Physical properties of plastid genomes

The plastid genome maps as a circular molecule of double-stranded DNA (ptDNA). In land plants, the genome size is typically in the range of 120-160 kb (Fig. 1), although some exceptions have been noted (see 3.1 and 3.2.4). Identical copies of this genome are present in all plastid types: the undifferentiated proplas-tids of meristematic tissues, the green chloroplasts in photosynthesis-performing cells, the colored chromoplasts of flowers and fruits and other plastid types specialized in storage of starch, proteins, or lipids.

Chloroplast DNA can be extracted from isolated organelles (which are purified by gradient centrifugation; Jansen et al. 2005) and was found to have physical properties distinct from nuclear DNA. The distinguishing features include different buoyant density in CsCl gradients, different melting and renaturation behavior, different GC content and the absence of 5-methylcytosine from plastid DNA (Te-wari and Wildman 1966). In spite of its small genome size, plastid DNA can make up a significant fraction of the total cellular DNA which is due to its presence in high copy numbers. For tobacco leaves, it was estimated that about 9% of the total DNA is chloroplast DNA representing about 4.7 x 10-15 g DNA per chloroplast (Tewari and Wildman 1966).

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