The humble plasmid. We now know it so well, but as little as 60 years ago the field of extra-chromosomal heredity was decidedly murky. Not only was it the subject of great debate, conflict and friction within the scientific community, it was even used as a politico-religious tool during the Cold War!
The origin of the term “plasmid”
The term “plasmid” was coined in 1952 by Joshua Lederburg. It is a hybrid of the terms “cytoplasm” (or “plasmagene”; a term put forward by Darlington in 1944 to define “self-reproducing cytoplasmic particles”) and the “id”, the latin for “it”, as in plastid or chromatid.
For Lederburg, the plasmid was a generic term for any extra-chromosomal genetic particle. Of course, that wide definition could describe viruses, mitochondria or many other things, but over the years this was honed down to reserve the term for double-stranded, extra-chromosomal, self-replicating DNA.
The debate on extrachromosomal inheritance
Early experiments seemed to show that certain genes could be infectively transmitted from cell to cell but the idea of “cytoplasmic inheritance” was dismissed by much of the community, who argued that the chromosome was the sole heritable unit and that the observed inheritance was due to infection of the cells by parasites. In fact, the split in opinion went straight (and simply) down the lines of who studied what: Microbiologists embraced the idea of extra-chromosomal inheritance, while those who worked on Drosophila dismissed it. From a distance, it seems quite obvious why this would be the case, but at the time it was a very hot debate.
Plasmids in the Cold War
In those politically charged times, this scientific argument even became embroiled in clash between the communist and capitalist ideologies. This was because followers of the Soviet Lysenkist doctrine, which criminalised the teaching of Mendelian genetics in the USSR, held up cytoplasmic inheritance as evidence that Mendelian genetic theory was wrong.
This in turn led to proponents of cytoplasmic inheritance being viewed suspiciously as anticommunist hysteria swept the USA, and the Genetics Society of America came close to dismissing the theory based purely on political grounds. Now that would have been something to talk about!
This is all impressive work for a small piece of circular DNA, but given the revolution plasmids sparked in bioscience in recent decades, perhaps we shouldn’t be surprised.
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For further reading on the early days of plasmid research, check out this reflective article by Lederburg himself.