Pna Chemistry And Properties

The first report concerning the design and properties of PNAs was published in 1991 by Nielsen et al.[1] PNAs are synthetic DNA analogs in which the phosphodiester backbone is replaced by repetitive units of N-(2-amino-ethyl) glycine to which the purine and pyrimidine bases are attached via a methyl carbonyl linker (Fig. 1). The PNA molecules can routinely be labeled with biotin or fluorophores such as fluorescein or rhodamine.

A subsequent generation of PNAs could involve modification of the N-(2-aminoethyl) glycine backbone (PNA analogs) or chimeric architecture, such as PNA-peptide chimeras or PNA-DNA chimeras developed to improve the solubility and the cellular uptake of PNAs or to exhibit new biological properties. The synthetic backbone provides PNA with unique hybridization characteristics. Unlike DNA and RNA, the PNA backbone is not charged. Consequently, there is no electrostatic repulsion when PNA hybridizes to its target nucleic acid sequence, giving a higher stability to the PNA-DNA or PNA-RNA duplexes than the natural homo- or hetero-duplexes. This greater stability is reflected by a higher thermal melting temperature (Tm) as compared to the corresponding DNA-DNA or DNA-RNA duplexes.

An additional consequence of the polyamide backbone is that PNAs hybridize virtually independently of the salt concentration. This property can be exploited when targeting DNA or RNA sequences involved in secondary structures, which are destabilized by low ionic strength. This facilitates the hybridization with the PNAs. The unnatural backbone of PNAs also means that PNAs are not degraded by nucleases or proteases. Because of this resistance to the enzyme degradation, the lifetime of PNAs is extended both in vivo and in vitro. Also, PNAs are not recognized by polymerases and therefore cannot be directly used as primers or be copied.

Peptide nucleic acids hybridize to complementary DNA or RNA in a sequence-dependent manner, according to the Watson-Crick hydrogen bonding scheme. In contrast to DNA, PNA can bind in either parallel or antiparallel fashion. However, the antiparallel binding is favored over the parallel one. PNA is able to adopt both A- and B-type structures when associating with RNA and DNA, respectively, and PNA-PNA duplexes formed an unusual helix conformation, called P type, and characterized by a large pitch of 18 base pairs.

Peptide nucleic acid probes can bind to either single-stranded DNA or RNA, or to double-stranded DNA (dsDNA). Homopyrimidine PNAs, as well as PNAs containing a high pyrimidine/purine ratio, bind to complementary DNA sequences to form highly stable (PNA)2-DNA triplex helices displaying Tm over 72°C. In these triplexes, one PNA strand hybridizes to DNA through standard Watson-Crick base pairing rules, whereas the other PNA strand binds to DNA through Hoogsteen hydrogen bonds. The resulting structure is called P loops. The stability of these triple helixes is so high that homopyrimidine PNA targeted to purine tracts of dsDNA invades the duplex by displacing one of the DNA

strands.[2]

Finally, PNA-DNA hybridization is significantly more affected by base mismatches than DNA-DNA hybridization. A single mismatch in a mixed PNA-DNA 15-mer duplex decreases the Tm by up to 15°C, whereas in the corresponding DNA-DNA complex, a single mismatch decreases the Tm by only 11 °C. This high level of discrimination at single-base level has indicated that short PNA probes could offer high specificity and has thus allowed the further development of several PNA-based strategies for molecular investigations and diagnosis.

Getting Started With Dumbbells

Getting Started With Dumbbells

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.

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