RNA quantification for appropriate concentration and quality (260/280 ratio) is an important step before downstream analysis (including sequencing, RT-qPCR, etc.). Having insufficient RNA quantities or a high salt or phenol in the RNA product can lead to variable or irreproducible downstream results. The various methods used for RNA quantification include: 1. UV spectrophotometric (challenges include: low sensitivity, cannot distinguish between nucleic acid species), 2. Fluorescence-based (challenges include: requires standards, cannot measure amplifiability, not sequence-specific), and 3. RT-PCR (challenges include: requires standards, time-intensive, costly). In order to overcome these challenges, and also to ensure the proper quantification and quality control for RNA product, it is important to use at least two or more methods in order to discard any inconsistencies. Using standards for calibrations increases the sensitivity range for RNA detention (fluorescence- and RT-PCR-based methods). When using RT- PCR, it is important to choose correct primers, aligning to the desired site on the template and of appropriate product length, along with positive, negative and loading controls. It is also important to have at least two primer pairs in order to confirm results.
Cell cytotoxicity assays measure the ability of certain compounds or chemical mediators to reduce the viability of the cells. The term cell cytotoxicity assay can sometimes be used interchangeably with cell proliferation assay. Healthy living cells can be identified by the use of formazan dyes, protease biomarkers or by measuring ATP content. The formazan dyes are chromogenic products formed by the reduction of tetrazolium salts by dehydrogenases, such as lactate dehydrogenase (LDH) and reductases that are released during cell death. Common tetrazolium salts include INT, MTT, MTS and XTT. Cell cytotoxicity can also be measured by using the SRB and WST-1 assays. These assays can usually be used in a high-throughput fashion and can be quantitated by measuring absorbance, colorimetry or luminescence. All these assays require similar numbers of cell plating at the initiation, a time course of treatment with the cytotoxic agent and at least triplicates for each condition at every point of analysis. Cell shrinkage, plasma membrane blebbing, cell detachment, externalization of phosphatidylserine, nuclear condensation and ultimately DNA fragmentation are well-described features of apoptosis. The assays that rely on cell membrane integrity for their function, may not be able to quantify early apoptosis. Therefore, in order to distinguish early apoptotic vs. late apoptotic or necrotic cells, additional flow cytometry techniques can be used. A combination of Annexin V and PI (propidium iodide) can be used to distinguish early (Annexin V+/PI-) and late apoptotic (Annexin V+/PI+) cells. Sometimes, caspase assays are used in order to differentiate the stages of apoptosis.
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The estimation of DNA methylation level heavily depends on the complete conversion of non-methylated DNA cytosines. It is crucial to ensure complete conversion of non-methylated cytosines in DNA. Therefore, it is important to incorporate controls for bisulfite reactions, as well as to pay attention to the appearance of cytosines in non-CpG sites after sequencing, which is an indicator of incomplete conversion.
The estimation of DNA methylation level heavily depends on the complete conversion of non-methylated DNA cytosines. It is crucial to ensure complete conversion of non-methylated cytosines in DNA. Therefore, it is important to incorporate controls for bisulfite reactions, as well as to pay attention to the appearance of cytosines in non-CpG sites after sequencing, which is an indicator of incomplete conversion.
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A PCR reaction consists of the template DNA, two primers covering the amplification site, an enzyme, and buffers. However, non-specific product amplification and primer-dimer formation during set-up are major causes of PCR failure. Nevertheless, high-quality DNA polymerase and optimize reaction buffers will certainly lead to a successful PCR reaction
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