An update on origin-of-life research: DNA self-assembly in the solid state using heat

DNA is the code of life. In order to better understand life, scientists are seeking an explanation for its beginnings.

A new approach for forming DNA base pairs with heat in the solid state has been demonstrated by a team of scientists from Croatia and Germany last month.

Since the discovery of DNA scientists have tested the ability of DNA nucleotides to self-assemble or form base pairs. The nature of nucleotides (hydrophobic) prevents self-assembly in water. In fact, even a single Watson-Crick base pair is difficult to achieve in any liquid state because of the multitude of H-bonding opportunities afforded between water and the molecular structures. 

DNA, or deoxyribonucleic acid, is a complex molecule found inside the cells of all living things. Its structure was first deduced in 1953 by James Watson and Francis Crick of Cambridge University, using the X-ray diffraction studies of Rosalind Franklin.

Watson and Crick showed that DNA consists of two interlocking spiral chains of organic molecules which form the famous double helix shape. The two spirals are composed of units called nucleotides. 

Nucleotides are four different nitrogen-containing molecules. Each of the four can base pair only with one of the other.

Adenine base pairs with thymine to form the AT pair, and guanine with cytosine to form the GC pair. These two pairs of nitrogenous bases form the connections between the two chains of the double helix, thus holding the large DNA molecule together. The sequence or arrangement of AT pairs with respect to GC pairs is the information or instruction code for life.

Living organisms copy DNA using a special molecular machine that base pairs the AT and GC pairs in the correct sequence using one strand of DNA as a template. This molecular machine not only accomplishes the correct ordering of the base pairs but also positions the molecules in such a way to favor only Watson-Crick base pairing.

But how could this enzymatic process have occurred in the days before life existed? Scientists aren’t sure, but they are learning more about the goldilocks conditions required for non-enzymatic DNA base pairing.

In a paper published in the journal Chemical Communications on Sept. 9, Tomislav Stolar and colleagues show that DNA base pairs can be formed by subjecting nucleotides to high heat in the solid state and under certain chemical conditions. 

Whether those conditions actually existed on ancient Earth before the advent of life remains a key question. 

In the experiment carried out at the German Electron Synchrotron in Hamburg, the research team used a modified form of cytosine and guanine as their raw materials to attempt to cause these molecules to self-assemble into the C-G pair, as they do in living DNA. 

In order to do so they had to impose three conditions not necessarily seen in the primitive prebiotic state. 

First, they had to alter the composition of the two nitrogenous bases by replacing the nitrogen-hydrogen (N-H) group with a methyl (CH3) group. This method prevented the tendency of the molecules to form links, known as hydrogen bonds, at places where they are not wanted.

Second, they had to dry the chemical solution from a liquid into a solid state, in this case a powder. This, again, reduced the likelihood of unwanted hydrogen bonding. 

Third, they had to heat the solid mixture to temperatures above 200ºC (392ºF). 

The authors note that the latter two conditions—heat and drying—could have been present in early Earth when earthquakes, volcanism and wet-dry climate cycles might have allowed chemical solutions to form, dry into a powder, and then become heated over long periods. 

The third condition, methylation or replacement of the N-H with CH3 groups, may be more problematic. 

The authors acknowledge the limits of their laboratory system for modeling prebiotic conditions. But they believe their experiments can still provide important insights into the DNA self-assembly process.