[ Pobierz całość w formacie PDF ]
.They also provide major resistance against malaria.There is no question that it is better to be anemic than to be dead.This major influence on the function of the blood - so striking as to be readily apparent in photographs of red blood cells - is the result of a change in a single nucleotide out of the ten billion in the DNA of a typical human cell.We are still ignorant of the consequences of changes in most of the other nucleotides.We humans look rather different than a tree.Without a doubt we perceive the world differently than a tree does.But down deep, at the molecular heart of life, the trees and we are essentially identical.We both use nucleic acids for heredity; we both use proteins as enzymes to control the chemistry of our cells.Most significantly, we both use precisely the same code book for translating nucleic acid information into protein information, as do virtually all the other creatures on the planet.* The usual explanation of this molecular unity is that we are, all of us - trees and people, angler fish and slime molds and paramecia - descended from a single and common instance of the origin of life in the early history of our planet.How did the critical molecules then arise?* The genetic code turns out to be not quite identical in all parts of all organisms on the Earth.At least a few cases are known where the transcription from DNA information into protein information in a mitochondrion employs a different code book from that used by the genes in the nucleus of the very same cell.This points to a long evolutionary separation of the genetic codes of mitochondria and nuclei, and is consistent with the idea that mitochondria were once free-living organisms incorporated into the cell in a symbiotic relationship billions of years ago.The development and emerging sophistication of that symbiosis is, incidentally, one answer to the question of what evolution was doing between the origin of the cell and the proliferation of many-celled organisms in the Cambrian explosion.In my laboratory at Cornell University we work on, among other things, prebiological organic chemistry, making some notes of the music of life.We mix together and spark the gases of the primitive Earth: hydrogen, water, ammonia, methane, hydrogen sulfide - all present, incidentally, on the planet Jupiter today and throughout the Cosmos.The sparks correspond to lightning - also present on the ancient Earth and on modern Jupiter.The reaction vessel is initially transparent: the precursor gases are entirely invisible.But after ten minutes of sparking, we see a strange brown pigment slowly streaking the sides of the vessel.The interior gradually becomes opaque, covered with a thick brown tar.If we had used ultraviolet light - simulating the early Sun - the results would have been more or less the same.The tar is an extremely rich collection of complex organic molecules, including the constituent parts of proteins and nucleic acids.The stuff of life, it turns out, can be very easily made.Such experiments were first performed in the early 1950’s by Stanley Miller, then a graduate student of the chemist Harold Urey.Urey had argued compellingly that the early atmosphere of the Earth was hydrogen-rich, as is most of the Cosmos; that the hydrogen has since trickled away to space from Earth, but not from massive Jupiter; and that the origin of life occurred before the hydrogen was lost.After Urey suggested that such gases be sparked, someone asked him what he expected to make in such an experiment.Urey replied, ‘Beilstein.’ Beilstein is the massive German compendium in 28 volumes, listing all the organic molecules known to chemists.Using only the most abundant gases that were present on the early Earth and almost any energy source that breaks chemical bonds, we can produce the essential building blocks of life.But in our vessel are only the notes of the music of life - not the music itself.The molecular building blocks must be put together in the correct sequence.Life is certainly more than the amino acids that make up its proteins and the nucleotides that make up its nucleic acids.But even in ordering these building blocks into long-chain molecules, there has been substantial laboratory progress.Amino acids have been assembled under primitive Earth conditions into molecules resembling proteins.Some of them feebly control useful chemical reactions, as enzymes do.Nucleotides have been put together into strands of nucleic acid a few dozen units long.Under the right circumstances in the test tube, short nucleic acids can synthesize identical copies of themselves.No one has so far mixed together the gases and waters of the primitive Earth and at the end of the experiment had something crawl out of the test tube.The smallest living things known, the viroids, are composed of less than 10,000 atoms.They cause several different diseases in cultivated plants and have probably most recently evolved from more complex organisms rather than from simpler ones.Indeed, it is hard to imagine a still simpler organism that is in any sense alive.Viroids are composed exclusively of nucleic acid, unlike the viruses, which also have a protein coat.They are no more than a single strand of RNA with either a linear or a closed circular geometry.Viroids can be so small and still thrive because they are thoroughgoing, unremitting parasites.Like viruses, they simply take over the molecular machinery of a much larger, well-functioning cell and change it from a factory for making more cells into a factory for making more viroids.The smallest known free-living organisms are the PPLO (pleuropneumonia-like organisms) and similar small beasts.They are composed of about fifty million atoms
[ Pobierz całość w formacie PDF ]