器官分化ㆍ進化ㆍ育種에 있어서 genome의 變化

Genomic changes in the organ differentiation evolution and breeding

The age-old concept of classical genetics on the genetic information has been that gene loci which are arranged linearly on the chromosomes are fixed and genome is stable and static. By contrast, in the new concept the genome is regarded to be dynamic, changing, in certain cases, abruptly or in response to the environment. Some of the example of genomic changes are the rearrangement of immunoglobulin genes, amplification of rRNA genes in the animal oocytes, and the presence of transposable elements. In the course of evolution genomes have changed from small to large and from simple to complex. In the differentiation of tissues and organs during the ontogeny the undifferentiated zygotic nucleus becomes differentiated and changes in complexity. The major mechanisms bringing about these changes in evolution and ontogeny are gene duplication by unequal crossing over, rapid amplification of nongenic repetitive DNA by rolling circle model, and movement from place to place of DNA segments by the specific structural organization of transposable elements. The genetic variations, sources for the evolution and breeding, are also changes in the nuclear genomes but different entirely in the mechanisms from the genetic changes mentioned above. They are the products of meiotic divisions which bring about the genetic diversities in the reproductive cells. The mechanisms resulting in these changes are (1) recombinations of chromosomes between the parental genomes, (2) gene recombinations by intergenic crossing over at the nongenic region of DNA, and (3) recombinations of exons by intragenic crossing over at the intron regions of the genes.