103 3.5 Organelle chromosomes plast chromosomes per cell can number in the hundreds or thousands, and the number can even vary somewhat from cell to cell. The DNA is packaged into suborganel-lar structures called nucleoids , which become visible if stained with a DNA-binding dye (Figure 3-40). The DNA is folded within the nucleoid, but does not have the type of histone-associated coiling shown by nuclear chromosomes. The same arrangement is true for mitochondria. Many organelle chromosomes have now been se-quenced. Some examples of relative gene size and spac-ing in mitochondrial DNA (mtDNA) and chloroplast DNA (cpDNA) are shown in Figure 3-41. Organelle genes are very closely spaced, and in some organisms genes can contain introns. Note that most genes are con-cerned with the chemical reactions going on within the organelle itself; photosynthesis in chloroplasts, and oxidative phosphorylation in mitochondria. However, organelle chromosomes are not self-sufﬁcient: many proteins that act within the organelle are encoded by nuclear genes (see Figure 1-9). Cytoplasmic organelle chromosomes are embraced
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