1. 21.2 Primary Structure of Nucleic Acids
In the primary structure of nucleic acids, each sugar in a sugar–phosphate backbone is attached to a base.
Learning Goal Describe the primary structures of RNA and DNA.

2. Primary Structure of Nucleic Acids
In the primary structure of nucleic acids,
the nucleotides are joined by phosphodiester bonds.
the 3ʹ — OH group of the sugar in one nucleotide bonds to the phosphate group on the 5ʹ carbon atom in the sugar of the next nucleotide.

3. Phosphodiester Linkage

4. Base Sequence, Primary Structure
Each nucleic acid has its own unique sequence of bases, which
is known as its primary structure.
carries the genetic information.
is read from the sugar with the free 5ʹ phosphate to the sugar with the free 3ʹ — OH group.
is often written using the letters of the bases to represent the correct sequence.
5ʹ A C G T 3ʹ

5. Primary Structure of RNA
In the primary structure of RNA, A, C, G, and U are connected by 3ʹ,5ʹ phosphodiester linkages.