Team:Johns Hopkins-BAG/Building Block synthesis

Building Block Synthesis

A general method of building block synthesis based on overlap extension PCR is an efficient and inexpensive way to construct Building Blocks. Building Blocks are assembled by overlap assembly PCR from standard 5’-OH, 3’-OH 60-80 base pair oligonucleotides purchased from a DNA synthesis company. The overlap extension assembly process is broken into two polymerase chain reactions:

• "Templateless" PCR- Oligos are mixed and standard overlap extension assembly PCR program is utilized. Mixed length products are formed.
• "Finish" PCR - Outer two primers are mixed with the Templateless PCR product, such that a higher concentration of outer primer is present in the mixture, and a second round of PCR is completed to enrich for full length products
• Cloning - Finish PCR product is cloned using PGEM-T easy vector, taking advantage of Taq polymerase single 3’ adenylation.
• Sequence verification - Sequencing is completed by outside sources and sequence alignment using programs such as “CloneQC” aids with correct clone verification

Templateless/Finish PCR assembly trouble-shooting

• PCR program conditions can be altered to optimize construction of Building Blocks. Varying the anneal and extension temperature MAY assist in the production f the Building Block

One of the current protocols of producing a specific sequence of DNA within our undergraduate course of Build-A-Genome involves overlapping, gapped oligonucleotides (70bp) forming the basis of the 750bp Building Block. The Ligation Chain Reaction (LCR) is an alternative procedure that produces a more specific overlap between two strands which eliminates the gaps between DNA strands altogether. In other words, most overlapping oligonucleotides will now have a region of around 35bp in which they complement each other. By using ungapped strands of DNA, the Taq DNA Ligase enzyme will be used to string the DNA backbone together. Like a normal PCR, the cycling process produces large quantities of feasible DNA Building Block. One important theoretical advantage is the extensive overlapping of complementary DNA that occurs during the annealing process. With the LCR, it is more likely for oligonucleotides to anneal correctly and result in a final product of the correct sequence. The LCR would also reduce the likelihood of problems involved with loxP sites and other anomalies found in the normal TPCR and FPCR protocols. However, the LCR should be emphasized as an alternative to the current methods due to the expensive enzymes and reagents involved in the process of phosphorylation of the oligonucleotides for use in the LCR cycling program.