Haushaltsgene (housekeeping genes)

Any DNA contamination will artificially inflate the SYBR Green signal, yielding skewed gene expression profiles and false-positive signals. The most common source of DNA contamination is from PCR products generated during previous experiments. Such contamination is most often due to the improper disposal of tubes, tips, and gels that previously came into contact with PCR products. Additionally, PCR products may also contaminate pipettors, racks, work pads, and commonly used reagents such as water and buffers. To minimize the risk of contaminating your experiment with extraneous DNA, the following steps should be taken:

• Remove a single aliquot of water from your PCR-grade stock, sufficient to complete the experiment. This minimizes the number of times that the stock container is opened, thereby minimizing contamination risks.
• Use only fresh PCR-grade reagents and disposable labware.
• Treat any labware (tubes, tips, and tip boxes) used in PCR with 10% bleach, before discarding.
• Maintain a dedicated workspace for PCR setup (perhaps a PCR-only hood), away from areas of the lab where post-PCR work is done, such as running gels, enzyme digestions, and cloning.
• Change the lab bench pads/papers often and decontaminate lab benches and labware (racks, pipettors, etc.) before each use by washing with 10% bleach, and/or exposing to UV light for at least 10 minutes. This serves to degrade and/or inactivate contaminating DNA.
• Before, during, and after the experiment, minimize the opening and closing of any tubes or plates used during the experiment.  

For real-time detection, the RT² Profiler PCR Array is currently available for most QIAGEN, ABI, BioRad, Eppendorf, Stratagene, TaKaRa, Fluidigm, Cepheid, and Roche real-time instruments. Please refer to the link below, to determine which RT² Profiler PCR Array plate format is compatible with your instrument.

http://www.sabiosciences.com/manuals/PCRArrayGuide.pdf

The 3 most common negative controls included in a qPCR and/or qRT-PCR experiment are as follows:

1. A no template control (NTC) omits any DNA or RNA template from a reaction, and serves as a general control for extraneous nucleic acid contamination. When using SYBR Green chemistry, this also serves as an important control for primer dimer formation. Within the RT2 Profiler PCR Arrays, the GDC well also serves as a no template control, as this assay is designed to detect Genomic DNA.

2. A no reverse transcriptase control (NRT) or minus reverse transcriptase control (MRT) involves carrying out the reverse transcription step of a qRT-PCR experiment in the absence of reverse transcriptase. This control assesses the amount of DNA contamination present in an RNA preparation.

3. A no amplification control (NAC) omits the DNA polymerase from the PCR reaction. This is a control for background fluorescence that is not a function of the PCR. Such fluorescence is typically attributable to the use of a degraded, dual-labeled probe. This control is unnecessary when utilizing SYBR-Green probe chemistries.

Assuming 100% amplification efficiency, each step increase in Ct value represents a doubling in the amount of qPCR template. Therefore, evaluating the difference in Ct values between the qPCR assay, and its matching NRT control, leads to the following predictions:

Dissociation curves are carried out at the end of a PCR experiment by following a 3-step procedure.

First, all the components are denatured at 95°C, followed by complete annealing at a set temperature (based on the primer Tm values), followed by a gradual increase in temperature up to 95°C. Fluorescence intensity is monitored during this final temperature increase, resulting in the generation of a melting curve or dissociation curve.

By analyzing the first derivative of such a curve, you can readily assess the homogeneity of the PCR products, including the presence of primer–dimers, thereby determining the specificity of the PCR reaction. It is important to carry out such post-PCR analyses when using SYBR Green probe chemistry due to this reagent's lack of sequence specificity.

It is critical to include appropriate positive controls in a qPCR experiment to determine if false negatives are being detected in the experiment. Positive controls fall into one of 2 classes.

1. Exogenous positive controls refer to the use of external DNA or RNA carrying a target of interest. If these positive controls are assayed in separate wells/tubes from the experimental sample, they serve as a control to determine whether or not the reverse transcription and/or PCR reaction conditions are optimal. Additionally, exogenous DNA or RNA positive controls may be spiked into the experimental sample(s), and assayed in parallel or in a multiplex format with, the target of interest. These control reactions assess whether the samples contain any components that inhibit reverse transcription and/or PCR.

2. Endogenous positive controls refer to the use of a native target that is present in the experimental sample(s) of interest, but is different from the target under study. These types of controls are often referred to as normalizers, and are typically used to correct for quantity and quality differences between samples.

Within the RT2 Profiler PCR Arrays, the Positive PCR Control (PPC) wells contain a plasmid with a primer assay that detects a sequence it produces. This allows for quick confirmation of the performance of the PCR steps.

The RTC wells include assays that detect the artificial RNA that is spiked in to each sample during the cDNA synthesis step. This ensures the Reverse Transcription step proceeded as needed.

The Reverse transcription control requires that the reverse transcription is done with the RT² first strand kit. No other cDNA synthesis method can use this control.