Polynucleotides adenine-uracil base pairs

Base pairing The hydrogen bonds formed between complementary bases that are part of the polynucleotide chains of nucleic acids. The base pairing is specific in that adenine will base pair with thymine (uracil in RNA) and guanine will pair with cytosine. 

Complementary structures two structures which define one another, e.g. the 2 polynucleotide chains in the DNA duplex. The base pairs adenine/thymine (or adenine/uracil in RNA) and guanine/cytosine are complementary, so that by base pairing the nucleotide sequence of one polynucleotide chain defines a unique sequence in the complementary strand. 

Two polynucleotides with sequences A U (where A and U denote adenine and uracil, respectively) interact through A-U base pairs, forming a double helix. When n = 5 and n = 6, the equilibrium constants for formation of the double helix are 5.0 x 10 and 2.0 x 10 , respectively, (a) Suggest an explanation for the increase in the value of the equilibrium constant with n. (b) Calculate the contribution of a single A-U base pair to the Gibbs energy of formation of a double helix between A U polypeptides. 

DNA, a constituent of the cell nucleus, consists of two strands of polynucleotides that are coiled to form a double helix. The strands are held together by H-bonding between the nitrogen bases. The pyrimidines always form H-bonds with a specific purine i.e., cytosine with guanine and thymine with adenine. However, in RNA the pairing is between uracil and adenine. 

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