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.And at the very heart of life on Earth the proteins that control cell chemistry, and the nucleic acids that carry the hereditary instructions - we find these molecules to be essentially identical in all the plants and animals.An oak tree and I are made of the same stuff.If you go far enough back, we have a common ancestor.The living cell is a regime as complex and beautiful as the realm of the galaxies and the stars.The elaborate machinery of the cell has been painstakingly evolved over four billion years.Fragments of food are transmogrified into cellular machinery.Today’s white blood cell is yesterday’s creamed spinach.How does the cell do it? Inside is a labyrinthine and subtle architecture that maintains its own structure, transforms molecules, stores energy and prepares for self-replication.If we could enter a cell, many of the molecular specks we would see would be protein molecules, some in frenzied activity, others merely waiting.The most important proteins are enzymes, molecules that control the cell’s chemical reactions.Enzymes are like assembly-line workers, each specializing in a particular molecular job: Step 4 in the construction of the nucleotide guanosine phosphate, say, or Step 11 in the dismantling of a molecule of sugar to extract energy, the currency that pays for getting the other cellular jobs done.But the enzymes do not run the show.They receive their instructions - and are in fact themselves constructed - on orders sent from those in charge.The boss molecules are the nucleic acids.They live sequestered in a forbidden city in the deep interior, in the nucleus of the cell.If we plunged through a pore into the nucleus of the cell, we would find something that resembles an explosion in a spaghetti factory - a disorderly multitude of coils and strands, which are the two kinds of nucleic acids: DNA, which knows what to do, and RNA, which conveys the instructions issued by DNA to the rest of the cell.These are the best that four billion years of evolution could produce, containing the full complement of information on how to make a cell, a tree or a human work.The amount of information in human DNA, if written out in ordinary language, would occupy a hundred thick volumes.What is more, the DNA molecules know how to make, with only very rare exceptions, identical copies of themselves.They know extraordinarily much.DNA is a double helix, the two intertwined strands resembling a ‘spiral’ staircase.It is the sequence or ordering of the nucleotides along either of the constituent strands that is the language of life.During reproduction, the helices separate, assisted by a special unwinding protein, each synthesizing an identical copy of the other from nucleotide building blocks floating about nearby in the viscous liquid of the cell nucleus.Once the unwinding is underway, a remarkable enzyme called DNA polymerase helps ensure that the copying works almost perfectly.If a mistake is made, there are enzymes which snip the mistake out and replace the wrong nucleotide by the right one.These enzymes are a molecular machine with awesome powers.In addition to making accurate copies of itself - which is what heredity is about - nuclear DNA directs the activities of the cell - which is what metabolism is about - by synthesizing another nucleic acid called messenger RNA, each of which passes to the extranuclear provinces and there controls the construction, at the right time, in the right place, of one enzyme.When all is done, a single enzyme molecule has been produced, which then goes about ordering one particular aspect of the chemistry of the cell.Human DNA is a ladder a billion nucleotides long.Most possible combinations of nucleotides are nonsense: they would cause the synthesis of proteins that perform no useful function.Only an extremely limited number of nucleic acid molecules are any good for lifeforms as complicated as we.Even so, the number of useful ways of putting nucleic acids together is stupefyingly large - probably far greater than the total number of electrons and protons in the universe.Accordingly, the number of possible individual human beings is vastly greater than the number that have ever lived: the untapped potential of the human species is immense.There must be ways of putting nucleic acids together that will function far better - by any criterion we choose - than any human being who has ever lived.Fortunately, we do not yet know how to assemble alternative sequences of nucleotides to make alternative kinds of human beings.In the future we may well be able to assemble nucleotides in any desired sequence, to produce whatever characteristics we think desirable - a sobering and disquieting prospect.Evolution works through mutation and selection.Mutations might occur during replication if the enzyme DNA polymerase makes a mistake.But it rarely makes a mistake.Mutations also occur because of radioactivity or ultraviolet light from the Sun or cosmic rays or chemicals in the environment, all of which can change the nucleotides or tie the nucleic acids up in knots.If the mutation rate is too high, we lose the inheritance of four billion years of painstaking evolution.If it is too low, new varieties will not be available to adapt to some future change in the environment.The evolution of life requires a more or less precise balance between mutation and selection.When that balance is achieved, remarkable adaptations occur.A change in a single DNA nucleotide causes a change in a single amino acid in the protein for which that DNA codes.The red blood cells of people of European descent look roughly globular.The red blood cells of some people of African descent look like sickles or crescent moons
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