eChapters

Transfer 1.5 mL overnight culture to a 1.5 mL microfuge tube, centrifuge for 30 sec, and decant supernatant.

Resuspend cells in 400 µL TE by vortexing, add 50 µL 10% SDS, 50 µL proteinase K (20 mg/mL in TE). Incubate for 1 hour at 37^oC.

Shear DNA by 3-5 passages through a 26 G needle.
Extract twice with 500 µL phenol: chloroform (1:1), and twice with 500 µL chloroform. 
Precipitate nucleic acids by adding 25 µL 5 M NaCl and 1 mL 95% EtOH, vortex, and centrifuge for 10 min, decant supernatant. Dry pellet.

Principle

Restriction digestion is one of the most common reactions performed in molecular biology. Restriction endonucleases (RE) are also known as restriction enzymes, which recognize short, specific DNA sequences. There are three types of RE: Type-I, Type-II and Type-III. Type II RE, most commonly used for DNA analysis and genetic engineering, cleave double-stranded DNA (dsDNA) at specific sites within or adjacent to their recognition sequences. All Type II RE recognises a unique nucleotide sequence at which it cuts a DNA molecule and nowhere else. The recognition sequence is often a six base pair palindromic sequence (the top DNA strand from 5' to 3' is the same as the bottom DNA strand from 5' to 3'), but others recognize four or even eight base pair sequences. Many RE will not cut DNA that is methylated on one or both strands of the recognition site, although some require substrate methylation.

REs can also differ in the way they cut the DNA molecule. Some enzymes cut in the middle of the recognition sequence, resulting in a flush or “blunt end”. Other enzymes cleave in a staggered fashion, resulting in DNA products that have short single-stranded overhangs (usually two or four nucleotides) at each end. These are often called “cohesive ends”, as these single-stranded overhangs could potentially come together again through complementary base-pairing, as shown below:

Principle of DNA Ligase Mechanism

The process of connecting two pieces of DNA together is called ligation, and is catalyzed by an enzyme called ligase. The ligase used most often in molecular biology is derived from the T4 bacteriophage, and uses ATP to supply the energy necessary for the reaction. In addition to ATP, T4 ligase requires DNA with a 5´-phosphate group and free 3´-hydroxyl groups.

Applications

Ligation of blunt or cohesive-ended DNA fragments.

Repair of nicks in double-stranded nucleic acids.

Principle

Plasmids requires hosts to replicate, hence it has to be inserted into bacteria. But efficiency of uptake of plasmid DNA by bacteria is low and will not do so without getting degraded. Therefore, to enhance the probability of uptake of the plasmid DNA by bacteria it is necessary to make the bacterial cells “competent” to absorb the plasmid DNA. One of the methods to increase efficiency of cell competence is given below. It should be noted that competent cells are more fragile than normal bacteria. Vortexing the cells, heating the cells above 42ºC or to 42°C for prolonged periods, or exposure of the cells to abusive treatments may kill them

Transformation

The uptake of DNA by competent cells is called transformation. Transformation involves:

Mix a small volume of DNA (usually 1-3 µl) with 100 µl of competent cells.

The mixture is incubated on ice for 30 minutes to allow the cells to take up the DNA.

The cells are then heat-shocked at 37 to 42°C for 1 minute, and cold shocked on ice for 2 minutes.

The heat shock/cold shock reverses the effect of the competent cell process, by healing the damage inflicted by the competent cell solution.

Add 0.5 ml of LB culture mediumz

Incubate the cells for 10 - 30 minutes at 37°C to aid the healing process.

This cell/medium mixture is then spread onto bacterial culture plates containing agar, LB, and a selection mechanism.