Tuesday, November 2, 2010

Life Lines 13


In 1926 geneticist H. J. Muller proposed “ the gene as the basis of life “ by which he meant that all other parts of a cell or organism are products of these genes. About five years earlier he proposed that scientists should study viruses by chemical and physical means because, at that time, one could not distinguish genes from viruses. Bacterial viruses were first discovered in 1919 and several diseases in humans like polio, yellow fever, flu, mumps, measles, and the common cold turned out to be caused by viruses. It took another 25 years for viruses to be studied as organisms and having a life cycle. Viruses are too small to be seen with optical microscopes. They are studied for their structure by using electron microscopes, which didn’t exit until the late 1930s. Experiments with viruses in the 1950s helped establish the field of molecular biology, demonstrating DNA as the genetic material.

When I was a student of Muller’s in the 1950s, I imagined an origin of life that included the formation of nucleotides by natural processes and the formation of nucleic acid strands that replicated themselves and their occasional errors (mutations). I imagined a viral origin of cells and that many of our own genes had an ancestry going back to these viral-like first forms of life.

It is exciting to know that this hypothesis is turning into a well-supported theory. Consider these discoveries about viruses, mostly published in the last twenty years. Viruses are more numerous in number and kind than all other forms of life. One of my colleagues, Martha Baylor, years ago at Stony Brook, used to go to the south shore beaches and hold up Petri dishes with agar on them. She harvested hundreds of new viruses that were blown in by the ocean winds. About 100 million varieties of viruses are estimated to exist, most of them in the oceans that occupy most of earth’s surface. About 1000 viruses have had their sequences worked out; most are small with ten or fewer genes. A few are large with several hundred genes. By contrast bacteria have about 1000 genes and animals like us have about 25,000 genes. Not all viruses are parasites that destroy the cells they infect. Some attach their viral chromosome to one of the host chromosomes and in some cases the spliced inserted viral chromosome contributes to the benefit of the cell it infects. About eight percent of our human DNA is composed of known viral DNA sequences and an additional forty percent of human DNA suggests it had a viral past. Some bacteria require inserted viruses to produce their toxins, including diphtheria, cholera, and bubonic plague. One such inserted virus sequence in human chromosomes produces an essential protein for placenta formation.

The idea that all of life might have a viral origin is thrilling enough, but equally exciting is the view that viruses continue to play in the evolution of life by carrying genes from one form of life into another. At a level we cannot see, these viruses move in and out of plants, animals, and bacteria and make all of life an integrated community at the molecular level.

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