Plastids (chloroplasts) possess their own genetic information and consequently, express heritable traits. The plastid genome (plastome) occurs at high copy numbers, with up to thousands of genome copies being present in a single cell. Although mapping as a single circular molecule, the plastid DNA shows great structural dynamics. Multiple copies of the plastome are packed together in large nucleoprotein bodies, referred to as plastid nucleoids. The plastomes of land plants harbor a rather conserved set of approximately 100-120 genes in a genome of 120160 kilobase pairs (kb). In contrast, size and coding capacity of plastomes in algae are much more variable. In most plant species, plastids and their genetic information are inherited maternally and thus excluded from sexual recombination. The cytological mechanisms leading to uniparentally maternal inheritance are surprisingly diverse and can involve organelle exclusion by unequal cell division, plastid destruction or selective degradation of the plastid DNA from the paternal parent. Exceptions from maternal inheritance, i.e., biparental or paternal plastid transmission, have arisen multiple times during evolution.
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