By now, horizontal gene transfer as a topic of discussion should be familiar to you. Horizontal gene transfer describes the transfer of genetic material between organisms in any way that is not by vertical transmission (or the transfer of DNA from parent to offspring). If it has not been mentioned before, become familiar with the fact that horizontal gene transfer is common in prokaryotes, but extremely rare in eukaryotes (Alberts et al. 2014).
An example of gene transfers between organisms that emphasizes this topic greatly is the common virus. Viruses, we’ve all had them before. From the common cold to chickenpox, it would be near impossible for any of us to make the claim that we’ve never experienced their effects. Though it is a bit of a controversial topic at times, the prevailing view is that viruses are not living things themselves, but rather they are small packets of genetic material. Viruses are more so vectors for gene transfer than they are living. Viruses can be aptly described as parasites to the reproductive and biosynthetic machinery of the host cell (Alberts et al. 2014).
The way that viruses work is by first replicating in one cell of the organism that it is infecting. Following this, the virus emerges with a protective wrapping, and is free to repeat this process for cells of the same or different species. When this occurs, it is possible for a cell to be killed due to the spread of virus particles inside of it. However, in some cases, the virus may operate differently. Rather than generating these particles directly, the virus can choose to persist inside of its host for quite. During this time, the virus remains in a state of apparent harmlessness, as either a separate intracellular fragment of DNA termed a plasmid, or as a sequence inserted into the regular genome of the targeted cell. In prokaryotes, it is common for viruses to pick up DNA fragments from the genome of one host cell and ship them to another cell (Alberts et al. 2014).
As mentioned previously, horizontal transfer of genes is rare in eukaryotes. Examples of horizontal gene transfer are much more common in prokaryotes. Prokaryotes, in fact, often times have a capacity to uptake even nonviral DNA molecules from their surroundings. When this occurs, genetic information from these molecules that are taken up is transferred to the prokaryote. In reality, bacteria and archaea in the wild acquire genes from neighboring cells relatively easily. Often, this is not good for the organisms that can be infected when dangerous strains of bacteria become more resistant to treatment with evolution. One example of this is Neisseria gonorrhoeae. If you’ve been looking at the news lately, you might have heard of new, difficult-to-treat strains of gonorrhea infecting people across the United States. Over the last 40 years, it appears, certain strains of gonorrhea have become penicillin-resistant, due to the horizontal gene transfer that we’ve discussed. Another example of this, occurring on a longer time scale of 100 million years, is the change in genes in the present-day genome of E. coli, of which 18% are said to have been acquired through horizontal gene transfer from another species (Alberts et al. 2014).
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