FEATURE ARTICLE
Biomolecules and Nanotechnology
Evolution has forced innovative solutions to biomolecular problems. Some may inform the growing field of nanotechnology
David Goodsell
Evolutionary Legacy
The process of evolution by natural selection places strong constraints on the form that biological molecules may adopt. Because genetic information is passed directly from generation to generation, cells must maintain a living line back to the earliest primordial cells. If a cell fails to generate a living descendent, all of its biological discoveries will be lost. This is far more limiting than the technology of our familiar world. If we create machines that don't function, we scrap them and go back to the drawing board. But if a cell takes a gamble and changes a critical machine, it had better get it right the first time or the result will be disastrous.
The picture is not entirely grim, however, as cells have several levels of redundancy within which to develop new machines. First, the plans for a given machine may be duplicated, which allows the duplicate to be modified and ultimately perfected to perform a function different from the original. This is very common in the evolution of life. Hemoglobin, the protein that carries oxygen in our blood, is an example. Our cells contain information for building several different types of hemoglobin. One is optimized for carrying oxygen in the blood of adults, whereas another is found in the blood of a fetus. The fetal hemoglobin has a higher affinity for oxygen, allowing it to capture oxygen from the mother's blood. About 200 million years ago, a gene duplication allowed the fetal hemoglobin to be perfected separately.
Second, biology seldom involves a single cell. A population of cells?billions, trillions?is the biologically relevant entity. Within this population there exists ample room for experimentation. Millions of modifications may be tried, even if most are ultimately lethal. The population will still survive and individuals with rare improvements may grow to dominate in later generations. Human immunodeficiency virus (HIV) shows the benefits of evolutionary change, accelerated so that we can see the effects in months instead of millennia. HIV reverse transcriptase, the enzyme that copies the virus's genetic information, is particularly error-prone. Because of this, the population of viruses within an infected individual contains viruses with all possible single-site mutations?thousands of variants on the wild-type virus. The best of these will dominate, but even the weakest are continually created and recreated in subsequent generations by the low-fidelity copying mechanism. Thus, when an infected individual is treated with anti-HIV drugs, the population has a wide range of different mutants to choose from, some of which may be resistant to the drug: The virus is made more efficient by its very inefficiency.
The hallmark of biological evolution is the plasticity provided by mutation and genetic recombination. Within a population, or through genetic duplication within a single cell, a great many variants may be tested and the occasional improvement saved.
Evolution carries with it one important drawback, however: the problem of legacy. Once a key piece of machinery is perfected, it is difficult to replace it or make major modifications without killing the cell. This is particularly true for major molecular processes, such as protein synthesis, energy production and reproduction, which require the concerted action of many different molecular machines. This leads to the remarkable uniformity of all earthly living things when observed at the molecular level. All are built of the same basic components.
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