Question

Two stretches of DNA, A and B are taken for a polymerase chain reaction. If DNA-A has more guanine-cytosine bases than adenine-thymine and DNA-B has more adenine-thymine bases than guanine-cytosine bases, which of the following statements is true regarding PCR of these two stretches of DNA?

• A. Both the stretches of DNA will have the same denaturation temperature
• B. DNA-B will have more denaturation temperature than DNA-A
• C. DNA-A will have more denaturation temperature than DNA-B
• D. The denaturation step of PCR is independent of the base content of the DNA stretch

Answer 1

The correct option is C DNA-A will have more denaturation temperature than DNA-B

There are four naturally occurring nucleotide bases found in the DNA strands. The four different types of DNA nucleotides are- adenine (A), thymine (T), cytosine (C) and guanine (G). When a DNA strand replicates complementary to a template strand, these four nucleotides get incorporated by the DNA polymerase and extend the nucleotide sequence. These nucleotide bases form specific pairs- A complementarily pairs with T with two hydrogen bonds ($A=T$) and G complementary pairs with C with three hydrogen bonds ($G\equiv C$).

The number of hydrogen bonds determines the melting temperature ($T_m$) of DNA; the higher the G-C content, the higher will be the hydrogen bonds and the higher will be the melting temperature of the DNA.
In the question above, two DNA stretches, A has more G-C content compared to B, hence A will have higher melting temperature/ denaturation temperature.