Unit two. The Living Cell
4.7. Organelles That Contain DNA
Eukaryotic cells contain several kinds of complex, cell-like organelles that contain their own DNA and appear to have been derived from ancient bacteria assimilated by ancestral eukaryotes in the distant past. The two principal kinds are mitochondria (which occur in the cells of all but a very few eukaryotes) and chloroplasts (which do not occur in animal or fungi cells—they occur only in algae and plants).
Mitochondria: Powerhouses of the Cell
Eukaryotic organisms extract energy from organic molecules (“food”) in a complex series of chemical reactions called oxidative metabolism, which takes place only in their mitochondria. Mitochondria (singular, mitochondrion) are sausage-shaped organelles about the size of a bacterial cell. Mitochondria are bounded by two membranes. The outer membrane, shown partially cut away in figure 4.9, is smooth and apparently derives from the plasma membrane of the host cell that first took up the bacterium long ago. The inner membrane, apparently the plasma membrane of the bacterium that gave rise to the mitochondrion, is bent into numerous folds called cristae (singular, crista) that resemble the folded plasma membranes in various groups of bacteria. The cutaway view of the figure shows how the cristae partition the mitochondrion into two compartments, an inner matrix and an outer compartment, called the intermembrane space. As you will learn in chapter 7, this architecture is critical to successfully carrying out oxidative metabolism.
Figure 4.9. Mitochondria.
The mitochondria of a cell are sausage-shaped organelles within which oxidative metabolism takes place, and energy is extracted from food using oxygen. (a) A mitochondrion has a double membrane. The inner membrane is shaped into folds called cristae. The space within the cristae is called the matrix. The cristae greatly increase the surface area for oxidative metabolism. (b) Micrograph of two mitochondria, one in cross section, the other cut lengthwise.
During the 1.5 billion years in which mitochondria have existed in eukaryotic cells, most of their genes have been transferred to the chromosomes of the host cells. But mitochondria still have some of their original genes, contained in a circular, closed, naked molecule of DNA (called mitochondrial DNA, or mtDNA) that closely resembles the circular DNA molecule of a bacterium. On this mtDNA are several genes that produce some of the proteins essential for oxidative metabolism. In both mitochondria and bacteria, the circular DNA molecule is replicated during the process of division. When a mitochondrion divides, it copies its DNA located in the matrix and splits into two by simple fission, dividing much as bacteria do.
Chloroplasts: Energy-Capturing Centers
All photosynthesis in plants and algae takes place within another bacteria-like organelle, the chloroplast (figure 4.10). There is strong evidence that chloroplasts, like mitochondria, were derived from bacteria by symbiosis. A chloroplast is bounded, like a mitochondrion, by two membranes, the inner derived from the original bacterium and the outer resembling the host cell’s ER. Chloroplasts are larger than mitochondria, and have a more complex organization. Inside the chloro- plast, another series of membranes are fused to form stacks of closed vesicles called thylakoids, the green disklike structures visible in the interior of the chloroplast in figure 4.10. The light-dependent reactions of photosynthesis take place within the thylakoids. The thylakoids are stacked on top of one another to form a column called a granum (plural, grana). The interior of a chloroplast is bathed with a semiliquid substance called the stroma.
Figure 4.10. A chloroplast.
Bacteria-like organelles called chloroplasts are the sites of photosynthesis in photosynthetic eukaryotes. Like mitochondria, they have a complex system of internal membranes on which chemical reactions take place. The internal membranes of a chloroplast are fused to form stacks of closed vesicles called thylakoids. Photosynthesis occurs within these thylakoids. Thylakoids are stacked one on top of the other in columns called grana. The interior of the chloroplast is bathed in a semiliquid substance called the stroma.
Like mitochondria, chloroplasts have a circular DNA molecule. On this DNA are located many of the genes coding for the proteins necessary to carry out photosynthesis. Plant cells can contain from one to several hundred chloroplasts, depending on the species. Neither mitochondria nor chloroplasts can be grown in a cell-free culture; they are totally dependent on the cells within which they occur.
Symbiosis is a close relationship between organisms of different species that live together. The theory of endosymbiosis proposes that some of today’s eukaryotic organelles evolved by a symbiosis in which one cell of a prokaryotic species was engulfed by and lived inside the cell of another species of prokaryote that was a precursor to eukaryotes. Figure 4.11 shows how this is thought to have occurred. Many cells take up food or other substances through endocytosis, a process whereby the plasma membrane of a cell wraps around the substance, enclosing it within a vesicle inside the cell. Oftentimes, the contents in the vesicle are broken down with digestive enzymes. According to the endosymbiont theory this did not occur; instead, the engulfed prokaryotes provided their hosts with certain advantages associated with their special metabolic abilities. Two key eukaryotic organelles just described are believed to be the descendants of these endo- symbiotic prokaryotes: mitochondria, which are thought to have originated as bacteria capable of carrying out oxidative metabolism; and chloroplasts, which apparently arose from photosynthetic bacteria.
Figure 4.11. Endosymbiosis.
This figure shows how a double membrane may have been created during the symbiotic origin of mitochondria or chloroplasts.
The endosymbiont theory is supported by a wealth of evidence. Both mitochondria and chloroplasts are surrounded by two membranes; the inner membrane probably evolved from the plasma membrane of the engulfed bacterium, while the outer membrane is probably derived from the plasma membrane or endoplasmic reticulum of the host cell. Mitochondria are about the same size as most bacteria, and the cristae formed by their inner membranes resemble the folded membranes in various groups of bacteria. Mitochondrial ribosomes are also similar to bacterial ribosomes in size and structure. Both mitochondria and chloroplasts contain circular molecules of DNA similar to those in bacteria. Finally, mitochondria divide by simple fission, splitting in two just as bacterial cells do, and they apparently replicate and partition their DNA in much the same way as bacteria do.
Key Learning Outcome 4.7. Eukaryotic cells contain complex organelles that have their own DNA and are thought to have arisen by endosymbiosis from ancient bacteria.