Paper I — 2021 February (Supplementary) (2010 Scheme) — Q6
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Paper I
2021 February (Supplementary) (2010 Scheme) · 40 marks · 120 min
Question
Plasmid
Q62 marksShort Notes
Answer
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.
Types: F (fertility) plasmid — encodes genes for conjugation (sex pilus formation) and can integrate into the chromosome to form an Hfr strain; R (resistance) plasmid — carries antibiotic-resistance genes, often alongside a Resistance Transfer Factor enabling conjugative transfer; Col plasmid — encodes bacteriocins (colicins), toxic proteins that kill competing bacterial strains; virulence plasmids — carry genes for toxins or other virulence factors (e.g., enterotoxin genes in some E. coli strains).
Key properties: capable of self-replication independent of chromosomal replication; can be transferred between bacteria by conjugation (and sometimes transformation/transduction); may exist in multiple copies per cell; can integrate into the host chromosome (episomes) in some cases.
Clinical/biological significance: plasmids, particularly R plasmids, are the principal vehicle for the rapid horizontal spread of antibiotic resistance between bacteria — even across different species/genera — since resistance genes on a transferable plasmid can move independently of, and much faster than, chromosomal inheritance; a single plasmid can carry resistance to multiple antibiotic classes simultaneously.
