Melosis - Sexual Reproduction
Table of contents
Abstract 3
Introduction 4
Sexual reproduction in fungi 4
Sexual reproduction in plants and algae 5
Sexual reproduction in animals 5
Sexual reproduction in bacteria and archea 6
Conclusion 6
References 7
Abstract
Sexual reproduction is a dominant feature of life on planet earth. For sexual reproduction to occur, fusion of two specialized reproductive cells called gametes needs to take place. The process of cell division leading to the formation of gametes is essential for sexual reproduction. Despite the importance of sexual reproduction for diversifying of genes of live organisms as they pass them along to progeny, there are exceptions to that rule: some species can reproduce asexually not requiring meiosis to take place, others undergo meiosis but do not necessary require fertilization. In this paper, meiosis and sexual reproduction in plants, animals, algae and other microbes is shortly reviewed.
Introduction
Meiosis is the process of cell division along which haploid cells called sex cells or gametes, sperm in males and eggs in females, are formed. It occurs in all eukaryotes, including unicellular (protists, algae, fungi) and multicellular (mammals and plants) organisms, reproducing sexually. Sexual reproduction contributes to the genetic variation of populations mainly by three means: crossing-over, which is the shuffling of genes, independent assortment of homologous chromosomes and different combinations of gametes in fertilization. All of these processes occur arbitrarily; most importantly, the first two take place during meiosis.
Meiosis is preceded by a preparatory stage called interphase, in which the chromosomes are replicated to produce sister chromatids. Meiosis is divided into meiosis I and meiosis II. The first and longest phase of meiosis is prophase 1; in this phase, the nucleoli and the nuclear membrane disappear, and homologous recombinantion occurs leading often in chromosomal crossover. Prophase I consists of five stages: leptotene, zygotene, pachytene, diplotene and diakinesis. In prometaphase, the chromosomes are aligned along the equator of the cell. In metaphase I, the chromosomes form the metaphase plate. Followed by metaphase I is anaphase I; in this stage, the chromatids are pulled apart in opposite directions. In the telophase, a nuclear membrane and nucleoli are formed, the chromosomes are decondensed and cell membrane is formed in animals and cell wall in plants. Meiosis II consists of the same stages as in meiosis I and end up with four haploid daughter cells (Cooper, 2000).
Sexual reproduction in fungi
Fungi possess a huge variety or reproductive strategies. Species in the phylum Ascomycota can reproduce asexually through budding, fission or asexual sporulation. Despite the fact that they are microbes, they have different means of recombining or reproducing sexually. The ratio of sexual to mitotic generations can vary significantly between species of this phylum. In the budding yeast S.paradoxus, sex occurs only once in 1000 asexual cell divisions. As for the pathogenic Sclerotinia species, sex occurs in every generation. Moreover, molecular data suggests that there are very few truly asexual fungi species. Life cycle of Ascomycota species consists of yeast and filamentous fungi. …