What is the difference between a nucleoside and a nucleotide?
A nucleoside is formed by the attachment of a nitrogenous base to the 1′ position of a pentose sugar. It consists of only two components: a base and a sugar. Examples include adenosine, guanosine, thymidine, uridine and cytidine.
A nucleotide is formed when a nucleoside is linked to phosphoric acid at the 5′-position of the sugar moiety. Thus, a nucleotide consists of three components: a base, a pentose sugar, and a phosphate group. Examples include adenylic acid, thymidylic acid, guanylic acid, uridylic acid and cytidylic acid.
The key difference is that a nucleoside contains only a base attached to sugar, whereas a nucleotide contains a base, sugar, and a phosphate group esterified to the sugar. Nucleotides are the monomers that make up nucleic acids like DNA and RNA.
Explanation
The textbook clearly distinguishes between nucleosides and nucleotides in section 10.5.2. A nucleoside is the simpler unit with just a base-sugar combination, formed at the 1′ position of sugar. A nucleotide is the complete unit with an additional phosphate group attached at the 5′ position. This distinction is fundamental to understanding nucleic acid structure, as nucleotides serve as the building blocks (monomers) of DNA and RNA polymers.
Solution Steps
Step 1: Define nucleoside as base + sugar (attached at 1′ position)
Step 2: Define nucleotide as base + sugar + phosphate (phosphate at 5′ position)
Step 3: Highlight the key difference - presence/absence of phosphate group