Bacterial Genetics
Plasmids
Characters
 Extra-chromosomal circular double stranded DNA
molecules.
 They are dispensable as they are Not necessary for
bacterial life.
 Capable of autonomous replication (independent of the
bacterial chromosome).
 Multiple copies of the same plasmid may be present in the
same bacterial cell.
 Different types of plasmids may co-exist within the same
bacterial cell.
Types
 According to transmissibility between the bacterial cells:
Conjugative plasmids
(transmissible)
Non-conjugative plasmids
(non-transmissible)
Large
Usually small
1-2 (stringent)
10-60 (relaxed)
Present
Absent
Contain tra gene
Yes
No
Sex pilus formation
Yes
No
By conjugation
By the help of a
conjugative plasmid
Common in Gram –Ve
bacilli
Common in Gram +Ve
cocci
Item
Size
Copy number
Fertility factor (F factor)
Transfer among bacteria
Host bacteria
Functions
F- factor plasmid
F-factor mediates gene transfer by conjugation.
R- factor plasmid
These plasmids carry genes encoding antibiotic resistance.
These plasmids are usually conjugative.
Virulence
Plasmids may carry genes that produce toxins which are
virulence factors of pathogenic bacteria.
Production of bacteriocins
Plasmids may carry genes that produce bacteriocins.
Bacteriocins or colicins are bactericidal substances produced by
certain strains of bacteria and kill other strains of the same or
closely related species.
Production of antibiotics
Plasmids in some bacteria carry genes that produce antibiotics.
For example: antibiotic production by streptomyces bacteria.
Transposons
Characters
 Fragments of extra-chromosomal DNA.
 Move inside the bacterial cell by transposition.
 Move from one location on the bacterial
chromosome to another location.
 Or between the chromosome and plasmid and
the reverse.
 Thus, transposons are called jumping genes.
Structure
Inverted
repeat
Transposase
gene
Repressor
gene
Inverted
repeat
Antibiotic resistance
gene
 Transposon is formed of 4 domains:
1) Inverted repeat at each end.
2) Transposase gene: produce the enzyme
responsible for excision and integration of the
transposon.
3) Respressor gene: regulates domains 2 and 4.
4) Gene coding for:
 Antibiotic resistance OR
 Toxin production
Insertion sequence
o Simple type of transposon.
o Carries only the genetic information for
its excision and integration.
o No antibiotic resistance gene.
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