DNA. Public Domain
(Click on the following link to listen to an audio version of this blog .. DNA is Everywhere
A living organism contains a great deal of DNA. In a single human cell, for example, there is, if you uncoiled and stretched out and connected all of the individual pieces, about 6 feet of DNA. The most recent estimate of the number of cells in the human body is about 10 trillion, so, multiplying these two values together, we have, on average, 60 trillion feet of DNA in our bodies. 60 trillion feet equals 10 billion miles. For an off-the-wall, astronomical comparison, the planet Neptune is “only” 2.7 billion miles from Earth! Your DNA could just about stretch, back-and-forth, two full round trips to Neptune! That’s a lot of DNA, and that’s just one individual of just one species! Considering 8 billion people on Earth, and all of the other species we share this world with, the DNA-math begins to get completely overwhelming!
Bacterial DNA. Figure by Spaully, Wikimedia Commons
DNA in a bacterium is a large, circular molecule that seems much too large to escape out through the bacterial cell membrane and cell wall. Bacterial DNA, though, is also found in very small, also circular, bits called “plasmids,” which, apparently, can easily transverse the boundaries of the bacterial cell. Plasmids are abundantly found in the micro-environments surrounding bacteria and can re-enter almost any type of bacterial cell in that environment! Plasmids can contain all sorts of genetic information including mechanisms to resist antibiotics and information on how to make toxins.
Eukaryoti cell. Figure by Medirian. Wikimedia Commons
In larger, “eukaryotic” cells (like the cells of humans and other animals and also fungi, protists and plants), DNA was once thought to be totally confined within the double membranes of the nuclear envelope inside of a cell or within the double membrane envelope of a mitochondrion. It was thought that these highly protected environments were the only places where the fragile, eukaryotic DNA could exist.
This locational restriction on DNA, though, is not accurate. It has been determined that DNA, both bacterial and eukaryotic, is everywhere! It is in water, in soil and even in the air. It can persist for many thousands of years in these non-nuclear environments and can be collected and identified down to the species that originally produced it. This DNA from the environment is called “eDNA.” (see Signs of Spring 3, March 24, 2022).
Cave in Caucasus. Photo by Codwiki, Wikimedia Commons
Collection and identification of eDNA is now a routine sampling method in aquatic ecology, and the feasibility of describing terrestrial biotic communities from soil and air eDNA is being perfected. In anthropology techniques have been developed to allow sweeping for eDNA in ancient human habitations. Recently, 25,000 year old human DNA and also DNA from extinct wolves and bison were isolated from the sediments and debris in caves in the Caucasus region of western Georgia. In forensics, human eDNA in both the air and on surfaces have been collected as part of crime scene investigations.
DNA can also be isolated and identified from blood feeding insects and other invertebrates (see Signs of Summer 10, July 21, 2022). This “invertebrate-DNA” (“iDNA”) has been used to map the movements of 86 different species (ranging from endangered Yunnan spiny frogs to Asiatic black bears) in the Ailoshan Nature Reserve in China. iDNA from collected mosquitoes has also been used in the Everglades to track down invasive Burmese pythons!
Photo by Aka. Wikimedia Commons
To illustrate just how abundant and diverse eDNA is, consider a study published last summer in Biology Letters (June 15, 2022). Researchers in Germany evaluated the eDNA in the dried tea packaged in tea bags. Amazingly, in a single tea-bag (100 to 150 mg) of green tea, they found eDNA from over 400 different species of insects!
Some plants have been shown to release micro RNA’s (miRNA’s) from their roots! Researchers speculated that these miRNA’s may function as communication molecules alerting adjacent plants of the secreting plant’s level of stress, or these miRNA’s may be chemical instruments of competition. Plants absorbing the miRNA’s may have some critical metabolic pathway interfered with or shut down thus giving the secreting plant a competitive advantage. miRNA’s are quite delicate molecules, and its plant to plant transfer has only been observed in plants growing in liquid growth media. Future experiments are planned to see if these miRNA’s can survive and be transferred in soil.
So, we have nucleic acids floating around in surface water systems, flying around in the air, accumulating in soils and sediments and , possibly, soaking into soil or ground (or drinking) water. It may even be floating around in our tea cups! Add all of these to the incredible number of viruses that are flying and floating about, and it is easy to say without exaggeration that we are being constantly being plastered with nucleic acids of all kinds. It may be very improbable that an individual nucleic acid molecule could get into a host cell and cause changes, but it seems absolutely inevitable that one of all of these billions and trillions of nucleic acids hitting each and every one of us could (and would) accomplish some sort of genetic “message” delivery!
HGT illustration. Figure by A. R. Burnmeister, Wikimedia Commons
The message that these DNA’s could deliver are collectively referred to as “Horizontal Gene Transfer” (HGT) (also called “Lateral Gene Transfer” (LGT)). HGT involves genetic material from one species moving into the genome of another species. HGT has been extensively describe in bacteria and can occur by direct plasmid exchange between bacterial cells, by plasmid uptake by one bacterium from its environment or by viral infusion of genetic material into a bacterium.
Until recently, though, the idea of HGT in eukaryotic cells had been met with skepticism by the scientific community. New observations, though, and increasingly effective methods to identify and classify DNA, have led to an increasingly widespread acceptance of eukaryotic HGT.
Virus in green dye. Photo by Quimira, Public Domain
Viruses have been shown to transfer genes between eukaryotic cells. Cell membrane vesicles have been shown to take DNA fragments from one eukaryotic cell and deliver them to another. Single cell eukaryotes (protists) that phagocytize bacteria or other protists have been shown to acquire and assimilate DNA from their prey. Cells (both bacteria and protist) that live in the cytoplasm of other, larger cells (either as endosymbionts or as endoparasites) exchange DNA with their hosts. For example, the protist Plasmodium vivax, which causes the disease malaria in humans, acquires human genes as it lives in the cells of its host which assist in its ability to evade the human immune system.
There is evidence of HGT occurring in fungi, protists, cnidarians, mites, insects, nematodes, fish, mammals and land plants. Quite possibly, the evolution of land plants depended upon HGT (see Signs of Winter 9, February 13, 2020). One paper recently asserted that of the 20,000 genes in the human genome, 100 came from HGT exchanges. Although many researchers feel that this is an overestimate of the HGT load in humans, almost everyone agrees that HGT has had and continues to have an impact on human evolution.
No one has yet described the specific mechanism by which eDNA reaches a cell’s nucleus and then is incorporated into the eukaryotic genome. The incredible abundance of eDNA around us, though, does make even unlikely mechanisms for uptake and assimilation seem almost inevitable.