Genetics - A. V. Sivolob 2008

Genetics of Multicellular Eukaryotes
Genetics of Individual Development

The Development of any multicellular Organism begins with The formation of a zygote, a Cell arising from the fusion of Gametes (an egg cell and a sperm cell). Following Fertilization, the sperm Nucleus (pronucleus) enters the egg Cytoplasm and fuses with its nucleus, thereby restoring the diploid chromosome set characteristic of the given species. In most organisms, immediately after fertilization, the zygote undergoes a series of mitotic divisions (known as zygotic Cleavage), resulting in A large number of small Cells called blastomeres. As they develop, each of these blastomeres (or a group of them) gives rise to functionally specialized cells with a unique Morphology and physiological profile. Thus, the fertilized egg is capable of generating various cell types, thereby driving development from a single cell into a multicellular organism. This property of the zygote is known as totipotency. It is also characteristic of the blastomeres formed During the first four cleavage divisions. With subsequent divisions, cellular totipotency decreases as each cell "selects" its developmental program, becoming determined. Such cells undergo stable internal changes (in both nuclear and cytoplasmic Structure) that distinguish them and their descendants from other embryonic cells and chart their future path of specialization. It should be noted that these changes do not initially cause significant morphological differences between various cell types. As a determined cell continues to develop, it acquires visible morphological distinctions and Functions specific to that particular cell type. Such a fully specialized cell is referred to as differentiated.

Cellular determination and differentiation are driven by the gradual shift in Gene activity during organismal development: certain groups of genes become actively expressed in some cells while entering a repressed state in others. This differential pattern of Gene Expression emerges even at Cytology/cytology/16.html">Early stages of zygotic development and is caused by the heterogeneity of the egg cytoplasm. Cytoplasmic heterogeneity refers to the uneven distribution within The Cell of cytoplasmic determinants—specific mRNA molecules and their protein products. Because these mRNAs and Proteins are synthesized and accumulated in the egg during oogenesis, the genes encoding these cytoplasmic determinants are called maternal-effect genes. Consequently, the nuclei of the blastomeres formed during zygotic cleavage find themselves in distinct cytoplasmic environments, which dictate selective gene METABOLISM/31.html">Transcription. By forming concentration gradients along various axes of the egg, cytoplasmic determinants establish the anteroposterior and dorsoventral axes of the embryo. During cleavage, the blastomeres located in these respective Regions of the embryo will give rise to the Organs and Tissues characteristic of those specific PARTS OF THE organism.

Gene products synthesized within specific groups of blastomeres induce even greater variations in the cytoplasmic environment of the cell nuclei, which in turn triggers the activation of other specific genes. This cascade of gene activation and inactivation is further amplified through interactions between neighboring cells, which can mutually influence each other's subsequent differentiation via cytoplasmic bridges or Gap Junctions.

The General Principles of genetic determination of development are similar across various organisms. Differential gene expression during development has been studied in greatest detail in Representatives of the genus Drosophila and the nematode Caenorhabditis elegans. The GENETIC ASPECTS OF Embryogenesis in these specific organisms are discussed below.