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Name: Shelby 
Status: Student
Age: 19
Location: N/A
Country: N/A
Date: April 2003


Question:
We have learned that plasmid DNA is the "transforming principle," but is the plasmid DNA the only determinant of an antibiotic resistance? If the DNA is degraded with DNAse and only plasmid RNA remains, will the cell still show a resistance to the antibiotic or will the resistance disappear when the DNA is destroyed?



Replies:
See a plasmid as a parasitic DNA molecule (not RNA). It invades a cell, replicates at the cost of that cell, and stays with the cell ever after. When the cell divides, the plasmid replicates too and both offspring cells will ocntain the plasmid. But in addition, a replicated plasmid can transform to another cell, thus reproducing 'independently' of its original cell. The transfer of plasmid DNA from one cell to another (or, in vitro, from the medium into a cell) we call 'transformation'.

Why does the plasmid do this? In order to exist. Cells try to protect themselves from such invaders, for instance by producing restriction enzymes which would cut any foreign incoming DNA. But the plasmids can protect themselves against that, just as the cell has to protect itself against its own restriction enzyme with the correct modification enzymes.

Plasmids also regularly contain genes that the cell can benefit from. Instead of being a neutral invader, the plasmid now becomes a profitable extra genetic moiety. These can be antibiotic resistance genes, or virulence genes (those are the most common examples but the variation is much larger). Would you remove the plasmid completely from the cell ('curing' the cell), that property (say antibiotic resistance, or virulence) would then be lost.

But antibiotic resistance genes are not always encoded on plasmids: there are many mechanisms how a cell can be resistant to an antibiotic and some are encoded on the chromosome.

When you would distroy the chromosomal DNA of the cell, the cell would die. Then the plasmid can also no longer replicate, since it requires the cell machinery to do so.

I hope this answers your questions.

Trudy Wassenaar


Typically, a plasmid contains an antibiotic resistance gene, sometimes more than one. The gene, in the form of DNA, must be transcribed into messenger RNA, and then translated into the protein that is the agent that counteracts the effect of the antibiotic. So, in your question, plasmid-encoded RNA could exist in the absence of the plasmid if the plasmid was transcribed sufficiently prior to the action of the DNAse. However, messenger RNA has a relatively short half-life (depending on the gene, developmental state, and the like), meaning that it is itself being degraded by RNAse enzymes, and so the amount of time that plasmid mRNA will outlive the plasmid itself would be short. In summary, in the absence of the antibiotic resistance gene encoded on the plasmid, a bacterial cell will not long survive the destruction of the plasmid; the plasmid is the sole source of the antibiotic resistance gene.

D. Silvert



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