Endosimbiosis

Endosymbiosis Theory By Qing-jun Wang Most eukaryotic cells contains a variety of organelles, including mitochondria, chloroplasts for plants and algae, nucleus, and et al. The bulk of genome if aneukaryotic organism is contained in the chromosomes in the cell nucleus while a much smaller portion is located in the mitochondria and chloroplasts. Non-photosynthetic eukarya do not havechloroplasts. Some early-branching eukarya, such as Giardia lamblia, have neither mitochondria nor chloroplasts. They are all anaerobic. These facts suggest that mitochondria and chloroplasts do not appear to beintrinsic components of eukarya cells. Rather, they could be integrated into the cells of eukarya at a later evolutionary stage. In addition, mitochondria and chloroplasts resemble simple bacteriafrom their appearances to the circular forms of their DNA. Therefore, a hypothesis concerning the origin of mitochondria and chloroplasts was raised that eukaryotic organelles such as mitochondria andchloroplasts were of bacterial origins. This hypothesis, known as endosymbiosis theory, can be best tested by molecular phologenetics. The idea is as follows. The sequences of a relatively conservedgene from nuclear, mitochondrial and chloroplast genomes of representative organisms of bacteria, archaea and eukarya are aligned and a universal phylogenetic tree is constructed. If the mitochondria andchloroplasts are originated from bacteria, their genes should branch from within the bacteria domain. If the mitochondria and chloroplasts are originated independently within the eukarya domain, thentheir genes should branch from within the eukarya domain. As examples, the phylogenetic trees of small-subunit rRNA, cytochrome c, cytochrome c553 and ferredoxin were built (Figures 1, 2, 3 and 4,respectively). Figure 1 (Taken from Evolutionary Analysis, 2001, Freeman & Herron) shows that both Zea Mays mitochondrial small subunit rRNA and chloroplast small subunit rRNA are branched from within...