PCR

Polymerase Chain Reaction (PCR)

DNA DNA is a nucleic acid that is composed of two complementary nucleotide building block chains. The nucleotides are made up of a phosphate group, a five carbon sugar, and a nitrogen base.

DNA DNA Sugar Deoxyribonucleic acid RNA Sugar Ribonucleic acid

DNA DNA has four nitrogen bases. Two are purines ( 2 ringed base ) Adenine ( A ), Guanine ( G ) Two are pyrimidines ( 1 ringed base ) Cytosine ( C ), Thymine ( T )

DNA These four bases are linked in a repeated pattern by hydrogen bonding between the nitrogen bases. The linking of the two complementary strands is called hybridization.

DNA Example of bonding pattern. Primary strand CC G AA T GGG A T G C GG C TT A CCC T A C G Complementary strand

DNA Molecule Adenine Thymine Guanine Cytosine

DNA A purine always links with a pyrimidine base to maintain the structure of DNA. Adenine ( A ) binds to Thymine ( T ), with two hydrogen bonds between them. Guanine ( G ) binds to Cytosine ( C ), with three hydrogen bonds between them.

What does PCR need? Template (the DNA you are exploring) Sequence-specific primers flanking the target sequence, Forward & Reverse. Polymerases Nucleotides (dATP, dCTP, dGTP, dTTP) Magnesium chloride (enzyme cofactor) Buffer Water, mineral oil

PCR Requirements Magnesium chloride: .5-2.5mM Buffer: pH 8.3-8.8 dNTPs: 20-200µM Primers: 0.1-0.5µM DNA Polymerase: 1-2.5 units Target DNA:  1 µg

Steps in PCR Denaturation 93 to 95°C 1min Annealing 50 to 55°C 45sec Elongation 70 to 75°C 1-2min

How does PCR work? Heat (94 o C) to denature DNA strands Cool (54 o C) to anneal primers to template Warm (72 o C) to activate Taq Polymerase, which extends primers and replicates DNA Repeat multiple cycles

Denaturation Denaturation is the first step in PCR, in which the DNA strands are separated by heating to 95°C. The Hydrogen bonds between the two strands breaks down and the two strands separates.

Annealing Annealing is the process of allowing two sequences of DNA to form hydrogen bonds. The annealing of the target sequences and primers is done by cooling the DNA to 55°C. Time taken to anneal is 45 seconds.

Elongation Taq polymerase binds to the template DNA and starts adding nucleotides that are complementary to the first strand. This happens at 72°C as it is the optimum temperature for Taq Polymerase.

Denaturing Template Heat causes DNA strands to separate Denature DNA strands 94 o C 3’ 5’ 5’ 3’ 5’ 3’ 3’ 5’

Annealing Primers Primers bind to the template sequence Taq Polymerase recognizes double-stranded substrate 3’ 5’ Primers anneal 64 o C 3’ 5’ 3’ 5’ 5’ 3’ 3’ 5’ 5’ 3’

Taq Polymerase Extends 3’ 5’ 3’ 5’ Extend 72 o C 3’ 5’ 3’ 5’ Taq Polymerase extends primer DNA is replicated Repeat denaturing, annealing, and extending 30 cycles 3’ 5’ 3’ 5’ 5’ 3’ 5’ 3’

The target product is made in the third cycle 3 ’ 5’ 3 ’ 5’ 3’ Cycle 1 Cycle 2 Cycle 3 3’ 5’ 5’ 3’ 5’ 5’ 3’ 5’ 3’ 5’ 3’ 3’ 3’ 3’ 3’ 5’ 5’ 5’ 5’

PCR Cycles Review Denaturation: 94°- 95°C Primer Annealing: 55°- 65°C Elongation of DNA: 72° Number of Cycles: 25-40 No target products are made until the third cycle. At 30 cycles there are 1,073,741,764 target copies (~1×10 9 ).

PCR Primers A primer is a strand of nucleic acid that serves as a starting point for DNA replication. A primer for each target sequence on the end of your DNA is needed. This allows both strands to be copied simultaneously in both forward and reverse directions.

Primer Problems Primers should flank the sequence of interest Primers that match multiple sequences will give multiple products Repeated sequences can be amplified -but only if unique flanking regions can be found where primers can bind A primer may form a dimer with itself or with the other primer. 5´-ACCGGTAGCCACGAATTCGT-3´ |||||||||| 3´-TGCTTAAGCACCGATGGCCA-5´

Primers That Form Hairpins Primers can have self-annealing regions within each primer (i.e. hairpin and foldbackloops) A primer may be self-complementary and be able to fold into a hairpin: 5´-GTTGACTTGATA ||||| T 3´-GAACTCT The 3´ end of the primer is base-paired, preventing it annealing to the target DNA.

PCR Taq DNA Polymerase Taq stands for Thermus aquaticus , which is a microbe found in 176°F hot springs in Yellow Stone National Forest. Taq DNA Polymerase (Taq Pol) is stable in high temperatures and acts in the presence of Mg. The optimum temperature for Taq Pol is 72°C

Disadvantages of Taq Pol Taq Pol lacks 3’ to 5’ exonuclease proof reading activity, commonly present in other polymerases. Taq mis-incorporates 1 base in 10 4 . A 400 bp target will contain an error in 33% of molecules after 20 cycles. Error distribution will be random.

Limitations of PCR Need for target DNA sequence information Primer Designing for unexplored ones. Boundary regions of DNA to be amplified must be known. Infidelity of DNA replication. Taq Pol – no Proof reading mech – Error 40% after 20 cycles Short size and limiting amounts of PCR product Up to 5kb can be easily amplified . Up to 40kb can be amplified with some modifications. Cannot amplify gene >100kb Cannot be used in genome sequencing projects.

How to overcome Difficulties? Pfu DNA Polymerase from Pyrococcus furiosus possesses 3' to 5' exonuclease proofreading activity. The error rate is only 3.5% after 20 cycles More amount of primer is added to avoid primer dimering. For unexplored genes, primers used in closely related species are used.

Designing PCR Primers Primer sequences should be unique Primers should be ~20 bases long. The G/C content should be 45–55%. The annealing temperatures should be within 1°C of one another. The 3´-most base should be a G or C. The primers must not base pair with each other or with themselves or form hairpins. Primers must avoid repetitive DNA regions.

Advantages of PCR Speed Ease of use Sensitivity Robustness

Applications of PCR Screening human DNA samples for mutations associated with genetic diseases such as thalassemia and cystic fibrosis. Can detect the presence of viral DNA before it turns in to a killer. PCR enables rapid amplification of template DNA for screening of uncharacterized mutations Can obtain sequences from hair, blood stain, bones, other forensic specimens, other remains preserved at archaeological sites.

Shanthilal.J III B.Tech., Biotechnology, K.S.Rangasamy College of Technology Tiruchengode. By,

PCR

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PCR