The average person has two sets of DNA. The first set is the nuclear DNA with which everyone is familiar. It is the unique genetic code of a person that contains the traits a person has and the traits a person can pass onto his or her offspring. The second set is an evolutionary clue, and its origin may surprise you.
Mitochondria and Inheritance
Cells contain organelles called mitochondria. Mitochondria are responsible for producing the energy needed for cellular processes. The mitochondrial DNA (mtDNA), or genetic code, is different from the nuclear DNA. This is due to the process of conception.
When a sperm fertilizes an ovum, the DNA from the sperm enters the ovum, but the spermatic mitochondria are left behind and destroyed. The mitochondria that are used for cell division are the mitochondrial DNA within the ovum, or the mother's mitochondria.
In the NOVA Online article "Tracing Ancestry with MtDNA," Rick Groleau says, "If you went back six generations in your own family tree, you'd see that your nuclear DNA is inherited from 32 men and 32 women. Your mtDNA, on the other hand, would have come from only one of those 32 women."
Mitochondrial DNA Is Not Unique
While different lineages may have slightly varying mtDNA due to mutations, mtDNA is not unique like nuclear DNA. When an ovum is fertilized, the nuclear DNA is formed from chromosome sets from the mother and father.
When cell division occurs, exact copies of the cell material are contained within the new cells. Since mitochondria only come from the mother, the mitochondria are copied rather than created from combined genetic material from the mother and father. For this reason, family members along the same female lineage will have the same mtDNA.
Mitochondria are thought to have originated from a symbiotic relationship between bacteria and humans, or their predecessors. That is one theory for the mitochondrial DNA being unique from the nuclear DNA. The splitting of the mitochondria for replication is similar to that of bacterial reproduction.
Mitochondrial Ancestors
Everyone currently living is said to be of one female ancestor, according to our mtDNA. This ancestor is not the first woman, however. It is possible that she is the only female to have passed her DNA onto female offspring during a time of line extinction. She and all of the people from her time received their mitochondrial DNA from another woman.
Origin of Mitochondrial DNA
The ability of mitochondria to synthesize RNA and the fact that mitochondria have their own DNA leads biologists to believe that mitochondria are descended from bacteria. "The endosymbiont hypothesis, first proposed by Lynn Margulies and now widely accepted, states that early in evolution, an energy-poor cell engulfed a bacterium with far more efficient energy-producing machinery and ultimately co-opted its functions; over time, the bacteria evolved into mitochondria," say Eric Schon and Salvatore DeMauro in their article "The other DNA: research on mitochondrial disease."
Cellular energy production by the mitochondria is a remarkable process in and of itself. It is fascinating that these cellular powerhouses can also be used to trace evolutionary lineages through mitochondrial DNA analysis.
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