Why should the particular polymer combinations of Earth reign supreme?
|Examples of typical polymer structures. Credit: Molecular Universe
Step outside and look around you. Chances are, your eye will light upon something made from polymers. Even your own body is composed of polymers - your bones, muscle fibers, your very DNA - polymers all. While polymers can be found everywhere, scientists also have learned to manipulate polymers to create things not found in nature, such as plastic and nylon.
Polymers are composed of molecules called monomers. Monomers are defined by their ability to link up and form chains, lining up like beads on a string to form the polymers.
This is the hidden structure of life on Earth. Boiled down to essentials, all life is based on the two-polymer system of protein and nucleic acid. Nucleotides (monomers) make up the nucleic acids RNA and DNA (polymers). Nucleic acid holds an organism's genetic code - storing the information and passing it on to descendants - and a mutation in these polymers is the basis for evolution. Nucleic acids also direct the formation of protein. Proteins, which are made up of amino acids, are essential for the growth and repair of tissue. Proteins are the workhorses of the cell, acting as enzymes, hormones, and antibodies.
This two-polymer strategy for life has lasted on Earth for billions of years, surviving radical climate changes and asteroid impacts. But as we search for life on other worlds, can we expect to find a similar arrangement of polymers? Why should the particular polymer combinations of Earth reign supreme throughout the Universe?
There are nearly infinite ways that monomers could combine to form polymers. Look at proteins: although proteins vary in size, the average protein chain is made up of 300 amino acids. With 20 amino acids available in nature, there are 20^300 possible protein polymer molecules. To give you some idea of the immensity of that number, the Universe is 10^19 seconds old.
On Earth, however, most living organisms make use of less than 100,000 types of protein molecules. Nucleic acid also has a range of different possible combinations. The nucleotides that make up DNA and RNA are sugar, phosphate, and bases (compounds related to caffeine). Bases are known by their letters: A (adenine), C (cytosine), G (guanine), T (thymine), and U (uracil). Three of the bases (A, C, G) are found in both DNA and RNA. However, T is found only in DNA and U is found only in RNA. While the two types of nucleic acids each use 4 types of bases, there are 100 to 10,000 base units in each polymer molecule. Thus, each nucleic acid molecule has a huge number of possible base combinations. In addition, the sugars and bases may be connected to each other in hundreds or thousands of different ways.
|Thymine structure - one of the bases found in DNA and not in RNA. Credit: Richard B. Hallick
With such a wide range of biopolymer combinations possible, how would we be able to recognize alien life if we came across it? Life composed of other polymer combinations would probably evolve into forms unlike anything we can conceive.
To try to figure out whether life in the Universe would have any form we could recognize, Steven Benner at the University of Florida has developed a model he calls the "Universal Genetic Biopolymer."
"A NASA workshop defined 'life' to mean a self-sustaining chemical system capable of Darwinian evolution," says Benner. "We asked, 'What chemical structures would be universal in the genetic molecules that support Darwinian evolution?'"
Assuming that organic molecules behave the same across the galaxy, Benner looked for structural features in a molecule that could mutate without interfering with replication. In addition, this mutation would have to be inheritable.
Benner and his team assumed that all life in the Universe would need liquid water. The chemical reactions of all living things on Earth take place in an aqueous solution, and water is the ideal solvent. It helps living things maintain homeostasis - a stable level of internal conditions. All living things need energy, and a common source of energy - oxidation-reduction, or "redox" reactions - occurs most easily in a water solution. The need for liquid water also infers a need for moderate temperatures ranging between zero and 100 degrees Celsius.
Benner decided to focus on the more complex polymers than on monomers, because monomers can not carry and pass on genetic information. "Monomers could not carry the amount of genetic information that is needed to encode life, at least not life having any degree of complexity, and support Darwinian evolution, " says Benner.
Benner found that only a small subset of polymeric materials are capable of suffering mutation without losing the molecular features that are essential for reproduction. "Most organic molecules change their physical properties significantly, sometimes dramatically, when their structure is changed, even only slightly," says Benner. "We concluded that one feature of the 'universal' genetic molecule is a repeating charge, either negative charge - which is what DNA has - or a repeating positive charge."
|The 'polarity' of the DNA strands. Credit: Centre for Biomolecular Sciences
DNA has a negative electric charge because it is composed of many polar molecules. Polar molecules are molecules with an uneven charge distribution. For instance, water molecules are polar: one side of the molecule has a positive charge while the other side has a negative charge. The negative charge in DNA is due to the phosphates in the sugar-phosphate backbone of each DNA strand.
"We have made modified backbone DNA analogs that miss the repeating charge," says Benner. "This tests our hypothesis that a universal genetic molecule, in water, will carry repeating charges."
The repeating charge of DNA sets it apart from all other polar molecules, and thus provides a reliable biomarker in the search for life. Benner says that we could currently look for these positive or negative repeating charges in the polymers found in meteorites.
Ken Nealson, an astrobiologist with NASA's Jet Propulsion Laboratory, agrees that a polymer with a repeating charge may be a quality universal to life.
"For the most part, it seems perfectly normal to me that some repeating charged polymer would be the informational molecule," says Nealson. "Although, if one imagines that life might exist in other solvents, or use energy sources other than those converted to redox energy, there may be many possibilities."
The environment in which the polymers form will influence their development. For instance, some polymers are stable in water, while others are not. The different environments of alien worlds would probably yield different kinds of polymer formations - and thus different creatures - than those found on Earth.
Nealson cautions that while he likes this model, it rests on the assumption that life must be capable of Darwinian evolution - just as it is on Earth. He suggests that life elsewhere may not be constrained by such a requirement.
"We may not want to restrict ourselves to all life doing as ours does," says Nealson. "It may not be so, and then the requirement for a repeating charge polymer may not be so crucial. I might prefer a definition based on more general properties of complexity and energy."
To test his theory of the Universal Genetic Biopolymer, Benner and his team have developed an artificially expanded genetic system. This system has 12 base letters in the genetic alphabet, as opposed to DNA's 4 base letters (A, C, G and T).
"We have used this alternative genetic system to direct the synthesis of proteins with more than 20 amino acids," says Benner. "We are presently trying to generate artificial Darwinian chemical systems that incorporate these, in collaboration with Chris Switzer at the University of California, Riverside."