Why does our genome have so many blank spaces? According to some calculations, up to 80% of our DNA does not code for proteins or functional RNAs, or has any known regulatory functions. DNA is neatly packed in chromosomes to facilitate its separation during cellular division. We are able to perfectly manipulate genetic information from practically every organism at both small (gene level) and big (chromosome and genome) scale. Yet again, we have absolutely no idea what does all this DNA do and why is it there!
Logically, many people think that the number of chromosomes (karyotype) directly corresponds to the complexity of a species. But what if I told you that you and I have 46 chromosomes, but a sheep has 54! Not saying that sheep are less evolved than humans, but it will probably make you reconsider “the more, the better” perception of the karyotype. It happens so that many plants have a surprisingly high number of chromosomes. In the case of domesticated plants this is explained by a large number of chromosomes making up for the increased beneficial trains of those plants, for which they get selected – higher yield, greater fitness to the environment. The general belief is that in the more evolved species selection has made better use of their DNA retaining, duplicating, alternating useful genes and DNA regions, and has tried to get rid of most of the useless, faulty, and damaged DNA. This is known as the “genome size paradox” – some of the more recent species have smaller genomes and more genes encoded in it – making for a higher concentration of genes per chromosome. But this is pretty much where all logic ends – chickens for example have twice as many chromosomes as cats, and pigs have half as many as the wolf.
There are many ways via which genomes change their size. Gene, chromosome or whole genome duplications increase the size of the genome while deletory mutations, chromosome aberrations, errors in chromosome copying and separation, all decrease the size of a genome. After such event occurs, the final word has the natural selection – if an organism with such genome rescaling is able to survive and more importantly – to propagate and give viable progeny, several millions of year later, this species might end up with a completely different karyotype and probably looking very different too.
One of the logical beliefs scientists have about the size of genomes and especially about the huge amount of non-coding DNA in it, is that it serves as a buffering zone for mutations and pathogens. Viruses for example survive by inserting their own genome in the genome of the host, often in a completely random spot, then they start using the host cells’ synthetic machinery as their own. Many chemicals and UV radiation cause random mutations in the DNA of each and every organism. More evolved organisms in general have sophisticated enough sets of enzymes to cope with these mutations. But having more than half of the DNA as a non-essential stretches to be targeted by those, often bad, mutations is a relatively easy to maintain safety buffer preferred by a number of species.
Only recently scientists rediscovered their fascination by the karyotype and started working in the lab and behind the computers towards unraveling the mysteries hidden in our chromosome number.