Team:UCL/Project/Experiments

Creating Competent Bacteria

E. coli are not naturally transformable, which means they lack the ability to take up plasmids (competency). Competency is induced by divalent cations such as calcium. These alter the permeability of the membranes enveloping the bacterium to plasmids. Normally macromolecules on the outer surface of bacteria are negatively charged which means the negative charges of incoming DNA would be repelled. The addition of calcium chloride facilitates the movement of DNA into the cell.

Transformation

To insert foreign DNA into our competent cells we used the heat shock treatment. Our competent bacteria are stored in -80C. To transform, DNA with a selectable marker and competent bacteria are mixed together and kept on ice for thirty minutes to allow interactions between calcium ions and the negative charges on the bacterial envelope. The mixture is exposed to a brief period of 38C (heat shock). The rapid shift in temperature alters the fluidity of the membrane therefore allowing DNA to enter the cell. Afterwards, the bacteria containing the foreign DNA are streaked on selective plates. Bacteria containing the foreign DNA with the selectable marker, such as ampicillin resistance, would be the only bacteria growing on the selective plates.

Minipreparation

This step involves purifying plasmid DNA from bacteria. This allows us to prepare a stock of DNA ready for digest, ligation or further transformation. This is done by first lysing the cells then applying centrifugation on the sample in spin filters. Eventually after the process DNA would be purified as it elutes through the filter along with elution buffer.

Analytical Digest

Gel electrophoresis exploits the fact that the negatively charged DNA fragments would move in a electric field. Furthermore, in the gel matrix, smaller fragments of DNA would move with more ease. This allows separation of shorter fragments of DNA from longer fragments. Our DNA sample is first digested into fragments with restriction enzymes. Finally the digested DNA sample is added to the gel. After exposure to an electric field for an hour, separated fragments can be observed under UV.

Nanodrop

The nanodrop, or spectrophotometer, quantitatively measures the purity of DNA in a sample. It uses the fact that nucleic acids absorb UV light in a specific pattern. The Beer-Lambert Law is used to determine the concentration of DNA as a function of absorbance.

Preparative Digest

This experiment is used to build up a stock of a specific fragment of DNA. Afterwards, the digested sample is run on gel electrophoresis. Compared to the analytical digest, this is done in larger quantities of DNA sample and enzymes. To isolate the wanted DNA fragment we extracted the band with the correct length on the gel.

Polymerase Chain Reaction

The polymerase chain reaction uses enzymes and primers to generate a large quantity of copies of a DNA sequence. This allows us to amplify a gene or sample to a more workable quantity. In addition, with the correct use of primers, we would be able to modify a gene so that illegal sites could be mutagenised and addition of the iGEM standardized prefixes and suffixes.

Zeocin Kill Curve

Before doing a transfection experiment, it is important to determine the concentration of selection reagent required for efficient selection. For this, we growed HeLa cells in various concentrations of Zeocin - 0 nM, 50 nM, 100 nM, 250 nM, 500 nM and 1000 nM. We conducted the kill curve in a 6-well plate. A T25 flask of HeLa is split in 10 ml medium. Each well is filled with 1 ml of cells. Incubation is carried out at 37C overnight to allow cells to attach before adding inhibitor. Medium is removed and replaced with 2 ml of medium containing antibiotic. Confluence of cells is observed in each well every day for 5 days.