Paper I — 2023 March (Supplementary) (2010 Scheme) — Q13
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Paper I
2023 March (Supplementary) (2010 Scheme) · 40 marks · 120 min
Question
Define plasmid. Give two clinical significance of plasmid.
Q132 marksShort Notes
Answer
Definition: A plasmid is a small, extrachromosomal, double-stranded, circular DNA molecule capable of independent (autonomous) replication within a bacterial cell, distinct from the bacterial chromosome — not essential for basic survival, but often carrying genes that provide a selective advantage under specific conditions.
Two clinical significances:
Spread of antibiotic resistance — R (resistance) plasmids carry genes conferring resistance to one or more antimicrobial agents and are transferable between bacteria by conjugation, allowing rapid horizontal spread of multidrug resistance through a bacterial population, even across different species/genera — a major driver of the emergence of multidrug-resistant pathogens.
Carriage of virulence factors — some plasmids carry genes encoding toxins or other virulence factors (e.g., enterotoxin genes in enterotoxigenic E. coli), directly contributing to a strain’s pathogenicity.
(A third valid point: plasmids are also the basis of important laboratory/biotechnology tools, e.g., as cloning vectors in recombinant DNA technology.)
