D-loop replication is a proposed process by which circular DNA like chloroplasts and mitochondria replicate their genetic material. An important component of understanding D-loop replication is that many chloroplasts and mitochondria have a single circular chromosome like bacteria instead of the linear chromosomes found in eukaryotes. However, many chloroplasts and mitochondria have a linear chromosome, and D-loop replication is not important in these organelles. Also, not all circular genomes use D-loop replication as the process of replicating its genome. In many organisms, one strand of DNA in the plasmid comprises heavier nucleotides (relatively more purines: adenine and guanine). This strand is called the H (heavy) strand. The L (light) strand comprises lighter nucleotides (pyrimidines: thymine and cytosine). Replication begins with replication of the heavy strand starting at the D-loop (also known as the control region). A D-loop is a short portion in circular DNA that has three strands instead of two. The middle strand, which is complementary to the light strand, displaces the heavy strand and forms a displacement loop (D-loop). Circular DNA is stable with this small D-loop and can remain in this formation, but the middle strand, or the displacing strand, is replaced frequently due to its short half-life, and is very energetically expensive to the cell. When diagramed, the resulting structure looks like the letter D. The D-loop was first discovered in 1971 when researchers noticed that many DNA in the mitochondria they were examining under microscope contained a short segment that was tripled stranded. Replication process Each D-loop contains an origin of replication for the heavy strand. Full circular DNA replication is initiated at that origin and replicates in only one direction. The middle strand in the D-loop can be removed and a new one will be synthesized that is not terminated until the heavy strand is fully replicated, or the middle strand can serve as a primer for the heavy strand replication. As the heavy strand replication reaches the origin of replication for the light strand, a new light strand will be synthesized in the opposite direction as the heavy strand. There is more than one proposed process through which D-loop replication occurs, but in all of the models, these steps are agreed upon. The portions not agreed upon are what is the importance of maintaining a D-loop when replication is not in progress, because it is energetically expensive to the cell, and what mechanisms,