There are a variety of circumstances under which rapid results using multiple markers in PCR amplifications are highly desired, such as in forensics, pathogen analysis, or detection of genetically modified organisms. In multiplex PCR, multiple sets of primers are used, often with less success than when applied to PCR as individual sets. Numerous studies have been conducted to optimize procedures, but each has described the process as time consuming and often unsuccessful. Considering the information given in Problem, why should multiplex PCR be any different than single primer set PCR in terms of dependability and ease of optimization?
Most of the techniques described in this chapter (blotting, cloning, PCR, etc.) are dependent on intermolecular attractions (annealing) between different populations of nucleic acids. Length of the strands, temperature, and percentage of GC nucleotides weigh considerably on intermolecular associations. Two other components commonly used in hybridization protocols are monovalent ions and formamide. A formula that takes monovalent ion (Na+) and formamide concentrations into consideration to compute a Tm (temperature of melting) is as follows:
Tm = 81.5 + 16.6(log M[Na+]) + 0.41(%GC) – 0.72(%formamide)
(a) For the following concentrations of Na+ and formamide, calculate the Tm. Assume 45% GC content.
Na+
% Formamide
0.825
20
0.825
40
0.165
20
0.165
40
(b) Given that formamide competes for hydrogen bond locations of nucleic acid bases and monovalent cations are attracted to the negative charges of nucleic acids, explain why the Tm varies as described in part (a).
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