Describe the structure of DNA and briefly comment on its salient features. 

DNA, or deoxyribonucleic acid, is the molecular foundation of genetic information in living organisms. It is the carrier of genetic instructions that govern the development, functioning, growth, and reproduction of all living organisms, from single-celled bacteria to complex multicellular animals and plants. Understanding the structure of DNA is crucial because it helps explain how genetic information is stored, replicated, and passed on from one generation to the next.

Structure of DNA:

DNA is a polymer composed of monomers called nucleotides. Each nucleotide consists of three components: a phosphate group, a deoxyribose sugar molecule, and a nitrogenous base. The structure of DNA was first described by James Watson and Francis Crick in 1953, who proposed the double-helix model based on experimental data, including X-ray crystallography images produced by Rosalind Franklin.

1. Double Helix:
   The most distinctive feature of DNA's structure is its double-helix configuration. This means that the DNA molecule consists of two long chains of nucleotides that twist around each other in a spiral shape. These two strands run in opposite directions, which is known as antiparallel orientation. The helical structure of DNA allows it to be tightly packed inside the nucleus of cells, despite its length.

2. Sugar-Phosphate Backbone:
   Each strand of DNA is held together by covalent bonds between the phosphate group of one nucleotide and the deoxyribose sugar of the next. This forms a backbone that provides structural integrity to the molecule. The sugar-phosphate backbone is negatively charged, which makes DNA hydrophilic and soluble in water. The backbone serves as the support for the nitrogenous bases, which project inward and pair with bases from the other strand.

3. Nitrogenous Bases and Base Pairing:
   The nitrogenous bases in DNA fall into two categories: purines (adenine (A) and guanine (G)) and pyrimidines (cytosine (C) and thymine (T)). The two strands of DNA are connected by hydrogen bonds between complementary pairs of nitrogenous bases. Adenine always pairs with thymine (A-T), and guanine always pairs __________ _________ ________ ______ _____ ________ _______ _____ ___ ___ _________ ___.
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