Chloroplast genome organisation

Chloroplast genome organisation

Chloroplast DNA is also circular and present in multiple copies. Early studies of chloroplast DNA utilized isodensity centrifugation as the main tool for its isolation. Although this approach works well with unicellular plants in higher plants most of the isolated DNA components initially believed to be chloroplast DNA turned out not to be chloroplast DNA  at all. These early experiments, in which DNA prepared from isolated chloroplast was analyzed by cesium chloride isodensity centrifugation, revealed the presence of three DNA components: a major component with a density of about 1.696g/cm that was thought to represent contaminating nuclear DNA, and two minor DNA components denser than nuclear DNA that were thought to represent chloroplast DNA.

 But in 1971 the chloroplast DNA of the alga Euglena was isolated as a single large circle, suggesting that the linear typical plant leaf cell contains about 10,000 chloroplast DNA circles distributed among 50 to 100 chloroplasts, giving each chloroplast between 100 to 200 DNA molecules. Depending on the organism. chloroplast DNA contains anywhere from 70,000 to more than 5,00,000 base pairs, with an average of 1,50,000 base pairs being typical for the chloroplasts of higher plants the presence of DNA in chloroplasts is not indisputable evidence that this DNA contains genes governing chloroplast traits. Independent support for the existence of circular DNA in chloroplasts has come from the genetic studies of Ruth Sagar, who employed the antibiotic streptomycin to induce mutations in chloroplast genes of the green alga Chlamydomonas.