Write-Up #3- The Myth of Junk DNA
Hello everyone, I am back with another information post! This one is somewhat in response to a number of questions I’ve gotten that relate to “junk DNA”, and I wanted to clear up some misconceptions.
The concept of junk DNA is a fairly old one, and came from when scientists didn’t have a great understanding of how genes worked. When genetics got its start as a science, we looked primarily at bacterial genomes, because they are small, easy to get, and we can have lots and lots of identical organisms. And when we looked at bacterial genomes they looked something like this:
Lots of bacterial genomes are circular, like our friend A. arilaitensis here. And each of those colourful rectangles around the edge represent a coding region for a protein. Bacteria have extremely dense genomes with regards to the amount of protein coding regions and proteins that they produce, but they have to be. There just simply isn’t a lot of space inside the cell for them to have any extraneous DNA floating around. If a gene isn’t being used, then it is removed, which is in sharp contrast to higher organisms that can keep genes around just for the fun of it (and in case it becomes useful later, or if the environment changes).
So with bacterial genomes in mind, we can shift our attention over to some very different genomes. Let’s go back to my previous info post and look at the structure of a eukaryotic gene from there:
There’s quite a bit more here that wouldn’t be considered part of the “gene”, but are still important- the upstream and downstream enhancers/silencers, as well as the introns are not coding regions but are equally important to expression. However, when we first studying these genes, the people looking at it went “these are not coding, they produce no protein, what is the point?”, and they were labelled “junk DNA”. As well, a lot of these supposedly “non-coding” regions were found later to have other important functions related to DNA storage within the cell, as well as epigenetics.
To further this, not every non coding region has a function either. We do also have long stretches of basically nothing that still serves an important function in protecting us from harm.
The first and foremost of these are the telomeres, found on the ends of each chromosome. Because of how DNA replicates in our cells, a little bit is lost off of the end each time, just the space that it takes for protein to bind to begin replication. If it weren’t for these telomeres, we would very rapidly start losing coding regions of DNA, and interrupt some very important cellular processes.
The other areas that are more central in the chromosome mainly function as a buffer region in case of mutation (or we haven’t found the function yet!). There is a certain level of mutation expected with every replication (it varies from organism to organism, some are better at replication than others), as well as from environmental factors (UV rays, other types of radiation, stress on the organism, etc), so by having lots of areas where there are no genes being coded, it increases the chances that mutations will happen where it won’t make a difference to the function of the organism. Bacteria frequently pick up catastrophic mutations during binary fission, but because they replicate so quickly that organism will be quickly replaced (listen, the bacterial lifestyle is a harsh one). In more complex organisms, a lot more energy is invested into ensuring that DNA is replicated correctly, or at least in a way that doesn’t cause any harm. To lose these non-coding regions would be catastrophic, because it simply wouldn’t be possible for our DNA repair mechanisms to keep up with the number of mutations being thrown at them.
I think I’ve fairly effectively driven home this point now- there isn’t really anything that geneticists consider “junk DNA” (the term more commonly used now is “non-coding DNA”), and to lose these regions would have a negative impact on the organism that it affected. It’s unfortunately a myth that a lot of science education hasn’t quite caught up to correcting yet!
Thanks so much for reading, and I’ll be back in about a month with “How to Draw and Use Punnett Squares”!
