DNA, a polymer of deoxyribonucleoside monophosphates (dNMP), is held together by 3′→5′-phosphodiester bonds. Except for a small number of viruses that contain single-stranded DNA (ssDNA), DNA is generally found in a double-stranded form (dsDNA) where the two strands wrap around each other, forming a double helix. In eukaryotic cells, DNA is associated with different types of proteins collectively known as nucleoprotein, found in the nucleus. In prokaryotes, the protein-DNA complex is present in a non-membrane-bound region called the nucleoid.

Phosphodiester bonds are formed by linking the 3′-hydroxyl group of the deoxypentose of one nucleotide to the 5′-hydroxyl group of the deoxypentose of the adjacent nucleotide through a phosphoryl group. This results in a long, linear chain with polarity. The chain has a 5′-end with a free phosphate and a 3′-end with a free hydroxyl, both of which are not attached to other nucleotides. Bases along the deoxyribose-phosphate backbone are conventionally written in sequence from the 5′-end to the 3′-end. For instance, in the DNA shown in Figure 30.2D (5′-TACG-3′), the base sequence is read as "thymine, adenine, cytosine, guanine." Nucleases, such as deoxyribonucleases for DNA and ribonucleases for RNA, can enzymatically hydrolyze the phosphodiester linkages between nucleotides. Alternatively, the linkages can be cleaved by hydrolytic chemicals. It is important to note that only alkali can cleave RNA, not DNA.

The double helix structure of DNA is essential for its stability and function.
DNA is a polymer of deoxyribonucleoside monophosphates (dNMP) covalently linked by 3′→5′-phosphodiester bonds. With the exception of a few viruses that contain single-stranded DNA (ssDNA), DNA exists as a double-stranded molecule (dsDNA), in which the two strands wind around each other, forming a double helix. [Note: The sequence of the linked dNMP is primary structure, whereas the double helix is secondary structure.] In eukaryotic cells, DNA is found associated with various types of proteins (known collectively as nucleoprotein) present in the nucleus, whereas the protein–DNA complex is present in a non–membrane-bound region known as the nucleoid in prokaryotes.
A. 3′→5′-Phosphodiester bonds
Phosphodiester bonds join the 3′-hydroxyl group of the deoxypentose of one nucleotide to the 5′-hydroxyl group of the deoxypentose of an adjacent nucleotide through a phosphoryl group (Fig. 30.2). The resulting long, unbranched chain has polarity, with both a 5′-end (the end with the free phosphate) and a 3′-end (the end with the free hydroxyl) that are not attached to other nucleotides. By convention, the bases located along the resulting deoxyribose-phosphate backbone are always written in sequence from the 5′-end of the chain to the 3′-end. For example, the sequence of bases in the DNA shown in Figure 30.2D (5′-TACG-3′) is read “thymine, adenine, cytosine, guanine.” Phosphodiester linkages between nucleotides can be hydrolyzed enzymatically by a family of nucleases, deoxyribonucleases for DNA and ribonucleases for RNA, or cleaved hydrolytically by chemicals. [Note: Only RNA is cleaved by alkali.]
B. Double helix