As discussed in Chapter 12, it is thought that eukaryotic cells originated through a symbiotic relationship between an archaeon and an aerobic bacterium (a proteobacterium). The two organisms are postulated to have merged to form the ancestor of all nucleated cells, with the archeaon providing the nucleus and the proteobacterium serving as a respiring, ATP-producing endosymbiont—one that would eventually evolve into the mitochondrion (see Figure 12–3). This most likely occurred roughly 1.6 billion years ago, when oxygen had entered the atmosphere in substantial amounts (see Figure 14–56). The chloroplast was derived later, after the plant and animal lineages diverged, through endocytosis of an oxygen-producing cyanobacterium.
This endosymbiont hypothesis of organelle development receives strong support from the observation that the genetic systems of mitochondria and chloroplasts are similar to those of present-day bacteria. For example, chloroplast ribosomes are very similar to bacterial ribosomes, both in their structure and in their sensitivity to various antibiotics (such as chloramphenicol, streptomycin, erythromycin, and tetracyclin). In addition, protein synthesis in chloroplasts starts with N-formylmethionine, as in bacteria, and not with methionine as in the cytosol of eukaryotic cells. Although mitochondrial genetic systems are much less similar to those of present-day bacteria than are the genetic systems of chloroplasts, their ribosomes are also sensitive to antibacterial antibiotics, and protein synthesis in mitochondria also starts with N-formylmethionine.
Figure 14–59 Staining of nuclear and mitochondrial DNA. In this confocal micrograph of a human fibroblast, the nuclear DNa is stained with the dye DapI (blue) and mitochondrial DNa is visualized with fluorescent antibodies that bind DNa (green). the mitochondria are stained with fluorescent antibodies that recognize a specialized protein translocase specific to the outer mitochondrial membrane (red). Numerous copies of the mitochondrial genome are distributed in distinct nucleoids throughout the mitochondria that snake through the cytoplasm. (From C. Kukat et al., Proc. Natl Acad. Sci. USA 108:13534–13539, 2011. With permission from the National academy of Sciences.) 5 ˜m
The processes of organelle DNA transcription, protein synthesis, and DNA replication take place where the genome is located: in the matrix of mitochondria or the stroma of chloroplasts. Although the enzymes that mediate these genetic processes are unique to the organelle, and resemble those of bacteria (or even of bacterial viruses) rather than their eukaryotic analogs, the nuclear genome encodes the vast majority of these enzymes. Indeed, most present-day mitochondrial and chloroplast proteins are encoded by genes that reside in the cell nucleus.
Over time, Mitochondria and Chloroplasts have Exported Most of their Genes to the Nucleus by Gene transfer