Certain purification steps are inefficient at low concentrations. One such step is precipitation. To form a precipitate a number of molecules need to aggregate into a "particle". A single molecule will not precipitate (even when solubility is decreased), but an aggregate will. The carrier molecules, whether RNA or something else that mimics nucleic acid, add the concentration and therefore proximity, necessary to form the aggregates that can be precipitated.
Carrier molecules have the greatest benefit when the input concentration is very low. The proportional loss of input molecules decreases as the concentration increases.
With this, it is possible that my eluted nucleic acids also contain the carrier RNA, that adds to the concentration?
@Sedef: Copied from the Qiaamp Viral RNA manual
"Yields of viral RNA isolated from biological samples are normally less than 1 μg and therefore difficult to determine photometrically. Keep in mind that the carrier RNA (5.6 μg per 140 μl sample) will account for most of the RNA present. Quantitative RT-PCR is recommended for determination of viral RNA yield."
Carrier RNA helps to isolate RNA at low yield so that the downstream RT-PCR can be successful. It is also good for improving the efficiency of an RNA isolation kit
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