Types of PCR
Multiplex PCR employs different primer pairs in the same reaction for simultaneous amplification of multiple targets. This type of PCR often requires extensive optimization of annealing conditions for maximum amplification efficiency of the different primer–template systems and is often compromised by nonspecific PCR artifacts. A stringent hot-start procedure and specially optimized buffer systems are absolutely crucial for successful multiplex PCR.
Compared with standard PCR systems using only 2 primers, an additional challenge of multiplex PCR is the varying hybridization kinetics of different primer pairs. Primers that bind with high efficiency could utilize more of the PCR reaction components, thereby reducing the yield of other PCR products. This often results in unamplified DNA sequences and absence of expected PCR products. Commercial PCR kits are available that are specifically designed to overcome the challenges of multiplex PCR and it is recommended that, where possible, such a kit is used.
Methylation-specific PCR (MSP)
MSP enables the methylation status of target DNA to be determined after sodium bisulfite treatment. The method requires two sets of primers to be designed: one set that anneals to unchanged cytosines (i.e., methylated in the genomic DNA) and one set that anneals to uracil resulting from bisulfite treatment of cytosines not methlyated in the genomic DNA. Amplification products derived from the primer set for unchanged sequences indicates the cytosines were methylated and thus protected from alteration (6).
Stringent and highly specific PCR conditions must be used to avoid nonspecific primer binding and the amplification of PCR artifacts. This is particularly important as the conversion of unmethylated cytosines to uracils reduces the complexity of the DNA and increases the likelihood of nonspecific primer–template binding.
Hot start PCR
RAPD: Rapid amplified polymorphic DNA analysis
In RAPD, the primers are only ~10 bases long. As a result, annealing temperatures required are <40°C.
RACE: Rapid amplification of cDNA ends
RACE is a variant of RT-PCR and is a procedure for amplification of nucleic acid sequences from a messenger RNA template between a defined internal site and unknown sequences at either the 3' or the 5' -end of the mRNA. RACE only requires the knowledge of a short sequence within the mRNA of interest. It is often used for cloning the remainder of incomplete cDNAs. There are two techniques:
5' RACE — amplifies 5' cDNA ends
3' RACE — amplifies 3' cDNA ends
The first step is common to both types of RACE and involves the conversion of RNA to single-stranded cDNA using a reverse transcriptase. The second steps are unique to each type of RACE; although each generates information that may yield the full-length cDNA sequence.
Because RACE uses an “anchor site” within the mRNA as a point of reference, it is sometimes known as “anchored PCR”.
In situ PCR
In situ PCR is a PCR reaction that occurs inside the cell on a slide, thus combining the sensitivity of PCR or RT-PCR amplification with in situ hybridization. In situ PCR allows cellular markers to be identified and further enables the localization to cell-specific sequences within cell populations, such as tissues and blood samples. Therefore, it is a powerful tool in applications such as the study of disease progression.
Fresh or fixed cells or tissue samples can be used in the procedure, although preparation of the sample is critical to the result, with fixation having a direct influence on PCR signal. The procedure is suitable for use with radiolabeled, fluorescently labeled or biotin-labeled nucleic acid probes.
The PCR process is essentially the same as a standard PCR, but with some modified reaction conditions (e.g., Mg2+ concentration).
Differential display PCR
Differential display PCR is based on RT-PCR and is used to compare and identify differences in mRNA (and therefore gene) expression patterns between two cell lines or populations.
In this technique, first-strand cDNA synthesis is primed with an anchored primer complementary to ~13 nucleotides of the poly(A) tail of mRNA and the adjacent 2 nucleotides of the transcribed sequence. After reverse transcription and amplification, amplified products are visualized using gel electrophoresis. The banding patterns observed can be compared to identify differentially expressed cDNAs in the 2 populations.
Invented in the 1990s, the technique fast became a key tool in gene expression analysis. However, it has been more recently superseded by RNA-seq, microarrays, and qRT-PCR.