CELL DIVISION: MEIOSIS AND SEXUAL REPRODUCTION

Organisms that reproduce sexually are generally diploid: each cell contains two sets of chromosomes—a maternal chromosome set and a paternal chromosome set—one inherited from each parent. Google Images

the germ cells, the germ cells, or gametes—are haploid; that is, they each contain only one set of chromosomes. Typically, two types of gametes are produced.

Sexual reproduction involves both haploid and diploid cells Sexual reproduction involves both haploid and diploid cells. (a) Cells proliferate in the diploid phase to form a multicellular organism; haploid gametes—the egg and the sperm—are formed by meiosis. these gametes reunite at fertilization to generate a diploid zygote, which will develop into a diploid organism that is genetically distinct from either parent. In the multicellular organism, the germ-line cells (dark color) are the precursor cells that give rise to gametes; the somatic cells (pale color) are the other cells of the body. (B) the same process is diagrammed with a view of the chromosomes involved.

The haploid cells do not divide and are highly specialized for their function as gametes; they have to be freshly generated from diploid precursor cells within the organism. This precursor cell lineage, dedicated to the production of germ cells, is called the germ line. The mixing of genomes that characterizes sexual reproduction is achieved by fusion of two haploid cells to form a diploid cell.

Haploid Germ Cells Are Produced From Diploid Cells Through Meiosis

During fertilization, two gametes will unite, generating a diploid zygote that is genetically distinct from each of its parents .The zygote then develops into a multicellular organism through repeated rounds of cell division and cell specialization.

In mitosis,the replicated chromosomes line up in random order at the metaphase plate; as mitosis continues, the two previously joined sister chromatids then separate from each other to become individual chromosomes, and the two daughter cells produced by cytokinesis inherit one copy of each paternal chromosome and one copy of each maternal chromosome. Thus, both sets of genetic information are transmitted intact to both daughter cells, which are, therefore, diploid and genetically identical.

Meiosis phases Two successive nuclear divisions occur, Meiosis I (Reduction) and Meiosis II (Division). The old name for meiosis was reduction/ division. Meiosis I reduces the ploidy level from 2n to n (reduction) while Meiosis II divides the remaining set of chromosomes in a mitosis-like process (division). Most of the differences between the processes occur during Meiosis I

Prophase I has a unique event -- the pairing of homologous chromosomes. Synapsis is the process of linking of the replicated homologous chromosomes. The resulting chromosome is termed a tetrad, being composed of two chromatids from each chromosome, forming a thick (4-strand) structure. The occurrence of a crossing-over is indicated by a special structure, a chiasma During crossing-over chromatids break and may be reattached to a different homologous chromosome.