There are three references that I added today. They, as well as others, can be found under the page tab called Additional References.
In this post, I would like to focus on the one about horizontal gene transfer. As stated in the reference, this is different from sexual transfer (also known as vertical gene transfer) because it does not involve parents.
Here is an excerpt from the article. If you wish, just skip on down to the next paragraph which explains it in simple english:
“Prokaryotes can exchange DNA with eukaryotes, although the mechanisms behind this process are not well understood. Suspected mechanisms include conjugation and endocytosis, such as when a eukaryotic cell engulfs a prokaryotic cell and gathers it into a special membrane-bound vesicle for degradation. It is thought that in rare instances in endocytosis, genes escape from prokaryotes during degradation and are subsequently incorporated into the eukaryote’s genome.”
For those readers not of a biological scientific bent, let me explain this in plain english: Prokaryotes are bacteria, eukaryotes are cells like we have. Enough said about that for now. Note how this excerpt says the process of horizontal gene transfer in eukaryotes is not well understood. It goes on to suggest that it is possible gene donor cells like bacteria may be ingested via endocytosis and instead of being digested and “eaten”, somehow DNA from these cells manages to escape the process and enter the nucleus where it can hook up with the eukaryotic DNA. It provides no specifics on how all of this is done.
Now pay very close attention to what I am about to say here: For DNA to get out of a digested bacteria and somehow escape into a nucleus is virtually impossible. Its best chance of doing this is in the form of a plasmid which is a small circle of DNA that is little more than a naked DNA virus. However, cellular restriction enzymes will usually turn circular DNA into linear DNA which will be completely degraded by other enzymes. Horizontal gene transfer can be done via genetic engineering by flooding the cell with DNA and swamping out the cellular digestive enzymes, but this is artificial brute force imposed by humans, not the rule in nature. Viruses are really just plasmids with a protective coating that allows them to invade the nucleus without getting destroyed in the process.
So, the question still remains. How can some hapless bacterium or small eukaryote like a yeast or protist (think amoeba or algae) be ingested by a large eukaryotic cell and digested just enough to release fragments of its DNA such that they can be incorporated into the host DNA? It can’t be done under natural conditions because the DNA needs to be protected in some way. This can happen if the bacterium or other cell simply invades the nucleus intact, just like a virus. Once there, it needs to find a mechanism to attach it’s DNA to the nuclear DNA if it is going to be replicated as part of the cellular DNA. Otherwise, it will simply be diluted out during the next cell division (see abortive transduction). How this could happen is the crux of this entire blog. Suffice it to say here that there is a DNA attachment site somewhere on the donor DNA that plugs into the host DNA, just like a virus. During subsequent cell replications, this donor DNA may get pared down to the bare essentials or completely removed based upon whatever desirable traits it provides to the host. Such is the basis for natural selection based upon environmental pressures.
Ok, please don’t freak, I’ll explain, as I always try to do. I hesitate to go into great length about the term endosymbiosis because it has been explained in great length throughout this blog, so I’ll be brief here. Any cell that incorporates into another larger cell and survives the process becomes an endosymbiont. If it causes a disease in the process, it is a parasite. If it provides some benefit to the host, the relationship is called mutalism. When I say endosymbiotic “memory”, what I mean is that even though the original cell donor may have been pared down over time to the point that it can no longer function as as an independent cell, it still “remembers” where it came from. During the course of host cell death, this memory may be invoked much like a virus that emerges from dying bacteria or eukaryotic cells (see temperate phage). In other words, the donor DNA tries to “escape” from the dying host cell and become independent once again. In the case of temperate viruses, it works because they still have the means to form a new virus particle. In the case of a degenerate endosymbiont, this results in a separation of the donor DNA from the host DNA, much like a virus, but it does not have the capacity to form a new donor cell because it no longer contains all of it’s original DNA. Such DNA may have the potential to escape from a dying cell and enter a new host cell at some random attachment site. By doing so, it can corrupt the genetic machinery of the new host cell and even impose its “will” upon it by forcing it to replicate in order to replicate the degenerate endosymbiont. In the human body, as well as in others, this could lead to uncontrolled cellular replication or cancer. Our cells use a remarkable process called apoptosis to sequester DNA during cell death so that it can be safely degraded.
Notice the elephant in this post? I never talked about how a bacterium could penetrate the nuclear envelope using nuclear pores that are far too small for this to occur. Maybe I’ll do that in the next post.