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pcr techniques in molecular biology
Citation preview
http://www.ncbi.nlm.nih.
gov/books/NBK7571/
http://www.ncbi.nlm.nih.
gov/books/NBK21128/
To familiarize students with the PCR technique and how its performed.
To illustrate how PCR is performed and how its results are analyzed.
To illustrate the importance of the PCR technique via its many applications.
Polymerase: DNA polymerase (enzyme) ◦ DNA polymerase duplicates DNA
◦ Before a cell divides, its DNA must be duplicated
◦ Chain Reaction: The product of a reaction is used to amplify the same reaction
◦ Results in rapid increase in the product
Polymerase Chain Reaction (PCR) uses principles of DNA replication to exponentially copy or amplify genes or DNA segments
Sequence to be copied called “target DNA/sequence”
Viral RNA can also be used
Amplified fragment called an amplicon
1. Template DNA ◦ The target DNA usually originates from the
cells of an organism ◦ Genomic DNA is extracted from the organism
which contains the target sequence ◦ whole cells can also be used ◦ Strands must be separated ie single stranded
for amplification to occur
…TCGGGAGGTCCTGTCCGACGTAGTCTTCTCCGGTAGTTCGTCCAGACAAGGTTCCCGGAA… 3’ 5’
…AGCCCTCCAGGACAGGCTGCATCAGAAGAGGCCATCAAGCAGGTCTGTTCCAAGGGCCTT… 3’ 5’
Target DNA
2. dNTPs ◦ Raw materials necessary
to synthesize DNA ◦ 4 dNTPs – for each base
found in DNA
T
C
A
A
C
T
A
T
G C
T C G
3. Primers ◦ Short sequences of chemically synthesized DNA (10-30
bases)
◦ Bind to the ss template DNA (or RNA) ◦ 2 primers used, usually called forward (F) and reverse (R)
◦ F & R primers flank and define the target DNA (gene) to be copied
◦ Serve as recognition sites for the polymerase
GGGAGGTCCT 3’ 5’ GTTCCAAGGG
5’ 3’
4. Polymerase Enzyme ◦ Thermostable enzyme which synthesizes new DNA
◦ Recognizes primers as start point for synthesis
◦ Synthesizes new strand in 5’ to 3’ direction
◦ Requires buffer and MgCl2 to function
5. PCR Reaction Buffer ◦ Generally contains:
0-50mM Tris-HCl pH 8.3
up to 50mM KCl
BSA
◦ Stabilize polymerase and the amplicons during PCR process
6. MgCl2 – polymerase functioning
Thermal cycler/PCR Machine ◦ uses heat to melt (separate)
template DNA and anneal primers to template DNA
◦ Electronically controlled, capable of changing temperatures quickly and in a cyclic manner
◦ Inventor: Kary Mullis in 1983
got $10,000 for his invention
Nobel prize in 1993
Different protocols for extracting genomic DNA
DNA should not be contaminated with spurious DNA, carbohydrates, phenols, or other compounds as these would inhibit DNA amplification.
PCR components Amount
Template DNA (5-200 ng)
1 mM dNTPs (200 uM final)
10 X PCR buffer
25 mM MgCl2 (1.5 mM final)
20 uM forward primer (20 pmoles final)
20 uM reverse primer (20 pmoles final)
5 units/uL Taq DNA polymerase (1.5 units)
Water
Final Volume
3 uL
10 uL
5 uL
3 uL
1 uL
1 uL
0.5 uL
26.6 uL
50 uL
In a thin wall Eppendorf tube assemble the following:
PCR occurs in a 3 step process called a cycle
1. Denaturation ◦ Occurs at approx. 90-95C, 1min
◦ The H-bonds in isolated DNA is denatured & made ss
◦ ss DNA serves as template for DNA cloning
Heating separates the double stranded DNA ◦ Denaturation
Slow cooling anneals the two strands ◦ Renaturation
Heat Cool
2. Annealing ◦ Temperature quickly reduced to about 40-65C
◦ Allows primers to bind to complimentary regions at the beginning and end of target sequence of target DNA in ssDNA, 45s
◦ 2 primers reqd, 1 for each strand
◦ Annealing temp. dependent on the length & base sequence of primer
◦ Allows primer to bind to template at high specificity
…TCGGGAGGTCCTGTCCGACGTAGTCTTCTCCGGTAGTTCGTCCAGACAAGGTTCCCGGAA… 3’ 5’
…AGCCCTCCAGGACAGGCTGCATCAGAAGAGGCCATCAAGCAGGTCTGTTCCAAGGGCCTT… 3’ 5’
GGGAGGTCCT 5’ 3’
GTTCCAAGGG 5’ 3’ T C
A
A
C
T
A T G
C
T C
G
3. Elongation ◦ Taq recognises primer and synthesises
complimentary strand using dNTPs ◦ 70-75C optimum, temp for taq polymerase
activity, 2min
T
G
…TCGGGAGGTCCTGTCCGACGTAGTCTTCTCCGGTAGTTCGTCCAGACAAGGTTCCCGGAA… 3’ 5’
…AGCCCTCCAGGACAGGCTGCATCAGAAGAGGCCATCAAGCAGGTCTGTTCCAAGGGCCTT… 3’ 5’
GGGAGGTCCT 5’
GTTCCAAGGG 5’
T C C G
A
A
C
T
A G T C
C
T
C G G
The above steps is referred to as a cycle.
At least 30 cycles are done
Each cycle increases the copy no. exponentially
Fragments are resolved on either agarose/ polyacrylamide gels
Calculates the predicted yield of DNA
PCR product yield = (input target amount) x (1 + % efficiency) cycle number
Molecular Identification Sequencing Genetic Engineering
• Molecular Archaeology • Molecular Epidemiology • Molecular Ecology • DNA fingerprinting • Classification of organisms • Genotyping • Pre-natal diagnosis • Mutation detection • Paternity testing • Sex determination • Detection of pathogens
• Bioinformatics • Genomic cloning • Genome Projects
• Site-directed mutagenesis • Gene expression studies
Primers are necessary ◦ To synthesize primers, knowledge of the
DNA sequence of interest must be known ◦ PCR cannot be used to study/amplify
fragments of genes never studied before or be used in areas of the genome where no sequence info is available
◦ **Exception- Arbitrarily Primed PCR (AP-
PCR), RAPDs or PCR where small known sequences are added to fragments to which primers are then made – AFLP
Size of amplified Fragment (amplificon) ◦ PCR can clone fragments up to 5 kb without much
difficulty, 40kb with some modification
◦ Some applications such as genome sequencing requires fragments>100kb to be amplified, therefore PCR cannot be used