Plastid DNA (ptDNA), also known as chloroplast DNA (cpDNA or ctDNA) in photosynthetic organisms, is the DNA located in chloroplasts, which are photosynthetic organelles located within the cells of some eukaryotic organisms, as well as some reduced plastids, such as apicoplasts. Chloroplasts, like other types of plastid, contain a genome separate from that in the cell nucleus. The existence of chloroplast DNA was identified biochemically in 1959, and confirmed by electron microscopy in 1962. The discoveries that the chloroplast contains ribosomes and performs protein synthesis revealed that the chloroplast is genetically semi-autonomous. The first complete chloroplast genome sequences were published in 1986, Nicotiana tabacum (tobacco) by Sugiura and colleagues and Marchantia polymorpha (liverwort) by Ozeki et al. Since then, tens of thousands of chloroplast genomes from various species have been sequenced.
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Chloroplast DNA in the context of Genome
A genome is all the genetic information of an organism or cell. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as regulatory sequences (see non-coding DNA), and often a substantial fraction of junk DNA with no evident function. Almost all eukaryotes have mitochondria and a small mitochondrial genome. Algae and plants also contain chloroplasts with a chloroplast genome.
The study of the genome is called genomics. The genomes of many organisms have been sequenced and various regions have been annotated. The first genome to be sequenced was that of the virus φX174 in 1977; the first genome sequence of a prokaryote (Haemophilus influenzae) was published in 1995; the yeast (Saccharomyces cerevisiae) genome was the first eukaryotic genome to be sequenced in 1996. The Human Genome Project was started in October 1990, and the first draft sequences of the human genome were reported in February 2001.
Chloroplast DNA in the context of Circular chromosome
A circular chromosome is a chromosome in bacteria, archaea, mitochondria, and chloroplasts, in the form of a molecule of circular DNA, unlike the linear chromosome of most eukaryotes.
Most prokaryote chromosomes contain a circular DNA molecule. This has the major advantage of having no free ends (telomeres) to the DNA. By contrast, most eukaryotes have linear DNA requiring elaborate mechanisms to maintain the stability of the telomeres and replicate the DNA. However, a circular chromosome has the disadvantage that after replication, the two progeny circular chromosomes can remain interlinked or tangled, and they must be extricated so that each cell inherits one complete copy of the chromosome during cell division.