Double-chromosomeDNA

Double life: Why life prefers doubles

Let me start at the outset by saying that this article has more questions than answers or explanations. It started with the question of “why does nature love doubles”? I mean, think about it; from double stranded DNA and two of each chromosome, to two matching sides that give us two of almost every organ. Throughout the living world, doubles seem to rule. But why?

If we start at the genetic level there are already several unanswered questions. Why the double helix? Although there are many theories, the most well-accepted theory regarding the origin of life claims that it all started with a bunch of RNA and proteins and that somehow DNA emerged as a more stable alternative to RNA. During the transition period, viruses evolved to use RNA, single-stranded and double-stranded DNA. However, it has been suggested that the high intensity of UV radiation hitting the Earth at the time would favour double-stranded DNA above other forms of genetic material as it was more resistant to the damaging effects than its counterparts. Alright, so that seems like a reasonable explanation. But then why have two of each chromosome?

My quest to find the answer to this question led me first to some more questions. Firstly, why and how did diploidy evolve? Even organisms that are not strictly diploid tend to have chromosome numbers in multiples of two. Bacteria were perfectly happy in their haploid lives but once the eukaryotes began to evolve, at some point diploidy became the norm as gene transfer and sexual reproduction became favoured. There are a few theories as to how this transition to increased ploidy may have occurred, but again, there is no final verdict on what exactly led to the shift. The working theories are that the early eukaryotes would take up bacteria and retain them if useful (mitochondria are evidence of this process) or fuse with other cells. Certain genetic elements may have found replicative advantages from this mixing of material and led to it becoming more prevalent. Thus, sexual reproduction evolved! The increased ploidy allowed for some genes to remain unused in the organism perhaps until a time when they could become useful. Still, experiments in yeast have shown that haploid and diploid cells can be considered equally “fit” depending on the conditions in which they are put. Either way, sexual reproduction sort of explains the whole double chromosomes thing.

So actually, this whole piece started with the question: why do we have two of most organs? The concept of being able to divide (more or less) equally down the middle has extended from the molecular level to the organismal level. Of course, we know that most eukaryotic, sexual, vertebrates such as ourselves are bilaterally symmetrical and can be neatly folded into two along a vertical axis. But why? What makes bilateral symmetry so beneficial? As it turns out, according to some fancy calculations and complicated formulae, in relation to movement and manoeuvrability, the other forms of symmetry just do not allow for the same mechanics as bilateral. That is to say, if you need to move, being bilaterally symmetrical is best. Since it confers directionality along the length of the body, it allowed for optimisation of the sensory organs and central nervous system. Specific advantages include depth perception by two eyes and the appropriate field of vision to either run from a predator or stalk prey. Two ears have similar advantages allowing the listener to sense directionality of sounds. Overall, it still seems a little unclear as to why some organs were needed in pairs and not others. But I suppose we just have to trust that this design worked best for the longest time and that’s why it’s still around!




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