Mutations

Mutations

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.

We humans look rather different from a tree, but down deep, at the heart of life, are we different? Image: Green Haired Beauty © Megan Jorgensen (ElenaB.)

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. Sickle cells carry less oxygen and consequently transmit a kind of anemia. 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 of the function of the blood – so striking as to be readily apparent in photographs or 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.