PCR Primer Design

Primer design for PCR
General Design considerations.  Make sure that:

• The primer length is between 15-30 bp homologous to the target DNA sequence. I suggest starting with 20-25 bp primers.
• The Tm of each primer is between 55-65 °C
• The GC content of each primer is between 40-60%
• The Tm of both primers are very similar, i.e., within ~2 °C
• The GC content of both primers are very similar, i.e., within ~5 %
• Either primer will not form a stable internal hairpin structure, i.e., &Delta;G <-3 kcal/mol
• Either primer will not form a stable dimer with itself, i.e., &Delta;G <-3 kcal/mol
• The forward and reverse primers do not combine to form a stable hairpin structure or dimer
• If possible the 3′ end of each primer should end with a GC
• If you are installing restriction sites at the ends of the pcr product so that the pcr fragment can be digested and ligated into a plasmid. ensure that the amplified region does not include the restriction enzymes which you will digest with in your next step.
• Include a few nucleotides (~4) followed by your restriction sites as 5’flanking regions (regions which are at the 5′ primer end, but are not complementary to the template) to your primer.

Specific considerations for Golden Gate Parts

• Please edit

Specific considerations for BioBrick Parts

• Ensure that the genomic DNA to be amplified does not contain any EcoRI, PstI, SpeI, or XbaI sites.
• I typically create a PCR product which has an XbaI site upstream of the part, and SpeI, NotI, and PstI sites downstream of the part.
• The Biobrick part starts with a start codon (ATG) and ends with two consecutive stop codons (TAATAA).
• Then the forward primer should be of the form:

5′ CCTTTCTAGAG (15-20 bp of the coding strand, starting ATG) 3′

and the reverse primer should be of the form:

5′ AAGG’CTGCAGCGGCCGCTACTAGT’A (15-20 bp reverse complement, starting TTATTA) 3′

Here there are a four nucleotides (in italics) flanking the restriction sites (in bold); such spacers are required to allow the restriction enzymes to cut properly.

Specific considerations for BioFusion Parts

• Ensure that the genomic DNA to be amplified does not contain any EcoRI, PstI, SpeI, or XbaI sites.
• I typically create a PCR product which has an XbaI site upstream of the part, and SpeI, NotI, and PstI sites downstream of the part.
• The insert for the forward primer does not begin with TC (or else a DAM I site (GATC) is formed, and XbaI cannot cut).
• The Biofusion construction does not begin with a start codon, nor does it end with a stop codon.
• Then, the forward primer should be of the form:

5′ ”CCTT””’TCTAGA”’ (15-20 bp of the coding strand) 3′
and the reverse primer should be of the form:
5′ ”AAGG””’CTGCAGCGGCCGCTACTAGT”’ (15-20 bp reverse complement) 3′
Here there are a four nucleotides (in italics) flanking the restriction sites (in bold); such spacers are required to allow the restriction enzymes to cut properly.

• Note: if it is not possible to make a good set of primers with the flanking regions described above, try changing the first 4 bases – which are external to the restriction site – of each primer, e.g. <br>5′ ”AAGG””’TCTAGA”’ (15-20 bp of the coding strand) 3′ <br>

• Note: if you are still not able to get a good set of primers, try using a completely different set of flanking regions to improve the primers. For example, you can also use a PCR product that has the EcoRI, NotI, and XbaI sites upstream of your part, while the SpeI site is downstream of your part.

In this case, the forward primer would be of the form:  5′ ”CCTT””’GAATTCGCGGCCGCATCTAGA”’ (15-20 bp complement to coding strand)3′ and the reverse primer should be of the form: <br> 5′ ”AAGG””’ACTAGT”’ (15-20 bp complement to coding strand) 3′.

 

Designing Primers Using Vector NTI
An easy way to design primers is to use Vector NTI.

• Find the genomic DNA sequence that you want to amplify as your part at [http://www.yeastgenome.org yeastgenome.org] or [http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed PubMed] and save it into Vector NTI.
• Highlight the sequence that you want as your part, and select Analyses -> Primer Design -> Amplify Selection.
• Under the Primer tab, set “Before” and “After” to 0 bp. Adjust the T_m, primer length, GC content, et c. as noted above.  Also click More>> and insert the flanking sense and anti-sense sequences (given at top) in the boxes “Attach to 5′ terminus of Sense primer” and “Attach to 5′ terminus of Anti-sense primer”. Lastly, click “Apply” then “OK.”
• Three possible sets of designed primers will appear in a folder on the left side of the screen, ranked by their score. Usually, the best possible score is 171.  The lowest score that I (Caroline) have successfully used is ~110.
• If the score is poor, look at the individual primer attributes to determine why this is and adjust the input conditions appropriately.
• If the two primers have very different GC content, try altering the ”flanking sequences” to equalize the GC content.
•   If the T_m is low, increase the minimum and maximum length of the primers. Vector NTI typically does not scan all possible primer lengths; this forces it to search longer lengths.
• Double-check your primers for hairpins & dimers by highlighting a given sequence, then right-clicking and selecting “Analyze”.
• For more information, check out Vector NTI’s [http://www.invitrogen.com/content.cfm?pageid=10141 user manual], Chapters 8 and 20.
• Order 25 nmol DNA oligo with standard desalting from IDT.