By the time Gregor Mendel began the pea-breeding experiments in 1856 that would lead to his discovery of the basic laws of inheritance, scientists already knew that the cell was a complex heterogeneous unit consisting of a nucleus and an indeterminate umber of additional subunits. In 1897, the German biologist Carl Benda identified one of those subunits, which he called the mitochondrion, Greek for "filament granule."
Aided by advanced microscopes and methodology, scientists soon discovered that the term inaccurately describes the small, sausage-shaped structures. In the post-World War II years the electron microscope revealed the complex inner structure of the mitochondrion, enabling biochemists to identify the enzymes that control the chemical reactions providing the energy that allows plants and animals to function.
Thereafter, investigation of the mitochondria was progressing slowly until 1971, when Douglas C. Wallace, a 25-year-old biochemist doing predoctoral research at Yale University, began studies that he continues today as the head of a 12-member team at Emory University in Atlanta. During the last six years, those studies have yielded striking results, offering new insights into inheritance, the origins of humankind, and the causes of a series of degenerative diseases.
Well over 99 percent of the DNA in each cell resides in the nucleus, but each cell contains several hundred mitochondria in the cytoplasm surrounding the nucleus. Each mitochondrion contains several copies of its own DNA (mtDNA). This mtDNA has a unique genetic code and is capable of mutation. However, the mtDNA differs radically from the nuclear DNA (nDNA). The former contains only 16,569 base pairs, or nucleotides, compared with the 2.5 billion or more base pairs of its nuclear cousin; it forms a closed circular pattern compared with the tangled staring of nDNA; and, of key importance for Wallace's studies, if is a female-specific DNA - while mothers transmit mtDNA to all offspring, only daughters pass it on to the next generation.
Since the mtDNA of all animals is similar and since the mitochondrium is about the size of a bacterium, scientists believe that mitochondria originated when a bacterium-like parasite invaded a primitive eukaryotic cell - that is, a cell with a nucleus.
Wallace compares the nuclear/mitochondrial relationship to "a city with multiple power plants around it, and within each power plant there is a blue print on how to build the power plant. Well, it would be exactly the same idea. In each mitochondrion there is a blueprint of genes on how to build the energy generation system of that mitochondrion.
"I started out with the concept that something that provided 90 percent of the energy of the body couldn't trivial," he says, "and therefore, even though few others were interested in studying it, I was bound and determined to understand its genetics. I've striven to this day to try to understand mitochondrial genetics and its implication for human evolution and disease. And so all of what we've done has been a linear progression of this basic concept - that if we understand the mtDNA we can have a better sense of what it means to be human."
Wallace's research splits into three distinct but closely related branches, the most
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