Nucleic Acids

To determine the correct linkage that joins two nucleotides, we need to understand the structure of nucleotides and how they connect to form nucleic acids like DNA or RNA.

1. Nucleotide Structure:
   • A nucleotide consists of three components: a sugar, a phosphate group, and a nitrogenous base.
2. Linkage Between Nucleotides:
   • Nucleotides are joined in a strand by bonding through the phosphate group of one nucleotide and the 3' -OH group of the sugar in the next nucleotide. This bonding creates a long chain, with a backbone made of sugar-phosphate-sugar-phosphate and so on.
   • The specific type of bond forming this linkage is known as a phosphodiester linkage. In a phosphodiester bond, the phosphate group links the 5' carbon of one sugar to the 3' carbon of the next sugar in the chain.
   • This phosphodiester linkage is crucial for creating the backbone structure of nucleic acids, allowing them to assume the correct geometry and function.
3. Ruling Out Other Options:
   • Disulphide linkage: Typically found in protein structures, particularly in the stabilization of protein tertiary and quaternary structures, and not in nucleic acids.
   • Glycosidic linkage: Common in carbohydrates, linking sugars by connecting an anomeric carbon of a sugar to another molecule, such as forming part of nucleoside structure, not between nucleotides.
   • Peptide linkage: Found in proteins, forming the backbone of proteins by linking amino acids, not nucleotides.

Thus, the correct answer is that two nucleotides are joined together by a phosphodiester linkage.