This video is a nice illustration of the incredible advances of molecular biology in the past 25 years. When I was a medical student, a long time ago, we learned all the anatomic structures of the cell but had no idea what many of them did. We knew that mitochondria made energy from oxygen but, aside from basic genetics (very basic) we didn’t understand most of what went on in the cell. Over the past six or seven years, I have spent some time reading about molecular biology so I could appreciate what has been learned and in an attempt to appear better informed to my students. Along the way, I got very interested in mitochondria.
First, a nonbiologist must learn the difference between a Eukaryote and a Prokaryote. A eukaryote is a cell, or an organism made up of cells, that has a nucleus (containing the chromosomes) enclosed by a membrane and a structure of cell organelles that carry out cell functions. Plants, for example, are eukaryotes and have a larger number of genes than humans do. They also have mitochondria. The prokaryote has its genetic material, often a single chromosome, lying free in the cytoplasm. Bacteria are prokaryotes. Yeasts are eukaryotes with nuclei.
If you don’t believe in evolution, it would be best if you stopped reading here.
It is generally accepted that the first living cells were some form of prokaryote and may have developed out of protobionts. Fossilized prokaryotes approximately 3.5 billion years old have been discovered (less than 1 billion years after the formation of the earth’s crust), and prokaryotes are perhaps the most successful and abundant organism even today. Eukaryotes only formed later, from symbiosis of multiple prokaryote ancestors; their first evidence in the fossil record appears approximately 1.7 billion years ago, although genetic evidence suggests they could have formed as early as 3 billion years ago.
Protobionts are thought to be the precursors of living cells. Maybe the prion, which causes mad cow disease is actually the ancestor of all life. Because of the extreme conditions existing early in the Earth’s history, proteins may have been the original genetic material. Christian de Duve, a Nobel Prize winning biologist, has written a book on the subject. Until the discovery of DNA, proteins were thought to be the genetic material by most biologists. Frederick Griffith began the modern field of genetics when he discovered Transforming Material, which was DNA. DNA “melts” at 87 degrees centigrade, however, and RNA, which will tolerate higher temperature, does not seem to be able to replicate itself without DNA. The origins of life may involve self replicating proteins, like prions.
There has been considerable interest in Archea, a class of organisms found in extreme conditions, because they may have been able to survive in those conditions. Perhaps, they were the first life forms. Originally thought to be bacteria, and first called “Archaeabacteria”, they are quite different and have a different cell membrane composition. First discovered in extreme conditions, like steam vents in the ocean floor or geysers, they are now recognized as widely distributed in all conditions, including ocean plankton. Craig Venter, whom I have previously discussed is collecting ocean water samples looking for useful Archaea samples to study their genome. They may hold the solution to the energy problem, for example.
Mitochondria were probably early prokaryotes in the evolution of life. They carried a unique characteristic. They can create energy from oxygen. As the earth cooled and plants began to develop, the atmosphere, at first made up of methane (which Archaea love) and carbon dioxide (which plants use as fuel), began to contain measurable oxygen. The ability to use that oxygen became desirable. The origin of mitochondria has stimulated intense research. Mitochondria may have been ingested by early eukaryotes and, because they carried the ability to use oxygen and produce more energy than anerobic metabolism, the relationship may have changed from predator to cooperation. Genome sequencing has allowed proof of theories that were only speculation 25 years ago. The mitochondrion has its own DNA. It was almost certainly once freeliving but, as it adapted to symbiosis, it lost unused genes until some types have only three. The study of mitochondrial DNA has contributed to the study of human origins. The “African Eve” theory is derived from the fact that all mitochondria are inherited from the mother. There are no mitochondria in the sperm.
UPDATE: An astute reader pointed out that my statement above is incorrect. Actually, it is a sign of how old I am as this was the previous understanding. However, sperm do have mitochondria but they are tagged for destruction and do not survive in the egg. Why this is, is not explained although the paternal mitochondria may be harmful in some fashion.
Other evidence that mitochondria were once free living come from the study of Rickettsia, cause of diseases such as typhus (which defeated Napoleon’s Grand Army in Russia) and Rocky Mountain Spotted Fever. The organism is named for Ricketts who discovered the organism and lost his life in the process.
That’s enough cell biology for a Saturday morning. The basics of evolution are contained in this story, however.