Understanding Nucleotide Structure

A nucleotide is the fundamental building block of nucleic acids like DNA and RNA. It is a composite molecule formed by three specific components.

Step 1: Identify the Three Components

Every nucleotide consists of three parts:

1. A Nitrogenous Base: This can be a purine (Adenine, Guanine) or a pyrimidine (Cytosine, Thymine, Uracil).
2. A Pentose Sugar: This is a 5-carbon sugar. In DNA, it is deoxyribose; in RNA, it is ribose.
3. A Phosphate Group: This is a functional group consisting of phosphorus and oxygen atoms (${\mathrm{PO}}_4^{3-}$).

Step 2: Understand How They Connect

The three components are covalently bonded together in a specific way:

• The nitrogenous base is attached to the 1' carbon of the sugar molecule.
• The phosphate group is attached to the 5' carbon of the same sugar molecule.

Therefore, the correct formation is: Base – Sugar – Phosphate.

Step 3: Evaluate the Options

Let's analyze the given choices:

• Base – Sugar – OH: Incorrect. An OH group is part of the sugar itself, not the third component.
• (Base – Sugar – Phosphate) – n: Incorrect. This describes a polynucleotide chain (nucleic acid), not a single nucleotide.
• Sugar phosphate: Incorrect. This is missing the essential nitrogenous base.
• Base – Sugar – Phosphate: Correct. This accurately describes the structure of a single nucleotide.

Final Answer

The correct answer is: Base – Sugar – Phosphate.

Related Topics & Formulae

Formation of a Polynucleotide

Nucleotides polymerize to form nucleic acids (DNA/RNA). The phosphate group of one nucleotide forms a phosphodiester bond with the 3' carbon of the sugar on the next nucleotide. The general formula for a chain of 'n' nucleotides is:

$[\text{(Base - Sugar - Phosphate)}{]}_n$

Key Theory

Nucleoside vs. Nucleotide: It is crucial to distinguish between these two terms. A nucleoside consists of only a base + sugar. A nucleotide is a nucleoside + phosphate group(s). The addition of the phosphate group is what gives a nucleotide the energy and functionality needed to form polymers.