referred to as germ-line cells.  Both somatic cells and germ-line cells are diploid, but somatic cells undergo mitosis to form genetically identical, diploid daughter cells, and germ-line cells undergo meiosis to produce haploid gametes (see figure above).
Some organisms do not reproduce sexually and never produce gametes. Reproduction in these organisms is referred to as asexual reproduction. The cell division of yeasts  is an example of asexual reproduction, and some plants can reproduce asexually.

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Meiosis