3. 5/7/2018 Gene.Editor18 3
Muhammad Khurram Shahzad
Founder Of #Gene.Editor Page
Member Of Pakistan Biotechnology Student Society
Founder OF #Gene editor Group on Facebook
https://www.facebook.com/gene.editor18/
4. Basic Over view Of this Presentation :
• Discuss about PCR
• Some Basic Information About Primer
• Discuss some basic rule of primer designing
• Also discuss some tools which we can use for
designing a primer
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5. What is PCR?
• PCR is a polymerase reaction which is used to
amplify the DNA .
• Most vital creations of the twentieth century
in molecular biology.
• Little amount of a genetic material can be
amplified by a PCR to identify and manipulate
DNA as well as detect infectious organisms,
including the viruses that cause, hepatitis,
AIDS, tuberculosis.
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6. PCR
• It can also be used to detect genetic
variations, including mutations in human
genes and many other tasks.
• So There is a different Steps in PCR :
• Denaturation
• Annealing
• Extension
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8. What is primer ?
• Primer is a short oligonucleotide sequence
which consist of 18-28bp
• Used in many molecular techniques ranging
from PCR (polymerase chain reaction) to DNA
sequencing.
• Primers are designed to sequence the region
of a template which are to be annealed.
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9. Primer
• Primer are essential for the DNA amplification
• For the purpose of detection
• Cloning and
• Sequencing
• That’s why it is important to know the primer
designing
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10. Primer
• Primer that is designed impact the entire DNA
amplification process
• DNA polymerases, Enzyme that catalyze
replication of DNA only initiate replication
process by adding nucleotide to primers.
• It is also important to choose the right primer
for a successful DNA amplification.
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12. General rules for primer designing
Primer and amplicon length
Primer length determines the specificity and
significantly affect its annealing to the template
Too short -- low specificity, resulting in non-specific
amplification
Too long -- decrease the template-binding efficiency
at normal annealing temperature due to the higher
probability of forming secondary structures such as
hairpins.
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13. Continue
Optimal primer length
18-24 bp for general applications
30-35 bp for multiplex PCR
Optimal amplicon size
300-1000 bp for general application, avoid > 3 kb
50-150 bp for real-time PCR, avoid > 400 bp
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14. General rules for primer Designing
Melting temperature (Tm)
Tm is the temperature at which 50% of the DNA duplex
dissociates to become single stranded
Determined by primer length, base composition and concentration.
Also affected by the salt concentration of the PCR reaction mixture
Working approximation: Tm=2(A+T)+4(G+C) (suitable only for
18mer or shorter).
• Melting Temperature Tm (K) = {ΔH/ ΔS + R ln(C)}
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15. Continue
Optimal melting temperature
52°C-- 60°C
Tm above 65°C should be generally avoided because of the potential
for secondary annealing.
Higher Tm (75°C-- 80°C) is recommended for amplifying high GC
content targets.
Primer pair Tm mismatch
Significant primer pair Tm mismatch can lead to poor amplification
Desirable Tm difference < 5°C between the primer pair
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16. General rules for primer designing
Specificity and cross homology
Specificity
Determined primarily by primer length as well as sequence
The adequacy of primer specificity is dependent on the nature of
the template used in the PCR reaction.
Cross homology
To improve specificity of the primers it is necessary to avoid
regions of homology. Primers designed for a sequence must not
amplify other genes in the mixture. Commonly, primers are
designed and then BLASTed to test the specificity
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17. General rules for primer designing
GC content, repeats and runs
Primer G/C content
Optimal G/C content: 45-55%
Common G/C content range: 40-60%
Runs (single base stretches)
Long runs increases mis-priming (non-specific annealing)
potential
The maximum acceptable number of runs is 4 bp
Repeats (consecutive di-nucleotide)
Repeats increases mis-priming potential
The maximum acceptable number of repeats is 4 di-
nucleotide
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18. Continue
Hairpins
Formed via intra-molecular interactions
Negatively affect primer-template binding, leading to poor or no
amplification
Acceptable ΔG (free energy required to break the structure): >-2
kcal/mol for 3’end hairpin; >-3 kcal/mol for internal hairpin;
Self-Dimer (homodimer)
Formed by inter-molecular interactions between the two same primers
Acceptable ΔG: >-5 kcal/mol for 3’end self-dimer; >-6 kcal/mol for
internal self-dimer;
Cross-Dimer (heterodimer)
Formed by inter-molecular interactions between the sense and
antisense primers
Acceptable ΔG: >-5 kcal/mol for 3’end cross-dimer; >-6 kcal/mol for
internal cross-dimer;
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19. General rules for primer design
GC clamp and max 3’ end stability
GC clamp
Refers to the presence of G or C within the last 4 bases from
the 3’ end of primers
Essential for preventing mis-priming and enhancing specific
primer-template binding
Avoid >3 G’s or C’s near the 3’ end
Max 3’end stability
Refers to the maximum ΔG of the 5 bases from the 3’end of
primers.
While higher 3’end stability improves priming efficiency, too
higher stability could negatively affect specificity because of
3’-terminal partial hybridization induced non-specific
extension.
Avoid ΔG < -9.
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20. Annealing Temperature And Other
consideration
Ta (Annealing temperature) vs. Tm
Ta is determined by the Tm of both primers and amplicons:
optimal Ta=0.3 x Tm(primer)+0.7 x Tm(product)-25
General rule: Ta is 5°C lower than Tm
Higher Ta enhances specific amplification but may lower yields
Primer location on template
Dictated by the purpose of the experiment
For detection purpose, section towards 3’ end may be preferred.
When using composite primers
Initial calculations and considerations should emphasize on the
template-specific part of the primers
Consider nested PCR
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21. Resource for primer designing
Primer3
Primer3Plus
PrimerZ
PerlPrimer
Vector NTI Advantage 10
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