Member-only story
by Chaotropy
This article is the second in a series on the origins of life, abiogenesis, chemical evolution, and the RNA world hypothesis.
Now let us look at evolution from the other direction: In life today, there is a highly conserved code that we humans (partially) share with all living creatures, be they fruit flies or forest elephants. This code is not stored in RNA, but in the even more stable DNA, or deoxyribonucleic acid, in which ribose is replaced by the energetically more stable deoxyribose. Two molecules of DNA form a double strand, which also contributes to its stability: The nucleic bases cytosine and guanine, as well as thymine and adenine, have such a matching spatial structure that they attract each other electrostatically and hold the two strands together. However, one condition must be met for this to happen: Wherever there is cytosine on one strand, there must be guanine on the other strand, and vice versa. In the same way, where there is thymine, there must be adenine on the other strand, and vice versa. This means that the DNA of one strand within a double strand is always a copy of the other, with both running in opposite directions. If we abbreviate the nucleic bases by their initial letters, we can easily imagine such a possible double strand:
...AGCTAGCTAGCTAGCTAGCTAGCTAGCTAGCTAGCTAGCTAGCTAGCTAGCTAGCT...
...TCGATCGATCGATCGATCGATCGATCGATCGATCGATCGATCGATCGATCGATCGA...A double strand of DNA then forms the well-known double helix structure, which is particularly stable, as described by James Watson and Francis Crick in 1953. Both were awarded the Nobel Prize in 1962.
In organisms known to us, the double strand of DNA can be separated for a short time by specific enzymes to generate a matching strand of RNA (it contains uracil instead of thymine, which matches also adenine). This new single strand of RNA — known as messenger RNA — can then be translated into a chain of amino acids (or a polypeptide chain) by a system in which three consecutive nucleic bases code for one amino acid. The memory code of DNA, which uses four different letters, thus becomes the functional alphabet of proteins with twenty letters (20 amino acids). The linear sequence of nucleic bases in DNA can thus be transcribed via RNA into an equally linear sequence of amino…
