Bacterial Transformation(细菌转化)

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Introduction
Transformation is a technique to introduce DNA into bacterial Cell s. There are many variations on a common theme, but the key points are listed below. Check details with supplier of competent bacteria and note that variations in timings and volumes will vary with application and bacterial strain.

There are four stages:
A Mix DNA/bacteria and incubate on ice Ð do not use an excessive amount of DNA, both in terms of concentration and actual volume (less than 1 µg and less than 10 µl). Note,
protein (e.g. Ligase) will reduce transformation efficiency, but it is not always necessary to remove prior to transformation.
B Heat shock Ð necessary for DNA uptake. Time heat shock carefully Ð excessive heat shock will kill the bacteria and the transformation will fail.
C Recovery Ð prior to selecting for transformed bacteria with antibiotics, it is necessary to allow them to recover in rich medium (e.g. LB, SOC or 2YT) for 30-60 mins at 37 ℃.
D Selection Ð essential to isolate (as single colonies) the bacteria which have taken up DNA.
This is usually performed on solid medium (LB-agar) in the presence of antibiotics. Cell s are incubated at 37 ℃ overnight.

Competent bacteria are extremely fragile Ð always thaw slowly on ice and do not hold the base of the eppendorf tube. The compency of the bacteria is also important-“sub-cloning efficiency” means about 106 colonies are produced per µg of (purified) DNA. “Library efficiency” can mean in excess of 109 colonies produced per µg of (purified) DNA. For subcloning and mutagenesis is normally sufficient, although “Library efficiency” bacteria may be useful if problems arise.Materials required
Competent Cell s nutrient broth
DNA Antibiotic plates
Plate spreader Bunsen burner
L-broth agar plates Ethanol

Caution
Bunsen burners have a naked flames Ð do not leave unattended.

Three transformations must be performed:
Experimental reaction(s)
Plasmid or positive control Ð this shows that E. coli are competent for DNA uptake
H2O or negative control Ð this shows that there are no false positives
Bacterial Transformation
1. Add 1-10 µl of the DNA (Experimental reaction or positive/negative control) to a vial(20-200 µl) of competent E. coli Cell s and mix gently. Do not mix by pipetting up and down.
2. Incubate on ice for 30 min.
3. Heat shock the cells for 30 sec at 42ûC without shaking (time varies by strain).
4. Immediately transfer the tubes to ice and incubate for 2 min.
5. Add 50-500 µl nutrient broth (room temperature).
6. Cap the tube tightly and shake the tubes at 37℃ for 30 min. Place on ice.
7. Spread 50-500 µl from each transformation on a L-broth agar plates containing antibiotics at the appropriate concentration. Incubate plates for 5-10 mins at room temperature, then invert the plates and incubate overnight at 37℃.
Most strains require 12-18 hours to form colonies. Do not incubate for excessive times as satelite colonies will form. Plates/colonies can be stored for a few days at 4 ℃ if not to be used immediately.
Appendix
Gibco, New England Biolabs and Stratagene have comprehensive guides to transformation.

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