Going viral – how much of our DNA is truly human?

The human genome contains all the genes that make us human. They are the instructions for our cells to act like human cells rather than the cells of any other organism. Given how differently we consider ourselves from other organisms (even our closest relatives – the primates), you might be surprised of how much of our genome is not really ours!

The basic of our DNA

DNA is an array of just four different basic building blocks named A, T, G, and C. The specific pattern in which they repeat through the length of the DNA molecule is what is important for the function of each gene they make up.

Our genes are stored in the double helix molecule of DNA. Different lengths and stretches of this DNA molecule contain our genes. Each gene is a concise instruction unit for our cells. The specific combinations and arrangements of different genes along the strand of DNA in our cells is what makes each species different from one another. 

Throughout evolution, many genes were conserved in the evolutionary tree – the ones important for survival at the conditions of the given era. Others became obsolete since they didn’t serve a function anymore in the new environment are were gradually lost. The evolution of species is essentially the evolution of the genetic code 

The ever-changing code

Here’s a brilliant video that explains some of the ways genes evolve. It is produced by 
Stated Clearly. You might want to check some of their other videos on genetic evolution – it’s fascinating stuff and I couldn’t possibly put it all in words here because it would end up being a whole college course in molecular evolution (or two).

The genes that came from someone else

Escential to remember – all our genes come from our ancestors one way or another, but have evolved through time. 

Some genes though came through more than just natural evolution. Parts of our DNA actually come from invaders of our bodies, whose genes or sometimes even whole genomes get imported in our own DNA.

Viruses infect us, they enter our cells and make them produce more copies of the virus. That way it can leave this cell and infect other cells. That’s how the virus spreads through our body and to other hosts too.

There is a special type of viruses though – the so called retroviruses

The invaders that took home

Retroviruses, unlike other viruses, need an extra step to be able to replicate and spread. They need to insert their own genes into the DNA of the host cell and via this route trick the cell into producing more viral particles. 

If such retrovirus happens to infect a sperm cell or an egg cell, it can get incorporated into the genome of the new individual made when these sperm and egg fuse. The virus can further get inherited in the DNA of this individual’s children and their children, and the children of their children… You get the picture!  After thousands and millions of years, it might turn out that this viral DNA is still found in the genome of many many people, ancestors of this original host.

That, for the record, is a totally wrong representation of how retroviruses look like, in case you were doubtful. I honestly wanted to give you the credits to the person who first drew it, but it's become so popular, that was impossible for me to trace the creator of the cartoon.

Becoming one with our viral enemies

Such infection and inheritance events have happened many many times in human history and the history of our hominid ancestors. So many times, that in fact scientists today know that about 8% of our DNA is actually viral in origin. 

Viral genes behave differently as part of our genomes. In some cases we’ve made them our own. Some have no function anymore and have also lost their viral properties. Such sequences are often referred by scientists as “junk DNA”.

Others, have mutated in a way to get new functions of relevance for our organism. In some cases they regulate the behaviour of other genes nearby. In other cases, they are responsible for such major evolutionary leaps that without them we wouldn’t be the species we are today.

"I am the virus and the virus is me"

How much of us is human after all?

This graph shows the average distribution of sequences throughout our genome:

Source: NHS National Genetics and Genomics Education Centre

Before you freak out that you don’t understand what all these labels mean, let me draw your attention to the part of the graph that matters:

The yellow piece of this pie contains all the protein-coding genes which make us human!

Proteins are responsible for building many parts of our cells, they are enzymes we use to digest food, proteins transport oxygen and CO2 in our blood so we can breathe… Without proteins we wouldn’t exist. 

The truth is, some of the other pieces are also vital – for the regulation of how genes and respectively proteins work in our body and when. But it still gives you an idea of how little of what defines us is truly ours! Some of those appropriated viruses I talked about above are also part of this yellow section, others are part of the green and purple sections.

And for big part of the rest of this pie-chart, we have no clue what the role of these sequences is for our existence. For some parts we know from experiments that they are vital, because removing them causes severe malformations or death. But we don’t yet know how are they important exactly.

Don't take yourself so seriously!

Now that we know how much we owe on viruses for being at all a thing, maybe we should remember to be more humble and grateful for what nature has given us. Also, if it makes you feel better, next time you are down with a viral infection, imagine that just may be you are setting the evolutionary path so something amazing!

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