"What exactly does it mean when it is said that life is carbon-based? Could life on another planet or moon be based on a different element?"
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How is mitochondrial DNA different from chromosomal DNA?
While chromosomal DNA and mitochondrial DNA are indeed distinct, both accumulate random mutations over time, and these changes can serve as a measure of time and thus evolutionary distances. However, the mitochondrial clock ticks about 10 times faster than the chromosomal clock. Therefore, it is suitable for tracking fairly recent events (thousands of years rather than millions). In addition, mitochondrial DNA is easier to trace because in vertebrates, mitochondria are of maternal origin and not subject to sexual recombination. In a study done at NASA’s Ames Research Center, mitochondrial DNA seem to indicate that the eukaryotic cell arose not as a result of fusion of two prokaryotes (endosymbiosis), but rather first evolved independently of each other. However, this research also suggested that mitochondria and chloroplasts were of endosymbiotic bacterial origin. Just as you guessed, mitochondria research is key in furthering our understanding of the evolution of life on Earth. For more information on the research mentioned above, see http://astrobiology.arc.nasa.gov/workshop/speakers/pace/pace_abstract.html
May 1, 2002
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