Chapter 8: Harvesting Energy: Glycolysis and Cellular RespirationIssues in Biology |
Next time you are running to catch a bus, think about those wonderful mitochondria, nestled deep in the cytoplasm of nearly every cell in your body. Without them, you wouldn’t be able to efficiently convert that bagel you had for breakfast into the ATP that your muscle cells need to contract properly and in the right direction. Even when you are sitting quietly on the bus, your cells require an input of energy simply to stay alive. In light of how important mitochondria are for your survival, it is perhaps startling to realize that mitochondria arose from an ancient truce between a predator cell and its prey. Our mitochondria, without which we could not survive, are actually the products of an enormously successful symbiosis that began about 1.5 million years ago.
Look carefully at mitochondria and you will find several clues that point to their original existence as independent, bacteria-like cells. For example, mitochondria reproduce by a type of division that appears similar to that used by bacteria. Like bacteria, mitochondria contain ribosomes and synthesize proteins from messages produced within them. Furthermore, each mitochondrion contains circular DNA molecules, as do bacterial cells. However, unlike bacteria whose chromosome contains several thousand genes, mitochondrial DNA contains just a few protein-encoding genes—about 13 in humans—and none of these genes are actually needed to build a mitochondrion or to help it reproduce! Instead, mitochondrial DNA encodes proteins that have essential roles in the Krebs cycle or in the electron transport system, which play key roles in harvesting the nutrients for the production of ATP. Thus, mitochondrial DNA is needed for mitochondrial function in cellular respiration, but not for mitochondrial existence.
Until fairly recently, no one would have imagined that defects in mitochondrial DNA could cause human disease. It was reasoned that, even if a mutation occurred in one of the DNA molecules, each mitochondrion has dozens of other copies that should mask the defect. Furthermore, even if all of the DNA in a single mitochondrion were mutated, cells contain many mitochondria and these should cover for the defective one sufficiently to prevent any symptoms. However, in 1988 it became clear that these assumptions were invalid when a human disease syndrome known as MERRF was found to result from defects in mitochondrial DNA. Subsequently, a variety of human disease syndromes have been traced to defects in mitochondrial DNA.
Mitochondrial diseases are highly varied in terms of their onset and the range of symptoms they produce. For example, they can occur in infancy or develop later in life. They can cause symptoms ranging from muscle fatigue and weakness to death. They typically have their greatest effects on tissues with high energy demands. Consequently, symptoms of mitochondrial diseases are frequently related to brain function (stroke, epilepsy, mental retardation), to sight and hearing, and to muscle function (weakness, twitching). However, mitochondrial DNA abnormalities have also been discovered in certain types of diabetes and are even suggested to be important for the symptoms of Alzheimer’s disease and aging. Clearly, having mitochondria that function properly is essential for human health!
An interesting aspect of mitochondrial DNA is that we inherit it nearly exclusively from our mothers. Thus, your mitochondrial DNA was passed down to you from your mother, she received it from her mother, and so on. How is it possible to inherit mitochondrial DNA only from a single parent? The answer lies in the differences between your mother’s and your father’s cellular contribution to your initial existence. Eggs are enormous cells that are packed full of mitochondria. In contrast, sperm are tiny cells that have few mitochondria, and an egg may actively destroy the "invading" mitochondria. Evolutionary biologists have analyzed mitochondrial DNA to work out relationships between various populations of vertebrates, including humans. However, this DNA exclusively follows the maternal lineage. Some of these molecular analyses indicate that all human mitochondrial DNA in existence today can be traced back to a common maternal ancestor who lived in Africa about 100,000–200,000 years ago. (Of course, scientists don’t think that "Mitochondrial Eve" was the ONLY woman alive at the time!)
So, next time you run to catch that bus, think about mitochondria, and next Mother’s Day, thank your Mom for her extra contribution to your genetic inheritance: your mitochondrial DNA.