Plasmid DNA Function
Date: April 2003
We have learned that plasmid DNA is the
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?
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
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.
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.
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Update: June 2012