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HIGH AND LOW COPY NUMBER
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.
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
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.
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.
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.
CLASSIFICATION OF PLASMIDS.
Plasmids may be classified on the basis of the number of copies foundin the
Low copynumber- stringent copy number.
Moderate copy number.
High copy number- relaxedcopynumber.
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.
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.
EXAMPLES OF PLASMIDS WITHTHEIR COPY NUMBER.
PLASMID APPROXCOPY NUMBER
P1 prophage 1
ADVANTAGES AND DISADVANTAGES.
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.
Small size hence cannot acceptlarge fragments.
Largerthe size lower its replication rate.
GENE BIOTECHNOLOGY by S.N. JOGDAND.
PLANTBIOTECHNOLOGYby B.D. SINGH.