text: Secondary endosymbiosis results in the engulfment of an organism that has already performed primary endosymbiosis. Thus, four plasma membranes are formed. The first originating from the cyanobacteria, the second from the eukaryote that engulfed the cyanobacteria, and the third from the eukaryote who engulfed the primary endosymbiotic eukaryote. Chloroplasts contain 16S rRNA and 23S rRNA. 16S and 23S rRNA is found only in prokaryotes by definition. Chloroplasts and mitochondria also replicate semi-autonomously outside of the cell cycle replication system via binary fission. Consistent with the theory, decreased genome size within the organelle and gene integration into the nucleus occurred. Chloroplasts genomes encode 50-200 proteins, compared to the thousands in cyanobacterium. Furthermore, in Arabidopsis, nearly 20% of the nuclear genome originate from cyanobacterium, the highly recognized origin of chloroplasts. Recent studies have been able to identify the speed and size at which chloroplast genes are able to incorporate themselves into the host genome. Using chloroplast transformation genes encoding spectinomycin and kanamycin resistance were inserted into the DNA of chloroplasts found in tobacco plants. After subjecting the plants to  spectinomycin and kanamycin selection, some plants began to tolerate spectinomycin and kanamycin. Roughly 1 in every 5 million cells on the tobacco leaves highly expressed  spectinomycin and kanamycin resistant genes. By using the cells expressing resistances, they were able to grow tobacco from these cell to maturity. Once mature, the plants were mated with wild-type plants, and 50% of the progeny expressed  spectinomycin and kanamycin resistance genes. Pollen was thought not to be able to transfer chloroplast DNA in tobacco (which later turned out not to be as true as was thought at the time), thus leading to believe that the genes were incorporated into the tobaccos genome. Furthermore, 11kb of integrated chloroplast DNA was introduced to the host genome, transferring more DNA that previously predicted at a faster rate than previously predicted.