Humans posses more viral DNA in our genome than DNA that codes for human proteins. Only 1.5% of the our genetic code, called the haploid human genome, actually codes for proteins that form the building blocks of our bodies. Research indicates approximately 8% of the human genome is made up of “endogenous retroviruses“, DNA relics of ancient viruses that embedded their genetic code into our ancestors millions of years ago, meaning there is about four times more viral DNA in our genome than human protein-coding DNA.
Retroviruses
A Retrovirus is a virus that has RNA inside a protein capsule. These viruses operate by attaching to compatible protein receptors on the outside of cell membranes and injecting their RNA into the host cell. The RNA then undergoes a process known as reverse transcriptase (creating DNA from the original RNA coding) by utilizing viral enzymes and then integrating the new viral DNA into the host cell’s genome.
As a result of the mechanism by which retroviruses work, it is possible for their DNA to become incorporated into a species’ genome by way of infecting a germ line cell. These cells are the precursors to sperm and egg cells. A retrovirus that integrates with a germ line cell is called an endogenous retrovirus. Endogenous retroviruses become a part of the offspring’s DNA because it has been integrated into the reproductive cells of the host. Scientists are now taking this information a step further by investigating the possible ramifications this process of amassing viral DNA into animal species’ genomes may have on the evolution of life.
Endogenous Retroviruses and Evolution
It is staggering that 98.5% of the human genome is not used to code proteins. How does our cell manage to find the minute portions of meaningful genetic code amongst a sea of otherwise useless code? There is a regulatory network in place within the genetic code that tells enzymes the appropriate places to start and stop coding along the DNA strand. That code is made up of master, or regulatory, genes that tell enzymes when to turn other genes on or off. This led to greater control over gene expression in higher vertebrates, thus explaining why some animals (such as humans and chimpanzees) can have remarkably similar genomes while outwardly producing vastly different creatures.
Evolution is typically viewed as a long term, on-going process that requires millions of years to develop recognizable mutations. New evidence from a recent study led by researchers from the University of California – Santa Cruz indicates that interaction with endogenous retroviruses may have precipitated immediate mutations that, if beneficial, resulted in substantial changes to a species over a short period of time. This could change the way evolution and the tree of life is constructed.
Endogenous retroviruses produce quick mutations by way of the regulatory gene system. It appears that approximately one-third of our regulatory genes are actually the genetic remnants of ancient retroviruses long since extinct in the world around us. The on and off switches for our genes are actually viral DNA incorporated into our genome.
Retroviruses as the Precursor to Higher-Life on Earth?
Scientists have also discovered gigantic retroviruses that appear to share similar characteristics to viruses from hundreds of millions of years ago. These viruses contain vast genomes with many genes and large quantities of useless “junk” DNA, just like ours. It has been postulated that these ancient retroviruses may have infiltrated prokaryotic (think bacteria) cells and became the nucleus; this union created the eukaryote cell, the single-celled predecessor to vertebrates and ultimately all higher-life on Earth.
As the years passed by and endogenous retroviruses integrated with various species’ DNA, sometimes causing evolutionarily beneficial mutations, the viruses themselves vanished from the world. Their genetic relics live on in the species of the world around us, and in ourselves. These viruses are a large reason for much of the genetic diversity and splendor of the variety of life on this planet.
Image Used in this Post
Mural Andrew Vírus atacando Célula courtesy of flikr user Bruno Biagioni Neto under the CC license.
8 Comments
It’s kind of insane to think about how much activity goes on inside of our bodies without us ever knowing it.
What’s stunning is the shear tenacity and complexity of life that stems from basic fundamental principles. Considering that life could benefit immensely, and as the evidence is suggesting, almost instantaneously from viral mutation and presence in DNA is remarkable.
As you suggest J, this will have a profound effect on the understanding of evolutionary science – it shall be fun to which how this ripples through the scientific community. Also this piece plays nicely into the hands of the experimental astrophysicists who maintain that life is truly a virus that has an amazing ability to thrive in regions of space where just a few simple conditions are met.
This was good stuff.
***Crazy theory alert***
Parlaying off of Greg’s comment with viruses in space, what if the viruses here on Earth traveled from some distant region of space in a comet or some other stellar object? Could we be the hybrid of indigenous single-cell life combined with the instructions for more complex life from an alien source sent using viruses as the messenger? Sweet.
Interstellar pollination?
That is an excellent picture by the way.
Jason wrote: Parlaying off of Greg’s comment with viruses in space, what if the viruses here on Earth traveled from some distant region of space in a comet or some other stellar object?
A lot of people don’t consider that the entire “crust” level of Earth is the product of billions of years of cosmic dust settling(100 million tons of consmic dust settles on earth everyday). Since we have never drilled beyond the crust and into the mantle, human beings have technically never touched the native mass of out planet. We have only interpreted the mantle, outer, and inner core based on what we know from the crust(geographic sound waves, etc.) That being said, everything we know of is from outer space.
Example: There is a set amount of gold on Earth. It can be melted down and dilluted into less than 24K, however, the mass will never change until Earth passes through another cosmic cloud of gold in space. Imagine that? Golden rain? Gold, like everything is alien to this planet, especially life, water, and viruses.
sorry for the mistakes… *cosmic *our
Indeed – as Carl Sagan eloquently put it, “we are all made of star stuff.”