What would a genetic molecule look like if it had 3 strands?

I had a thought about astrobiology recently.

Given that RNA viruses exist and the RNA World hypothesis link to original article which would obviously be single stranded, and the standard double-stranded DNA genomes across the tree of life; could a triple-stranded genetically encoded molecule work?

This lead me down a rabbit-hole of “dipping my toes” into the current research in astrobiology. (Throughout this I will be using as much of current genetics terms as possible to properly convey my ideas.)

According to Benner (2017) link to paper, encoded genetic molecules need to have a minimum set of characteristics. It needs to be water soluble, is a polyelectrolyte (many charges, either positive or negative), and needs to follow the principles of Darwinism. Benner called this a biopolymer. I came up with three different potential conceptualizations of a genetically encoded biopolymer trimer: 1, all strands contain encoded genes; 2, one strand has promoters while the other two are encoded; and 3, there are two regulatory strands and one coding strand.

1. All strands contain encoded genes

This is probably the simplest way to envision a trimer. This would mean that is there was complementarity, it could only happen in a 3-based system. Instead of A to B and vice versa like in DNA, there might exist something like A to B to C to A in terms of their bases. This is much more complex than ours and that level of complexity might not be able to sustain itself. But there are many inefficient systems present in Earth’s lifeforms, so that may not be a relevant argument. Anti-parallelism couldn’t exist, so semi-conservative replication as we understand it also could not exist. A fully conservative replication model might make sense. There might also be different modules of information: alleles could possibly be described in the same way; and chromosomes are already more complex in plants (hap-, di-, and triploidy) so this might be more common. An interesting thought could also include the potential for a three-parent inheritance system.

2. One promoter strand, two coding strands

This could mean that there is complementarity between the two coding strands and they could be anti-parallel. The non-complementary (non-coding) strand could allow for regulation. Binding to the non-expresses “coding” strand might allow the expressed strand to be exposed and therefore undergo transcription into a the functional biomolecule (proteins for us). This proposed method mimics how histones can regulate genes and whole regions of an eukaryotic chromosome by tightly binding, leaving coding sequences unavailable for transcription initiation.

3. Two promoter strands, one coding strand

This last proposed method seems to be more of a stretch and is albeit the least developed here. The “regulatory” strands might act in opposite to what was proposed in the second method: the regulatory strands are bound together by default and when one or both transiently bind to the coding strand, the nature of being double or triple stranded would allow for transcription initiation.

Summary

The above musings are just a collection of thought experiments or “what if’s” about astrobiology, but it is interesting to test one’s knowledge of genetics if something, so fundamental we take it for granted, is altered slightly.