• Plasmids are extra-chromosomal genetic elements that replicate independently of the host chromosome.
• They are small, circular (some are linear), double-stranded DNA molecules that exist in bacterial cells and in some eukaryotes.
2. • Plasmids are extra-chromosomal genetic elements that
replicate independently of the host chromosome.
• They are small, circular (some are linear), double-stranded
DNA molecules that exist in bacterial cells and in some
eukaryotes.
• They are duplex, supercoiled DNA molecules. The sizes of
plasmids range from roughly one to more than 1000 kilobase
pairs.
• It naturally occur in bacteria, however sometimes present in
archaea and eukaryotes.
A typical plasmid is a circular double stranded DNA molecule less than 1/20 the size of the
chromosome.
3. • The word ‘plasmid’ was first coined by Joshua Lederberg in 1952. He used it to describe ‘any extra -
chromosomal hereditary element.
• He discovered plasmid while conducting experiment on Salmonella bacteria and virus P²² with his student
Norton Zinder.
• The number of copies of plasmid in a cell is referred to as copy number.
• The number of plasmids may vary from none to several per bacterial cell. Different plasmids are present in
a cell in a particular number.
• When there are 1 or 2 copies in a bacterial cell, the copy number is called low copy number.
• When there are 20 or more copies present in a bacterial cell, the copy number is called high copy number.
4. • Individual bacterial cells may contain several different types of
plasmids and in some cases more than 10 at a time.
• Plasmids are generally isolated from the bacterial cells in
the supercoiled configuration. So far, thousands of different
types of plasmids have been isolated.
• More than 300 different types of naturally occurring plasmids
have been isolated from E.coli alone.
• Though, plasmids are not considered as part of the cell’s
genome, when a bacterial cell divides each daughter cells
receives a copy of each plasmid.
• Certain plasmids do not show any phenotypic traits, they rely on
DNA replication enzymes of the host cell or their replication;
however the initiation of replication is controlled by plasmid
genes.
• Such plasmids are known as cryptic plasmids.
5. ORIGIN OF REPLICATION
PROMOTER REGION
ANTIBIOTIC RESISTANCE GENE
SELECTABLE MARKER
RESTRICTION SITE
PRIMARY BINDING SITE
INSERT
6. Plasmids can also be transferred from one bacterial cell to another by the process called conjugation.
Plasmids can be generally categorised into two, they are
This categorization of plasmids is based on the presence or absence of a set of transfer genes called ‘tra’
genes which promotes bacterial conjugation.
Conjugative plasmids - Plasmids that govern their own transfer by conjugation. They are of relatively high
molecular weight and are present as 1-3 copies per chromosome.
Non-conjugative plasmids - They cannot start the conjugation on their own, they can only be transferred
through sexual conjugation with the help of conjugative plasmids. They are of low molecular weight and
present as multiple copies per cell.
CONJUGATIVE
PLASMIDS
NON - CONJUGATIVE
PLASMIDS
GENERAL TYPES
7. Stringent plasmids (or) low copy number plasmids
• Stringent plasmids are those which replicate only along with the cell division, which is proceeded by protein
synthesis.
• If the cells are treated with chloramphenicol, protein synthesis is arrested and hence plasmids cannot
replicate.
Relaxed plasmids (or) high copy number plasmids
• Relaxed plasmids can replicate independent of chromosomal replication.
• When treated with chloramphenicol, chromosomes cannot replicate whereas relaxed plasmids can
replicate to produce more copies.
Plasmid incompatibility
• Generally, two closely related plasmids cannot coexist in a bacterial cell
• In the population of progeny cells derived from a cell containing two such plasmids, the proportion of cells
having only one of the two plasmids increases with every cell division.
Episomes
• Some plasmids have ability to integrate into the bacterial chromosome under such conditions they replicate
under the control of genes of bacterial chromosome.
• The F factor of E.coli is called an episome because it can alternately exist in the F+ or Hfr state.
8. • It plays a major role in conjugation in bacteria E. coli and was the first to
be described.
• It is this plasmid that confers ‘maleness’ on the bacterial cells; the term
‘sex-factor’ is also used to refer to F-plasmid because of its this property.
• F-plasmid is a circular ds DNA molecule of 99,159 base pairs.
• One region of the plasmid contains genes involved in regulation of the
DNA replication (rep genes).
• The other region contains transposable elements (IS3, Tn 1000, IS3 and
IS2 genes) involved in its ability to function as an episome.
• The third large region, the tra region, consists of tra genes and possesses
ability to promote transfer of plasmids during conjugation.
9. • R-plasmids are the most widespread and well-studied group of plasmids
conferring resistance to antibiotics and various other growth inhibitors.
• R- plasmids typically have genes that code for enzymes able to destroy
and modify antibiotics.
• They are not usually integrated into the host chromosome.
• Some R-plasmids possess only a single resistant gene whereas others
can have as many as eight.
• Plasmid R 100, for example, is a 94.3 kilobase-pair plasmid that carries
resistant genes for streptomycin and spectinomycin, chloramphenicol,
tetracycline etc.
• It also carries genes conferring resistance to mercury.
• Many R-plasmids are conjugative and possess drug- resistant genes as
transposable elements, they play an important role in medical
microbiology.
10. • Virulence-plasmids confer pathogenicity on the host bacterium.
• They make the bacterium more pathogenic as the bacterium is
better able to resist host defence or to produce toxins.
• For example,
• Ti-plasmids of Agrobacterium tumefaciens induce crown gall
disease of dicot plants; enter toxigenic strains of E. coli cause
traveller’s diarrhoea because of a plasmid that codes for an
enterotoxin which induces extensive secretion of water and
salts into the bowel.
11. • Col-plasmids carry genes that confer ability to the host bacterium to
kill other bacteria by secreting bacteriocins, a type of proteins.
• Bacteriocins often kill cells by creating channels in the plasma
membrane thus increasing its permeability.
• They also may degrade DNA or RNA or attack peptidoglycan and
weaken the cell-wall.
• Bacteriocins act only against closely related strains.
• Col E1 plasmid of E. coli code for the synthesis of bacteriocin called
colicins which kill other susceptible strains of E. coli.
• Hemolysin and enterotoxin property of enteropathogenic E.coli
(EPEC) are governed by the plasmids.
• Lactic acid bacteria produce bacteriocin NisinA which strongly
inhibits the growth of a wide variety of gram-positive bacteria and is
used as a preservative in the food industry.
12. • Metabolic plasmids possess genes to code enzymes that
degrade unusual substances such as toluene, pesticides,
xylene and salicylic acid, etc. Also known as dissimilation
plasmids or degradative plasmids.
• pTol is responsible for the ability of certain pseudomonas
species to break down industrial solvents such as toluene
and xylene.
• A combination of several plasmids, when transferred to
pseudomonas allows the bacteria to breakdown complex
hydrocarbons and other compounds present in crude oil.
• This bacteria have the potential for treatment of environments
contaminated with oil spills.
• However, some metabolic plasmids occurring in certain
strains of Rhizobium induce nodule formation in legumes and
carry out fixation of atmospheric nitrogen.
13. • Plasmids occur rarely in eukaryotic cells. Some plasmids have been found in yeast (Saccharomyces
cerevisiae) and in several plants.
• The only RNA plasmid discovered till now has been found in yeast. It is a double-stranded RNA having a
molecular weight of 15 x 106 Daltons.
• It contains 10 genes including one coding for a bacteriocin-like protein.
• The protein can kill other yeast cells lacking the plasmid. This yeast plasmid has been designated as
killer particle.
• Yeast also contains small DNA plasmids with high copy number. They are located in the nucleus and like
the chromosomal DNA are associated with basic proteins histones.
• Some yeast DNA plasmids have been genetically engineered in such a way that they are capable of
multiplication in both E. coli and yeast.
• This plasmid has been used in transfer of useful genes from other organisms into yeast cells via E. coli
for production of valuable therapeutically important proteins.
• A successful application of the Yep plasmid is the transfer of the gene coding the coat glycoprotein of
hepatitis B virus to yeast.
14. They can acquire chromosomal genes by several mechanisms
They have single site of a large number of restriction enzymes, preferably in genes with a readily scorable
phenotypes
They also carry genes for nitrogen fixation. These confer the phenotypic traits of plasmids
They may carry very important genes for antibiotic resistant toxin production, for antibiotic production, for
degradation of a large number of unusual substrates
15. They do not
have
extracellular
forms and exist
inside cells
simply as free
and typically
circular DNA
They replicate
independently
or
autonomously
They are easily
isolated and
purified from
the bacterial
cells
They do not
cause damage
to cells and
generally are
beneficial
They may affect
certain
characteristics
of the bacterial
cell
They act as
episomes and
reversibly integrate
into a bacterial
chromosome
They may pick-up
and transfer certain
genes of bacterial
chromosome
They are specific to
one or few particular
bacteria
They code for their
own transfer
16. They propagate
the DNA
fragment linked
to them by
invitro ligation
They are of low
molecular
weight
They have high
transformation
efficiency
They have
convenient
selectable
markers such
as antibiotic
resistance,
toxic production
They have an
origin of
replication
They are passed on to the daughter cells during cell division
They are genetically dispensable elements that they are non-essential for the growth of the cells
They have the ability to clone reasonably large pieces of DNA
17. They are very important tool in recombinant DNA technology or genetic engineering.
Production of hormones, enzymes, antigens for vaccines, etc
Vectors for gene replacement or regulation in mammalian tissues.
Identification of gene sequences or virulence properties, gene expression and function.
Plasmid profiling is used for epidemiological typing of bacterial strains.
Nodulation and symbiotic nitrogen fixation - Rhizobium
Transfer genetic information for a biochemical pathway for the degradation of organic compounds
such as octane, camphor, naphthalene, salicylate etc - Pseudomonas.
Pigment production - Erwinia, Staphylococcus
Lactose, sucrose, urea utilization, nitrogen fixation - Enteric bacteria
