Molecular Evolution

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The Story of Evolution, Spring 2005 First Web Papers On Serendip

Molecular Evolution

Jennifer Gerfen

Life as it exists on earth had developed into an array of complex characteristics. There are a variety of complex mechanisms that interact to give rise to the different species that exist. It is hard for some people look for a reason that they exist without there being some reason that gave rise to this complexity, however, it is possible that all of the adaptations that caused life to evolve to the point that it is stems from a chance encounter rather than an original purpose. The structure that life has been formed from shows a complexity that has many conserved elements, which indicated that the specialization was formed not through design, but through a long process of chance encounters.

Most biological molecules have the ability to form their structures spontaneously. The structure of macromolecules such as DNA and proteins fold directly as they are formed. The helices or other patterns tend to be the most stable and adaptive to the environment. Protein domains tend to form in order to reduce the amount of hinderance from the interaction of amino acids as well as counter the hydrophobic affect from the non-polar amino acids. Lipids also have a tendency to spontaneously form membranes when placed in water. These molecules currently interact in specific ways, although, the original formation might have been enough to initiate the beginning of life.

The primordial earth had the correct environment to form the basic molecules necessary for the spontaneous formation of life. Stanley Miller and Harold Urey were able stimulate the conditions of the early earth with a mixture of the ammonia, methane, water, and hydrogen gas exposed to electrodes, which stimulated lightning. Through repeated experiments these abiotic conditions were able to produce organic molecules such as aldehydes, amino acids, and molecules that were similar to RNA. (1) These experiments show that the components necessary for life can form without a preexisting structure.

These elements would be able to create a precursor to a cell since RNA is able to be the hereditary material. There are many viruses that currently exist with an RNA genome. Recently it has also been demonstrated that RNA molecules have catalytic properties. (1) The creation of ribozymes gave early life the ability to replicate since one of the functions is RNA replication. Another catalytic function of ribozymes is the ability to form peptide bonds, which would produce simple poly-peptides. The role of peptides would likely provide more functions as time went on. This would allow for the transfer of the molecule of heritability to change from RNA to DNA, which has a greater stability and therefore would be more able to handle greater amounts of coding regions.

Proteins are encoded for by building creating a chain of amino acids based on three nucleotide sequences of the RNA based on the genetic code. The genetic code is universal among most organisms including species ranging from bacteria to humans, however mitochondria and chloroplasts have their own genomes and own genetic code. (2) The current theory on the origin of these organelles is that they were originally endosymbionts that lived inside other cells and eventually lost most of their genomes as they lost the need to function on their own.

If the genetic code is not entirely universal it shows that the way the code is determined was likely due to random selection rather than some force to have the perfect genetic code. The evolution of this code likely took place early in evolutionary history, since it is imperative that the transfer RNA molecules function correctly, since mutations would cause problems in the formation of every protein which would cause the cell no possibility for survival.

The proteins themselves seem to be derived from a single set of original proteins. Despite the fact that there are a large number of proteins that are encoded for in many different organisms there tends to be certain domains that are incorporated into many different proteins. A common example is the immunoglobin domain. (3) It is likely these similarities between proteins could have arisen from mutations where a portion of the genome is replicated incorrectly and an extra copy of a gene is added to the genome. Mutations of the extra copies of the gene would not cause problems in the cell since there will still be a functional copy of the gene. If fact the mutation of the gene might be favorable since multiple copies of the gene might lead to slight problems due to the affects of gene dosage, since the organism would produce twice the amount of protein than was necessary.

Speciation events oftentimes occur due to these chromosomal mutations that cause gene differences to occur. This has led to synteny, conserved gene order, among similar organisms. Chromosome duplication and translocation produce the opportunity for important mutations. This conservation helps to show relatedness between organisms, for example human chromosome 9 and mouse chromosome 2 have portions where the order of the genes is conserved. (1) The incomplete conservation shows how organisms are able to be similar, though there is still a pronounced difference between organisms which is not consistent between organisms.

Organisms have conservation on the molecular level, which shows that it is likely that they evolved from a common ancestor. Though it is through the differences that highlight how basic molecular processes are able to create the diversity of life that we are able to observe. Actually observing life developing might not be observable in a laboratory due to time constraints we do have series of observations that have led to the theory that it is indeed possible that the only spark that caused the creation of life was constant lightning storms on a primordial earth.

References

(1) David Nelson and Michael Cox (2005) Lehninger Principles of Biochemistry. New York: W. H. Freeman and Company.

(2) Leland Hartwell et al (2004) Genetics. Boston: McGraw Hill.

(3) Thomas Pollard and William Earnshaw (2004) Cell Biology. Philadelphia: Saunders.

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