Genetics with the Fundamentals of Breeding - M.P. Myhun - 2008

CHAPTER II. Material Foundations and Molecular Mechanisms of Heredity

2.3. Asexual and Sexual Reproduction and Their Cytological Basis

All types of reproduction in unicellular and multicellular

organisms are based on a single universal process: Cell Division. Both asexual and sexual reproduction are inherent in living nature, and some organisms employ both modes of reproduction.

Asexual reproduction: a new Organism arises from a single or

several Cells (spores, vegetative parts). Under such conditions, all offspring are genetically identical to one another and to the maternal form. This involves the division of somatic cells, which in living nature occurs primarily through mitosis.

Mitosis is an indirect or equational division of somatic cells in which the chromosome number in the daughter cells remains the same as in the parent cell. This is the most common mode of division in animal and plant cells. As a deviation from normal mitosis, endomitosis sometimes occurs (Chromosomes double, but do not segregate, resulting in polyploid cells). In addition, some specialized cells divide by amitosis. This is direct division, where a cell splits into two halves, each functioning as a daughter cell. Amitosis also serves as the method of division for Prokaryotic Cells.

The genetic significance of mitosis lies in the uniform distribution of hereditary information between newly formed cells. The transmission of Genetic information is achieved through the autoreplication of each chromosome, which doubles to produce two identical chromatids. During mitosis, these chromatids separate into the daughter cells, thereby ensuring The transfer of identical hereditary factors (Fig. 2.1).

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Figure 2.1

Diagram of mitosis in a hypothetical cell with two pairs of homologous chromosomes

A characteristic property of mitotic cell division is that the parent cell, upon division, produces two daughter cells with fully identical genotypes; consequently, the entire population of cells is characterized by complete genotypic identity.

Sexual reproduction: a new organism arises from a zygote formed by the fusion of two haploid cells (Gametes). Although offspring resemble one another, they are genetically non-identical and exhibit a wide spectrum of Variability. This variability arises as early as The formation of gametes (Germ Cells). Gamete formation involves a specialized type of cell division known as Meiosis, which halves the chromosome Complement. In addition to reducing the chromosome number by half, meiosis involves a crucial phenomenon: the exchange of segments between homologous chromosomes (Crossing Over), which gives rise to one of the forms of hereditary variability known as combinatorial variability (Fig. 2.2).

Figure 2.2

Diagram of meiosis in a hypothetical generative cell with two pairs of chromosomes

The genetic significance of meiotic division boils down to the following key points:

1. Meiosis generates chromosomes with a novel genetic composition through crossing over.

2. Meiosis ensures the genetic diversity of gametes via the random reassortment of maternal and paternal chromosomes.

3. Meiosis acts as the mechanism that maintains a constant species-specific chromosome number.

A characteristic feature of meiotic cell division is that a single initial diploid cell gives rise to four haploid cells that differ genotypically from the parent cell and from one another (owing to crossing over). The fusion of such gametes during Fertilization produces organisms with distinct genotypes.

Meiosis, with its stages and phases of germ cell development, represents only a stage of sexual reproduction. The process by which mature germ cells are formed is called gametogenesis. Although the formation of female and male gametes in animals and plants exhibits various differences and specific details, they share a common underlying pattern (Fig. 2.3; Fig. 2.4).

Figure 2.3

Diagram of the gametogenesis process in animals and humans

During the maturation of pollen grains, following meiotic division, a tetrad of four haploid microspores is formed, which subsequently breaks apart into individual haploid cells. Each microspore then undergoes two successive mitotic divisions to produce a mature pollen grain.

During the maturation of the embryo sac in plants, meiotic division yields a tetrad of haploid megaspores, of which only one develops while the other three degenerate. The Nucleus of the surviving megaspore undergoes three mitotic divisions to form the female gametophyte (the embryo sac).

The fusion of a male gamete with a female gamete, culminating in the union of their nuclei and the formation of a zygote, is termed fertilization.

Figure 2.4

Diagram of male and female gametophyte development in flowering plants

Monospermic fertilization – only a single sperm penetrates the egg cell and effects fertilization.

Polyspermic type – several sperm penetrate the egg cell, but only one achieves fertilization.

Double fertilization is characteristic of higher plants.

Other modes of sexual reproduction also occur:

- apomixis – the embryo develops from an unfertilized egg cell (in plants, reproduction via seeds without fertilization);

- parthenogenesis – The Development of an embryo from an unfertilized egg cell in lower animals (such as aphids, bees, etc.);

- androgenesis – the development of an embryo from an egg whose own nucleus degenerates prior to fertilization, leaving the nuclear material solely contributed by the penetrating spermatozoa.