Hi,

Of the trillions of cells that compose our body, from neurons that relay signals throughout the brain to immune cells that help defend our bodies from constant external assault, almost every one contains the same 3 billion DNA base pairs that make up the human genome – the entirety of our genetic material. It is remarkable that each of the over 200 cell types in the body interprets this identical information very differently in order to perform the functions necessary to keep us alive. This demonstrates that we need to look beyond the sequence of DNA itself in order to understand how an organism and its cells function.

After the Human Genome Project, scientists found that there were around 20,000 genes within the genome, a number that some researchers had already predicted.  Remarkably, these genes comprise only about 1-2% of the 3 billion base pairs of DNA.  This means that anywhere from 98-99% of our entire genome must be doing something other than coding for proteins – scientists call this non-coding DNA.  Imagine being given multiple volumes of encyclopedias that contained a coherent sentence in English every 100 pages, where the rest of the space contained a smattering of uninterpretable random letters and characters.  You would probably start to wonder why all those random letters and characters were there in the first place, which is the exact problem that has plagued scientists for decades.

Why is so much of our genome not being used to code for protein? Does this extra DNA serve any functional purpose? To start to get an idea of whether we need all of this extra DNA, we can look at closely related species that have wildly varying genome sizes.  For instance, the genus Allium , which includes onions, shallots, and garlic, has genome sizes ranging anywhere from 10 to 20 billion base pairs.  It is very unlikely that such a large amount of extra DNA would be useful in one species and not in its genetic cousin, perhaps arguing that much of the genome is not useful.  Furthermore, these genomes are much larger than the human genome, which indicates either that an onion is highly complex, or more likely that the size of a genome says nothing about how complex the organism is or how it functions.

Hope this helps.

Good luck.