Describe the methods used for isolation of DNA.
In cloning experiments, two types of DNA molecules are isolated: plasmid DNA and genomic DNA from bacteria. Several methods are employed for their isolation.
One common method involves boiling the bacterial lysate. Boiling causes irreversible denaturation of chromosomal DNA and proteins, including deoxyribonuclease. The denatured chromosomal DNA forms a gel that is precipitated by centrifugation. In contrast, the plasmid DNA, which is only partially denatured, renatures into a circular double helix and remains soluble in the solution.
Another method uses an anionic detergent SDS along with NaOH solution to lyse the bacterial suspension. The broken cell wall, chromosomal DNA, and denatured proteins clump together as a large mass coated with SDS. This mass is precipitated by replacing sodium ions (Na⁺) with potassium ions (K⁺) and separated through centrifugation. The plasmid DNA is then isolated from the supernatant by ethanol precipitation.
The general process of nucleic acid extraction involves four essential steps: disruption of biological samples, protection of nucleic acids from degrading enzymes, separation of nucleic acids from other molecules, and assessment of purity and quality of isolated nucleic acids.
Finally, the isolated nucleic acids are precipitated and concentrated by adding ethanol or isopropanol.
Explanation
The textbook describes two specific methods for DNA isolation - the boiling method and the SDS-NaOH method. Both methods exploit the structural differences between plasmid DNA (circular) and chromosomal DNA (linear) to separate them. The boiling method relies on differential renaturation, while the SDS-NaOH method uses selective precipitation. The context also outlines the four general steps of nucleic acid extraction that apply broadly to isolation procedures.