Another strategy to provide RNA fingerprints employed an arbitrary primer instead of an anchored oligo-dT primer in the first step of RT, selecting those regions internal to the RNA that have 6-8 base matches with the 3' end of the primer. As in DD-PCR, this is followed by arbitrary priming of the resulting first strand of cDNA with the same or a different arbitrary primer, and then PCR amplification. This method is known as RAP-PCR. This method samples anywhere in the RNA, including opening reading frames, and can be used on RNAs that are not poly-adenylated (such as bacterial RNA). A limitation of this method too is that rare RNAs will be underrepresented. Differential display strategies designed to target specific sequences in bacteria, such as highly iterated palindromic (HIP) elements found in half of the genes in the genome of cyanobacteria, provided a convenient global expression strategy for identifying light intensity-regulated genes in bacteria using a limited number of primers. Another area of wide application of DD is a study of scarcely sequenced and complex plant genes, which contain large families of homologous genes.
In order to improve the specificity of DD, attempts were made to replace the 10-13-base arbitrary primers— which hybridize nonspecifically to cDNA templates causing mismatches—with primer sites produced by restriction enzyme digests of double-stranded (ds) cDNAs; thus allowing adapters for priming sites to be ligated into specific regions on the cDNA. These ds cDNAs are then amplified with either anchored oligo-dT
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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.