An odd fact about viruses and mammals, including human ones

In researching the Ebola virus, I came across an interesting fact. Our very existence, birth even, depends on a virus and an enzyme its DNA codes for.

Those easily bored with biology can page back or forth to something more interesting. Those with some curiosity, please bear with me.

A virus is essentially DNA or RNA, usually with a protein coat. RNA viruses include a ‘reverse transcriptase’ which takes advantage of the infected cell’s machinery and makes a DNA copy of the RNA. Once there is DNA with viral information present, other enzymes attempt to insert that DNA into the host cell. Once there, the host cell is usurped to create viral proteins specified by the viral DNA. Eventually a large number of viral particles are produced, the host cell lysed, and the virus goes on to infect other cells.

It is possible for the viral insertion to go slightly awry, leaving viral DNA in the host cell but not effective for viral reproduction. It is possible for one or more viral proteins to still be ‘read out’ from this DNA and produced in the host cell.

It is possible for the above to occur in gametes – reproductive cells – of the host, and to be passed on to future generations. It is possible for this extra DNA to be transcribed, producing novel protein(s) based on the viral DNA.

Here you will find an article about this. I will summarize.

Mammals develop (embryogenesis) with the assistance of a placenta. The placenta invades the mother’s blood vessels to permit exchange of nutrients and wastes. (Mother becomes stomach and kidneys for the embryo.) The developing embryo will have half its DNA from the other parent – the father. It will produce proteins that the mother’s immune system has never seen – simple things like hair colour variants, and more complex things like blood type and predilection to health and/or disease.

If the mother’s immune system ‘sees’ these foreign proteins, it will attack them. To prevent this, the placenta must be impervious to white blood cells. White blood cells can penetrate between cells at their junctions. To prevent this, the outer layer of the placenta is ‘synctial.’ This means, there are no dividing cell walls: it’s all one big thin manifold-complex cell with many many nuclei, as required to manage that much cytoplasm.

The enzyme that makes it possible for the placenta boundary to be synctial is called synctin. It is coded for by a piece of DNA that is clearly viral in origin. Various mammals have different versions of this genomic necessity, implying that it was acquired several times in different lineages from distinct viruses.

So we owe our ‘birth right’ to a piece of viral DNA.

The complexity of life on earth is mind-boggling. The more I learn, the larger I estimate my ignorance to be.

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