DNA's secondary structure is predominantly determined by base-pairing of the two polynucleotide strands wrapped around each other to form a double helix. A purine base always pairs with a pyrimidine base (guanine (G) pairs with cytosine (C) and adenine (A) pairs with thymine (T) or uracil (U)). It has a single ring structure, a six-membered ring containing nitrogen. The pyrimidines are cytosine and thymine. Purines consist of a double ring structure, a six-membered and a five-membered ring containing nitrogen. The bases in the DNA are classified as purines and pyrimidines. The secondary structure is responsible for the shape that the nucleic acid assumes. The nucleotides on one strand base pairs with the nucleotide on the other strand. In DNA double helix, the two strands of DNA are held together by hydrogen bonds.

Secondary structure is the set of interactions between bases, i.e., which parts of strands are bound to each other. Main article: Nucleic acid secondary structure DNA Therefore, the complementary sequence will be to the sense strand. DNA is double-stranded containing both a sense strand and an antisense strand. An example of a complementary sequence to AGCT is TCGA. Sequences can be complementary to another sequence in that the base on each position is complementary as well as in the reverse order. Sequences are presented from the 5' to 3' end and determine the covalent structure of the entire molecule. Ī nucleic acid sequence is the order of nucleotides within a DNA (GACT) or RNA (GACU) molecule that is determined by a series of letters. Nucleic acids are formed when nucleotides come together through phosphodiester linkages between the 5' and 3' carbon atoms. The polarity in DNA and RNA is derived from the oxygen and nitrogen atoms in the backbone. For both the purine and pyrimidine bases, the phosphate group forms a bond with the deoxyribose sugar through an ester bond between one of its negatively charged oxygen groups and the 5' -OH of the sugar. Cytosine, thymine, and uracil are pyrimidines, hence the glycosidic bonds form between their 1 nitrogen and the 1' -OH of the deoxyribose. The nitrogen bases adenine and guanine are purine in structure and form a glycosidic bond between their 9 nitrogen and the 1' -OH group of the deoxyribose. 5-carbon sugar which is called deoxyribose (found in DNA) and ribose (found in RNA).It is this linear sequence of nucleotides that make up the primary structure of DNA or RNA. Primary structure consists of a linear sequence of nucleotides that are linked together by phosphodiester bond.

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