<P> The inverted repeats vary wildly in length, ranging from 4,000 to 25,000 base pairs long each and containing as few as four or as many as over 150 genes . Inverted repeats in plants tend to be at the upper end of this range, each being 20,000--25,000 base pairs long . </P> <P> The inverted repeat regions are highly conserved among land plants, and accumulate few mutations . Similar inverted repeats exist in the genomes of cyanobacteria and the other two chloroplast lineages (glaucophyta and rhodophyceae), suggesting that they predate the chloroplast, though some chloroplast DNAs have since lost or flipped the inverted repeats (making them direct repeats). It is possible that the inverted repeats help stabilize the rest of the chloroplast genome, as chloroplast DNAs which have lost some of the inverted repeat segments tend to get rearranged more . </P> <P> New chloroplasts may contain up to 100 copies of their DNA, though the number of chloroplast DNA copies decreases to about 15--20 as the chloroplasts age . They are usually packed into nucleoids, which can contain several identical chloroplast DNA rings . Many nucleoids can be found in each chloroplast . In primitive red algae, the chloroplast DNA nucleoids are clustered in the center of the chloroplast, while in green plants and green algae, the nucleoids are dispersed throughout the stroma . </P> <P> Though chloroplast DNA is not associated with true histones, in red algae, similar proteins that tightly pack each chloroplast DNA ring into a nucleoid have been found . </P>

Where are chloroplasts located in a plant cell