In animal, yeast, and fungal cells, DNA is present in two organelles, the nucleus and the mitochondria. In plant and algal cells, DNA is present in plastids (of which chloro-plasts are one example) as well as in mitochondria and the nucleus. Unlike the DNA in the nucleus, which is packaged into chromosomes, plastid DNA and mitochondrial DNA are circular and thus resemble the DNA in prokary-otes (e.g., bacteria).

Mitochondrial DNA is small and codes for relatively few mitochondrial proteins. Although mitochondria contain their own protein synthesis machinery, the majority of the hundreds of mitochondrial proteins are coded for by nuclear genes. These proteins are synthesized in the cytoplasm and imported into the mitochondria. Plastid DNA is somewhat larger than that of the mitochondrion and contains the genetic information for more chloroplast proteins. However, as is the case for mitochondria, most of the proteins in a chloroplast are coded by nuclear genes and are synthesized in the cytoplasm. Proteins destined for mitochondria and chloroplasts have an extension on their N-terminal end that targets the proteins to the correct organelle and to the correct place within the organelle. These extensions, which, like the remainder of the proteins, are composed of amino acids, are usually cleaved off as the proteins find their proper place within the or-ganelle. Remarkably, some proteins composed of more than one polypeptide may contain a polypeptide coded for by nuclear DNA and synthesized in the cytoplasm and another polypeptide that is coded for by mitochondrial or chloroplast DNA. Ribulose 1,5-bisphosphate carboxy-lase/oxygenase is a prominent example of such a protein in chloroplasts.

The discovery that mitochondria and chloroplasts contain DNA, coupled with a wealth of sequence information about both DNA and proteins, added credence to the notion that these organelles arose from the engulfment of unicellular organisms by a primitive nucleated cell. Mitochondria may have been derived from a bacterium, and chloroplasts, from a unicellular alga. After the engulfment events, genes in the bacterium and alga coding for proteins that duplicated those in the nuclear genomes of the hosts were lost and other genes were transferred from the bacterial and algal genomes to the genomes of the hosts.

The distribution of proteins and lipids within biological membranes is asymmetric. Thus, one side of a membrane is distinct from the other. The coupling membranes of mitochondria and chloroplasts are opposite to each other. Protons are ejected from mitochondria during respiratory electron transport but are taken up by thylakoids during light-driven electron transport. The catalytic portion of the ATP synthase is located on the outside of the thylakoid membranes, whereas that of the mitochondrial ATP syn-thase is present on the inside of the inner membrane. As seen in Fig. 7, the orientation of the coupling membranes of mitochondria and chloroplasts is consistent with the hypothesis that these organelles are of bacterial and algal origin.

Each membrane in a cell has its distinct set of proteins and lipids. The most common membrane lipids are phospholipids. Phospholipids arediglycerides. Two of the three hydroxyls of glycerol are linked to long-chain fatty acids by ester bonds. The third position is occupied by phosphate. A number of different polar substituents are linked to the phosphate by anhydride bonds. The phospholipid composition of the mitochondrial inner membrane is virtually the same in plant mitochondria as in animal mitochondria and resembles that in the plasma membrane of some bacteria. The lipids in chloroplast membranes are very distinctive. The phospholipid content is unusually low and about 80% of the membrane lipids in thylakoids are diglycerides that have one or two galactose (a six-carbon sugar) on the third position of the glycerol. Galac-tosyldiglycerides are absent in the membranes of animal, yeasts, and fungi but are present in the photosynthetic membranes of all organisms that carry out oxygenic photosynthesis. The lipid compositions of mitochondrial and chloroplast membranes are consistent with the engulfment hypothesis for the origin of these organelles.

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