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DNA (Deoxyribonucleic Acid) is a molecule that carries the genetic information used in the development, functioning, and reproduction of all living organisms. It is composed of two long chains of nucleotides, which are twisted together to form a double helix.

Each nucleotide within a DNA chain is composed of three parts: a phosphate group, a sugar molecule (deoxyribose), and a nitrogenous base. There are four different nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T).

The double-helical structure of DNA consists of two chains of nucleotides running in opposite directions, arranged in a helical manner around a central axis. The sugar and phosphate groups on the nucleotides form the backbone of each DNA strand, while the nitrogenous bases face the center of the helix structure. The nitrogenous bases of each strand of DNA are connected by hydrogen bonds to nitrogenous bases on the opposite strand in a complementary manner. For example, adenine (A) always pairs with thymine (T) and guanine (G) always pairs with cytosine (C).

Within each strand of the DNA double helix, th`e sugar and phosphate groups are covalently bonded together, forming strong phosphodiester bonds. These covalent bonds create the strong backbone structure of the DNA strand.

Considering the interactions between the two strands of the DNA double helix, hydrogen bonds play a crucial role. Specifically, hydrogen bonds occur between the complementary nitrogenous base pairs (A-T and G-C) of the two strands. These bonds hold the two strands together, like rungs on a ladder, and provide the stability of the double-helical structure. However, these hydrogen bonds are weaker than the covalent bonds found within each DNA strand, which allows for the strands to be separated during processes such as DNA replication and transcription. In summary, the double-helical structure of DNA consists of two chains of nucleotides coiled around a central axis, with the sugar-phosphate backbone and complementary base pairs connected by hydrogen bonds.