RT-PCR-specific errors in the quantification of mRNA transcripts are easily compounded by any variation in the amount of starting material between samples. This is especially relevant when dealing with in vivo samples that have been obtained from different individuals, or when comparing samples from different tissues.
The most common method for minimizing these errors and correcting for sample-to-sample variation is to amplify a cellular RNA specified by a housekeeping gene that serves as an internal reference against which other RNA values can be normalized.1-32-1 However, because there is no single mRNA with a constant expression level among different tissues of an organism, its use as an internal calibrator is inappropriate.1-34-1 rRNA has been proposed as an alternative normalizer, but there are serious concerns regarding its expression levels, transcription by a different RNA polymerase, and possible imbalances in relative rRNA-to-mRNA content in different cell types that caution against its use as a normalizer. Copy numbers can also be normalized to total cellular RNA and reported as copies per microgram of total RNA. However, total RNA levels may be increased in highly proliferating cells, and this will affect the accuracy of any comparison of copy numbers between normal and tumor cells.
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The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.