Structure and Organisation of Chloroplast Genome
The chloroplast genomes of vascular plants and most algae are quite similar. In general structure and organization, especially by comparison to the wholesale variation seen in the nuclear and mitochondrial genomes. With one Possible exception, all known chloroplast genomes are circular DNA molecules. Size variation is greatest among green algae in which most chloroplast genomes range between about 85 and 300 kb.

A number of genes have been located on the circle and one of the important features is the presence of two copies of the ribosomal DNA sequences. These sequences are often but not always-present on a large inverted repeat. Other genes mapped include those for the large subunit of RuBP. Case, tRNAs, subunits of ATP synthase, and cytochrome.

Most of this size variation can be accounted for by the presence or absence of a portion of the plastid genome which has been duplicated and is present in an inverted orientation in the plastid DNA molecule. The location of this inverted repeat is relatively fixed with respect to other genes and it separates a small single-copy region from a large single copy DNA region.

For example, psaA and psaB encode polypeptides which are tightly associated in the reaction centre of PSI. Other genes, such as rbcL and some gene encoding tRNAs, are not part of polycistronic transcription units. While the plastid gene content of higher plants is very constant, variation in gene order is evident, which results primarily from DNA inversion. DNA.

Inversions have reshuffled plastid genomes such that distances between genes, and relative orientation of transcription units, vary considerably in genomes of higher plants. For example, rps16 is proximal to trnk in barley, whereas in pea rbcL occupies this position. The greatest variation in gene order is found in peas (at least 12 rearrangements), perhaps due to lack of an inverted repeat in this plasmid genome which might stabilize the genome.