Plasmid counts in cell.

1. HIGH AND LOW COPY NUMBER
PLASMIDS.
PRESENTEDBY
SHABNOORSHAIKH
M.SCPART 1
1203

2. JOSHUA LEDERBERG
The termplasmidwas first introducedby the American
molecular biologist Joshua Lederberg in 1952.
He was knownfor his workin genetics, artificial
intelligence and space exploration.
He was just 33 years old whenhe won the1958 Nobel
prize that bacteria can mate and exchange genes.
He sharedthe prize withEdward L. Tatumand
George Beadle who won for their work with genetics.

3. WHAT IS A PLASMID?
Like other organisms, bacteriauses its double stranded DNA as their genetic material.
Thesegenetic material exist naturally in the cytoplasmof thebacterial cells.
In addition to the nucleoidchromosome, bacterial cell often containplasmid as extra
chromosomal hereditarydeterminant.

4.  A plasmid is a DNA molecule that is separatefrom, and can replicate independently
of, the chromosomal DNA.
They are double strandedand, in many cases, circular.
Plasmids usually occur naturallyin bacteria, but are sometimes foundin eukaryotic
organisms (e.g., the 2- micrometre-ring in Saccharomycescerevisiae).
They are small (a few thousand base pairs) usually carry few genes.
PlasmidDNA in bacteriummay be 0.1 – 5%of total DNA.

5. GENERAL COMPONENTS OF PLASMID
Origin of replication:-50-100 bp seq where replicationis initiated.
Selectable marker:- antibioticresistant gene.
Multiple cloning site:- seq that
can be cleavedby restriction enzymes
for cloning purposes.
Repressor gene:- for regulationof copies.

6. PLASMIDCOPY NUMBER
Copy number is a parameter of bacterialpopulation.
Generallydefinedas theratio of number of moles of plasmidDNA to the number of
mole of chromosomal DNA in the cell.
It varies in rapidly growing bacterial culture.
Copy number is controlledby genes on the plasmidand by interactions between the
host and the plasmid.
It is primarily determinedby the replicon.

7. CLASSIFICATION OF PLASMIDS.
Plasmids may be classified on the basis of the number of copies foundin the
host cell.
Low copynumber- stringent copy number.
Moderate copy number.
High copy number- relaxedcopynumber.

8. STRINGENT PLASMIDS
Theseplasmids exists in small numbers
that is 1-4copies per cell.
Its replicationis closely tiedto host cell
chromosomal DNA replication.
Here plasmidDNA replicates only onceor
twice before cell division
Usually large plasmids havelow copy number.

9. RELAXEDPLASMIDS.
Theseplasmids existsin large numbers that
is more than100 copies per cell.
Its replicationis not dependent on host
cell’s chromosomal DNA replication.
Here plasmidDNA replicates repeatedlyuntil
the proper number is reached.
Usually small plasmids havehigh copynumber.
They are used for expressionof clonedgene
toget higher yields.

10. EXAMPLES OF PLASMIDS WITHTHEIR COPY NUMBER.
PLASMID APPROXCOPY NUMBER
F 1
P1 prophage 1
RK2 4-7
pBR322 16
pUC18 30-50
plj101 40-300

11. ADVANTAGES AND DISADVANTAGES.
 ADVANTAGES:-
Readilyisolated fromcells.
Can be reintroduced intoa bacterial cell.
Single restriction site for 1 or more res enzymes.
Multiple copynumber so that amplification of the plasmid DNAbecomes easy.
 DISADVANTAGES:-
Small size hence cannot acceptlarge fragments.
Largerthe size lower its replication rate.

12. REFERENCES
GENE BIOTECHNOLOGY by S.N. JOGDAND.
PLANTBIOTECHNOLOGYby B.D. SINGH.
 http://www.molecular-plant-biotechnology.info/bacterial-vectors/classification-of-
plasmids