Chirality A brief overview
All living organisms are based upon certain mirror isomers of amino acids. Although normal chemical reactions produce right and left mirrors in equal amounts, often called a racemic mixture, the proteins which constitute the organelles in living cells are composed entirely of right handed forms of sugars and left handed forms of amino acids (called enantiomers). Because such chemicals exist exclusively in one form, they are referred to as being homochiral. Any naturalistic explanation for the origin of life must, therefore, provide an account of how naturalistic mechanisms would produce the proper mirrored building blocks necessary for the generation of the first self-replicating life form.
Chirality A problem for neo-Darwinism
Chirality is a long-standing enigma for those who hold to a naturalistic explanation for the origin of life. DNA and RNA are not capable of complementary pair bonding in the absence of being homochiral. This means that racemic DNA or RNA cannot replicate. Living things possess molecular machinery to produce homochirality, but undirected chemistry would produce equal mixtures of the left-handed and right-handed isomers, called racemates. There is no known mechanism by which racemic polypeptides could form the specific shapes required for enzymes; rather, they would have the side chains sticking out all over the place. Moreover, a wrong-handed amino acid disrupts the stabilizing alpha-helix in proteins. DNA could not be stabilized in a helix if even a small proportion of the wrong-handed form was present, so it could not form long chains. Only a tiny fraction of wrong-handed molecules is required to terminate RNA replication.
The problem of chirality is crucial to the concept of abiogenesis -- that is, that life can originate from non-living matter. Proponents of such a model must answer questions such as Which came first, homochirality or life? If one holds that homochirality was first, it is an admission that without left-handed amino acids and right-handed sugars, lifes structures and processes would not have been possible. One then has to account for the origin of this homochirality. Conversely, if one contends that life was first, then one is saying that chirality was not important to the origin of lifes structures and processes as we now know them. One must enter a special pleading for a vastly different metabolism in the protobiont, ignoring for instance the pivotal role of polypeptide homopolymers in hydrogen-bonded networks for proton and electron transport. Moreover, one must also account for the successful transition to homochirality as we have it today -- a problem which is, arguably, even more insurmountable. The logical conclusion from these considerations is that homochirality and life emerged simultaneously.
The origin of homochirality is of fundamental importance in origin-of-life research, since non-homochiral mixtures of amino acids or sugars are not conducive to the composition of RNA, DNA and proteins -- the building blocks of all living organisms. There remains no naturalistic explanation that describes how homochirality could have arisen through entirely materialistic processes. Processes that can enhance the enantiomeric excess of appropriate amino acid or nucleic acid building blocks produce only modest increases in the percentage of those proteins, while requiring unrealistic, laboratory conditions.