Preparation of Plasmid DNA
A plasmid preparation is a method used to extract and purify plasmid DNA. Many methods have been developed to purify plasmid DNA from bacteria. These methods basically involve three major steps
- Growth of the bacterial cell
- Harvesting and lysis of the bacteria
- Purification of plasmid DNA
Growth of the bacterial cell
Plasmids are almost always purified from liquid bacteria cultures, usually E-coli, which have been transformed and isolated. Virtually all plasmid vectors in common use encode one or more Antibiotic resistance genes as a selectable marker, which allows bacteria that, have been successfully transformed to multiply uninhibited.
Harvesting and lysis of the bacteria
When bacteria are lysed under alkaline conditions both DNA and proteins are precipitated. Some scientists reduce the concentration of NaOH used to 0.1M in order to reduce the occurrence of ssDNA. After the addition of acetate-containing neutralization buffer the large and less supercoiled chromosomal DNA and proteins precipitate, but the small bacterial DNA plasmids can renature and stay in solution.
Purification of plasmid DNA
Purification of plasmids from 2 cultures of bacteria involves the same procedure like preparation of total cell DNA but there is an important distinction between plasmid purification and preparation of total cell DNA. The plasmid preparation, we have the plasmid DNA from large amount of bacterial chromosomal DNA. Several methods are available for removal of the bacterial DNA during plasmid preparation
General method
During cell lysis, the shearing of chromosomal DNA is very less; hence the large size of the chromosomal DNA tends it to be removed along with the cell debris by centrifugation
There is another method on the basis of conformational difference between plasmids and bacterial DNA. Before going to this you must prepare a clear lysate (plasmid, DNA) by centrifugation in which plasmid and DNA are present. Most plasmid resides in supercoil state.
For the separation, two different methods are commonly used
- alkaline Denaturation method
- EtBr-CsCl (cesium chloride) density gradient centrifugation
Alkaline Denaturation method
The basic concept of this method is to maintain a very narrow pH range in which the non supercoiled DNA will be denatured but not the supercoiled plasmid.
Initially the pH of the solution containing both is adjusted at 12-12.5. This in turn denatures the open circular DNA or the non supercoiled DNA.
When a bit of acid (acidic potassium acetate) is added, it neutralizes the base.
At neutral pH, the genomic DNA renatures and is trapped in the SDS/lipid/protein precipitate and forms a lump.
The plasmid DNA renatures into double stranded molecules that remain in the solution.
Diagrammatic representation
Density gradient Method
A solution of cesium chloride when centrifuged at a very high speed a density gradient is produced. A high centrifugal force pulls the CsCl ions towards the bottom. This method is also called isopycnic centrifugation.
Why is CsCl used?
Caesium chloride is used because, at a concentration of 1.6-1.8 gm/ml, its density is almost similar to that of the DNA.
The CsCl and DNA mixture is kept in a centrifuge for several hours at a high speed to create a force of almost 105 g. after sometime when a gradient of caesium ion is formed, it causes 2 forces to act simultaneously .i.e. the diffusion and the centrifugal force. The centrifugal force pushing the molecule outward where as diffusion pulls it towards the center. In turn, the denser molecules are pulled inside to the bottom of the tube. So, the density of the caesium ions moves in a decreasing order of density towards the surface. once the force has been created all the other ingredients of the misture are too stacked as per their decreaing densities.
In case of DNA, the GC bonding counts denser than the AT bonding. So, for 2 equal proportion of DNA run in a density gradient, the higher GC concentrated molecule will form the lower layer.
So if in case we have RNA, DNA, plasmid in a density gradient, then, the order of decreasing density would be
RNA > supercoiled plasmid DNA > DNA
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