Simon Anders
Valued Senior Member
Why do some species exhibit remarkably streamlined chromosomes, while others possess massive amounts of non-coding DNA?
C-value enigma
- Thread starter Simon Anders
- Start date
Actually the answer is very complex. One aspect is that streamlined genomes are mostly found in organisms in which there is selective pressure for maintaining the genome as small as possible. These restrictions might stem from morphological aspects as in viruses (their protein hull can only contain a limited amount of nucleic acids) or microorganisms (increased nucleic acid synthesis results in a higher burden of their fitness). However, as it becomes more and more evident, even non-coding DNA regions may provide a number of function, many of them of a regulatory nature.
The regulatory nature of non-coding DNA has become one of the hottest, perhaps the hottest, topic on genetics. The number of manuscripts dealing with the regulation of gene expression by microRNAs has exploded in recent years.
Yes indeed, the C-value paradox is a very complex question with no simple answers. There are a number of reasons why eukaryotic organisms have accumulated non-coding DNA. There are duplicated protein-coding genes and tandemly repeated genes, as well as multiple copies of other DNA sequences in the genome, generally referred to as repetitive DNA. Some of these sequences are quite short and occur as tandem repeats; others are much longer and are interspersed at many places in the genome.
So why is all this apparently superfluous DNA tolerated (in the evolutionary sense)? Well, as stated above, it can have a function in gene regulation as well as other identified functions such as attachment of chromatin to sites within the nucleus, and aspects of genome organization. There are almost certainly other not-yet-identified functions. So it is likely that non-coding DNA must be maintained through evolution because without it the cell would be non-viable..
A second possibility is that much non-coding DNA has no function but is tolerated by the genome because there is no selective pressure to get rid of it. Possession of non-coding DNA is neither an advantage nor a disadvantage and so the non-coding DNA is simply propagated along with the coding DNA.
Yes indeed, the C-value paradox is a very complex question with no simple answers. There are a number of reasons why eukaryotic organisms have accumulated non-coding DNA. There are duplicated protein-coding genes and tandemly repeated genes, as well as multiple copies of other DNA sequences in the genome, generally referred to as repetitive DNA. Some of these sequences are quite short and occur as tandem repeats; others are much longer and are interspersed at many places in the genome.
So why is all this apparently superfluous DNA tolerated (in the evolutionary sense)? Well, as stated above, it can have a function in gene regulation as well as other identified functions such as attachment of chromatin to sites within the nucleus, and aspects of genome organization. There are almost certainly other not-yet-identified functions. So it is likely that non-coding DNA must be maintained through evolution because without it the cell would be non-viable..
A second possibility is that much non-coding DNA has no function but is tolerated by the genome because there is no selective pressure to get rid of it. Possession of non-coding DNA is neither an advantage nor a disadvantage and so the non-coding DNA is simply propagated along with the coding DNA.
Simon Anders
Valued Senior Member
Thank you both for your answers. I have to say I found the issue odd. My impression is that there is not a lot of useless redundancy in nature - or? - so it seemed odd that our DNA had so much 'junk' and that for many in the field this was reacted to with a shrug.