A carrier molecule is typically involved in the transport of other biological compounds such as proteins, DNA or RNA, electrons, or protons including ions. For example, carrier proteins can transport other molecules such as ions, sugar, fat, or peptides through the cell membrane. For the isolation of DNA and RNA using mini purification kits, the use of carrier DNA is recommended when expected yields are low, for example, below ten (10) ng. DNA carrier molecules such as poly-dA (poly-(A)), poly-dT (poly-(T)) or poly-dA:dT (poly-(AT)) should be used for RT-PCR. Other carriers such as herring sperm DNA may interfere with PCR by possibly nonspecific binding of primers. Poly-(A) may interfere with oligo-dT primers. In this case, use a different carrier. Poly-(A) refers to the poly-(A) tail at the 3'-end of oligonucleotides consisting of a repetitive sequence of adenine nucleotides. The 3'-end of nearly all eukaryotic mRNAs include a string of 50 to 250 adenylate residues, called poly-A tail. Typical concentrations of carriers are >10 μg/ml. The size distribution of the carrier molecules ranges from 100 base pairs ten (10) kilobases (kb). But optimal amounts used need to be experimentally determined for each application. Beránek et al., in 2016, investigated the effect of carrier molecules on the extraction yields of circulating tumor DNA (ctDNA) from plasma samples. Their results revealed that poly-(A) worked best and therefore is the carrier molecule of choice when extracting low levels of DNA or RNA from biological samples. Alternatively, a linear polyacrylamide (LPA) is also an option for DNA and RNA template coprecipitation. Gaillard and Stauss precipitated DNA with ethanol in the presence of LPA to improve yields. Sachdeva and Simm demonstrated the precipitation effect of LPA during their isolation and characterization of X-DING-CD4 cDNA using touch-down PCR. Carrier molecules used for immunization are:
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