BOTANY WITH THE BASICS OF HYDROBOTANY (AQUATIC PLANTS OF UKRAINE) - B.Ye. Yakubenko - 2011

V. CYTOLOGY. THE CONCEPT OF THE CELL

Ways of Formation of New Cells

Meiosis

An indirect Cell Division in which four daughter Cells are formed from a single parent cell, containing half the chromosome number of the parent cell, is called Meiosis. It consists of two successive divisions: heterotypic (or reductional) and homeotypic (or equational). Each of these divisions comprises four phases: prophase I, metaphase I, anaphase I, telophase I, prophase II, metaphase II, anaphase II, and telophase II. The first (reductional, heterotypic) division results in the segregation of homologous Chromosomes into different cells and a twofold reduction (reduction) in the chromosome number.

During the second (equational) division, sister chromatids separate into single-chromatid chromosomes in a manner similar to mitosis. Crucial processes of meiotic division take place in prophase I, which is therefore subdivided into five stages: leptonema, zygonema, pachynema, diplonema, and diakinesis. It is precisely during prophase I that Crossing-over occurs, disrupting linked Gene inheritance.

Meiosis is characteristic only of sexually reproducing plants. During Fertilization, the chromosome number is doubled, whereas during meiosis it is halved, thereby ensuring a constant chromosome number. Meiosis occurs exclusively in diploid or tetraploid cells and is impossible in triploid and haploid cells. It takes place only once, after which cells multiply repeatedly through mitotic division. During meiosis, chromosomes and their constituent genes are distributed randomly among the daughter cells.

In Algae, meiotic division undergoes in the zygote or reproductive Organs. In the first case, haploid cells are formed from the zygote, giving rise to haploid plants (zygotic meiosis); In the second case, meiosis occurs within the reproductive organs of a diploid plant, producing haploid Gametes (gametic meiosis).

In higher plants, meiosis takes place within sporangia (sporic meiosis). Diploid Cells of the sporogenous tissue develop into haploid spores As a result of meiosis. In gametangia, sex cells are formed via mitotic division during gametogenesis.