Overlap PCR

Background

** Before attempting this somewhat advanced PCR technique, be sure to read the Standard PCR protocol and check out a reference describing PCR theory, like this one.

Overlap PCR is a technique commonly used to assemble two or more double-stranded DNA templates into a single, larger DNA fragment made up of these component pieces. Primers are designed to generate template DNA with 20-40 base pairs of sequence identity on the end you want splicing to occur. Typically, a template extension reaction is first performed with template in a reaction mix (without primers). This allows the overlapping region of template to anneal and be extended by DNA polymerase, creating a small amount of the desired product. Then, a PCR reaction is performed, either in the original reaction mix by simply adding primers, or by starting a new reaction and using a small amount of the first reaction as a template.

Primer Design

Template Extension

Overlap PCR

Template	10µM UF primer	10µM DR primer	10x buffer	2mM dNTPs	5U/µL taq	ddH2O	Total
X	1.5	1.5	3	3	0.3	20.7 - X	30 μl

10x dNTPs are 2 mM in each dNTP.

If the stock concentration of DNA is 1ng/μl, 3 μl would give the desired concentration of template. 1.5 μl of a 10μM primer gives a final concentration of 500nM. Water is added as needed to create a final volume of 30 μl.

When more than a few reactions are being made (5+), it is advisable to use a mastermix. A mastermix is a batch of all of the ingredients common to all the reactions, i.e., water, buffer, polymerase, dNTPs. To make a mastermix, take the amount of each of the individual ingredients needed (ex: 3 μl 10x dNTPs) and multiply it by (number of reactions + 1). So if 8 reactions are being run, the mastermix would contain 27 μl 10x dNTPs. When many PCR reactions are being run (15+) it often becomes necessary to add more than just 1 extra batch for the final volume.

Mastermixes not only save time and materials, but they also allow for much greater precision. Measuring out 0.3 μl of 100x Taq polymerase with a pipette is very inaccurate, however, 2.7 μl of the same Taq can go into the MM, from which volumes of higher precision can be withdrawn.

In the thermocycler, make sure that the program "PCR" contains the appropriate temperatures and times. The most important step to check, is that the 72°C elongation step should be roughly 1 minute for every 1000bp of the longest PCR product. If the longest expected product is 2500bp, then 2.5 minutes at 72°C should be appropriate. For most PCRs, 30-40 cycles should be appropriate.

Agarose Gel Electrophoresis

In order to analyze PCR results, the products are run on an agarose gel and the resulting gel is observed in UV light.

First, the gel has to be made. A standard 1% agarose gel uses 1g of agarose for every 100 ml of buffer. A different percentage may be used, and gels with less than 1% agarose may be used to clearly distinguish products of very similar sizes. For a standard 50 ml gel, add .5g agarose and 50 ml SB buffer to a 125ml flask and heat for 1:30 minutes. Meanwhile, assemble a gel rig and find a comb with an appropriate number of wells, then place the comb into the rig. After heating add 2.5 μl SYBR Safe (5μl SYBR Safe for every 100mL gel) and swirl to mix. Pour the liquid from the flask into the rig and wait about 30 minutes for it to solidify.

Once the gel has solidified, gather all PCR products that are to be run, an appropriately sized ladder, and 6x loading dye. The latter two may be found in the 4°C fridge in the computer room adjacent to the gel area. First, load 6-7 μl of ladder into the first well. The easiest way to combine dye and DNA is to cut out a 4x4 sheet of parafilm, make a drop of 1 μl of dye onto the parafilm for each sample to be run. Next, add 5 μl of PCR product to the dye and pipette up and down to homogenize. Once all samples are combined with dye, load them into the gel, making note of what sample goes into what lane.

Whole-Cell PCR

• Grow fresh overnight cultures of the cells to be used as template in LB.
  • Important: Do NOT use E. coli grown in Davis Minimal (DM) media as template for whole-cell PCR. Ingredients in DM interfere with reliable amplification. (Probably the high concentration of citrate added as a chelating agent).
  • Amplification from frozen cultures is also not as reliable as from freshly grown cells.
• Dilute cells 1000x fold into ddH₂O (NOT saline) in a 1.7 ml Eppendorf tube or 96-well microplate.
  • Too many cells or too much residual media is a common source of failure for whole-cell PCR.
• Add the diluted cell mixture as 1/10th the final volume of the PCR reaction.
• Add an initial denaturing step of 10 minutes to your PCR program at 94°C to lyse the cells.

-- Main.MichaelHammerling - 20 Sep 2011