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Real Time PCR

Real Time PCR

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Real Time PCR

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  1. Real Time PCR • Principles and Important Considerations

  2. Chemistry

  3. Chemistry

  4. Chemistry

  5. Primer/Probe design is really important • Use PrimerExpress • No mismatches are allowed (make sure you have the correct sequence) • MGB labeled with VIC or FAM (never ROX) • Primers/Probe need to be validated • For gene expression experiments it is ideal that one of the primers or probe sits in an exon junction (prevents amplification from genomic DNA).

  6. Endogenous control gene: • Present in all experimental samples • Expression does not vary between treatments, tissues, age, etc. i.e. constant expression levels • By using an endogenous control as an active reference you can normalize quantification of mRNA target for differences in the amount of total RNA added to each sample. i.e. loading control • Commonly used: 18S or 25S rRNA, actin, GAPDH, ubiquitin, etc

  7. Referencesample • Used in Comparative CT and relative standard curve experiments • Sample used as the basis for relative quantitation results. i.e. Everything gets expressed and compared relative to this sample. • Also called calibrator • It doesn’t matter which sample is used, however normally the negative control is used.

  8. Amplification curve: log view Always use during analysis Plateau Exponential Linear Amplification curve: linear view

  9. Ct It's all about Ct Threshold

  10. Threshold set too high Threshold set too low

  11. Threshold is important • Threshold determines Ct values • Ct values are used to calculate relative expression, presence/absence, etc. It's all about Ct values • Threshold should be set in the linear portion (parallel lines) in log view. Always check! • Do not use Ct values of 35 or higher. Repeat using more cDNA/DNA

  12. Endogenous control Gene of interest Good Comparative Ct example (for gene expression) • Small variation in endogenous control Ct values • Shape of curve, including linear phase (log view)

  13. 1 1 2 2 3 Bad Comparative Ct example • Too much variation in endogenous control Ct values • Sigmoidal curves. Check baseline levels • No amplification

  14. Other important considerations • Really sensitive technique: 1 DNA molecule can be detected • Contaminationand cross-contamination IS a problem • Always include no template (water) controls for each gene in each plate • Include no RT control (RNA) some of the time. Specially when you are starting using this technique • RNA/cDNA/DNA quality is critical. OD values (260/280≥ 1.8 260/230 ≥2.0) • At least 3 biological replicates

  15. Good Practices • Mortars and pestles should be treated with bleach and autoclaved • Pipettes and centrifuge should be decontaminated (RNAase Away) • Cover your working area and replace mats after each round of extractions • Change gloves frequently • If extracting RNA don't talk to your samples! RNases are present in your saliva. Use RNase-free water • Keep your working area clean!

  16. Reaction setup NEVER set your plates/tubes directly on the bench or ice ALWAYS set your plates/tubes on a rack

  17. Keep the machine clean and free of dust! Plates go directly on the heating block. Tubes go on a black rack.

  18. Online help • Real-Time PCR Systems. Chemistry Guide: • Guide to Performing Relative Quantitation of Gene Expression Using Real-Time Quantitative PCR: • • Troubleshooting guide: