Nucleotide Structure — The Building Blocks of Nucleic Acids

Nucleic acids are the information-carrying molecules of life. Both DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are polymers assembled from a single type of monomer: the nucleotide. This lesson covers the OCR A-Level Biology A specification point 2.1.3 (a) — the structure of a nucleotide as the monomer from which nucleic acids are made — and begins specification point 2.1.3 (b) — the synthesis of a nucleic acid via a condensation reaction between nucleotides forming a phosphodiester bond.

A thorough grasp of nucleotide chemistry is essential. Every later idea in this topic — the double helix, base pairing, replication, transcription, translation, ATP, even enzyme cofactors such as NAD and FAD — depends on the chemistry of nucleotides.

1. What Is a Nucleotide?

A nucleotide is a molecule made up of three covalently bonded components:

A pentose sugar (a 5-carbon monosaccharide)
A nitrogenous (organic) base
A phosphate group (derived from phosphoric acid, H₃PO₄)

Key Definition — Nucleotide: A biological monomer composed of a pentose sugar, a phosphate group and a nitrogenous base, joined by condensation reactions.

A nucleotide without the phosphate group is called a nucleoside (sugar + base only). You do not need to reproduce nucleoside terminology in exam answers, but it helps to know the distinction.

        Phosphate
            |
        [  O=P–O⁻  ]
            |
            O
            |
        5'  CH₂     Base (A, T, C, G or U)
            |       /
        Sugar (pentose) — attached at 1' carbon
           /   \
          3'    2'
          OH    (H in DNA, OH in RNA)

The numbers (1', 2', 3', 4', 5' — read "one prime", "two prime" and so on) refer to the carbon atoms of the pentose sugar. They are crucial: the phosphate is attached at the 5' carbon, the base at the 1' carbon, and the hydroxyl that accepts the next phosphate is at the 3' carbon. These positions define the directionality (5' → 3') of nucleic acid strands.

2. The Pentose Sugar

A pentose is a monosaccharide with five carbon atoms arranged in a furanose (five-membered) ring.

Two pentoses appear in nucleic acids:

Pentose	Found in	Key feature
Ribose (C₅H₁₀O₅)	RNA	Hydroxyl (–OH) group on the 2' carbon
Deoxyribose (C₅H₁₀O₄)	DNA	Hydrogen (–H) on the 2' carbon (one fewer oxygen than ribose)

The single chemical difference — an –OH versus an –H at the 2' position — has enormous consequences:

DNA is chemically more stable than RNA. The absence of the 2'-OH means DNA resists hydrolysis, which is essential for a molecule that must preserve the genome for decades.
RNA is less stable and easily hydrolysed — appropriate for a temporary information-carrier that must be made and destroyed quickly.

graph LR
  A[Pentose Sugar] --> B[Ribose - 2'-OH<br/>found in RNA]
  A --> C[Deoxyribose - 2'-H<br/>found in DNA]

Exam Tip: "Deoxy" literally means "without oxygen". Deoxyribose has one fewer oxygen than ribose because the 2'-OH is replaced by 2'-H.

3. The Nitrogenous Bases

The nitrogenous (organic) bases are planar, aromatic ring-containing molecules that contain nitrogen and have weakly basic chemistry (they can accept protons). Five bases appear in nucleic acids, grouped into two families by ring structure.

3.1 Purines — Double-Ringed Bases

Adenine (A)
Guanine (G)

Purines have a fused double-ring structure (a six-membered ring fused to a five-membered ring), containing four nitrogen atoms.

3.2 Pyrimidines — Single-Ringed Bases

Cytosine (C)
Thymine (T) — only in DNA
Uracil (U) — only in RNA (replaces thymine)

Pyrimidines have a single six-membered ring containing two nitrogen atoms.

Base	Abbrev.	Ring type	Found in
Adenine	A	Purine (double ring)	DNA and RNA
Guanine	G	Purine (double ring)	DNA and RNA
Cytosine	C	Pyrimidine (single ring)	DNA and RNA
Thymine	T	Pyrimidine (single ring)	DNA only
Uracil	U	Pyrimidine (single ring)	RNA only

Memory aid: Purines = Pure As Gold (Adenine, Guanine, fused double ring). Pyrimidines (CUT) = Cytosine, Uracil, Thymine — the single-ringed ones.

The base is always attached to the 1' carbon of the pentose sugar via a glycosidic bond (C–N bond between the 1' carbon and a nitrogen on the base).

4. The Phosphate Group

A phosphate group is derived from phosphoric acid (H₃PO₄). At physiological pH it carries one or two negative charges, making nucleotides (and nucleic acids) strongly acidic and negatively charged.

The phosphate is attached to the 5' carbon of the pentose sugar by an ester bond (a phosphoester linkage formed by condensation between the phosphate and the 5'-OH of the sugar).

The negative charge on phosphate groups is why DNA runs towards the positive electrode in gel electrophoresis, and why histone proteins (positively charged due to lysine and arginine residues) bind DNA so effectively.

5. Putting It Together — A Complete Nucleotide

A nucleotide is assembled by two condensation reactions:

Base + pentose → nucleoside (removes one H₂O — forms the glycosidic bond at 1')
Nucleoside + phosphate → nucleotide (removes one H₂O — forms the phosphoester bond at 5')

graph LR
  A[Nitrogenous base] -- condensation<br/>(−H₂O) --> B[Nucleoside]
  C[Pentose sugar] --> B
  B -- condensation<br/>(−H₂O) --> D[Nucleotide]
  E[Phosphate] --> D

Examples of complete nucleotides:

dAMP — deoxyadenosine monophosphate (a DNA nucleotide)
UMP — uridine monophosphate (an RNA nucleotide)
ATP — adenosine triphosphate (a nucleotide with three phosphates; energy currency; covered in Lesson 8)

6. Phosphodiester Bonds and Polynucleotide Chains

Nucleotides polymerise by a condensation reaction between the 3'-OH of one nucleotide and the 5'-phosphate of the next. The bond formed is called a phosphodiester bond — "phospho" (phosphate), "di" (two), "ester" (ester linkage), because a single phosphate is now linked by ester bonds to two different sugars.

      5' end
        |
     Phosphate
        |
       Sugar — Base
        |
     Phosphodiester bond (condensation, −H₂O)
        |
     Phosphate
        |
       Sugar — Base
        |
       3' end

Key points to remember:

Each phosphodiester bond formation releases one water molecule (condensation).
Hydrolysis of a phosphodiester bond requires a water molecule and is catalysed by enzymes called nucleases.
A polynucleotide has a 5' end (free phosphate) and a 3' end (free hydroxyl). This gives the strand directionality.
The sugar–phosphate backbone is the same along the whole strand; only the bases vary. Information is encoded in the sequence of bases, not in the backbone.

Exam Tip: Always write "phosphodiester bond", never "phosphate bond" or "ester bond". Always mention that it is formed by condensation, releasing water.

7. Comparing DNA and RNA Nucleotides

Feature	DNA nucleotide	RNA nucleotide
Pentose sugar	Deoxyribose	Ribose
Bases used	A, T, C, G	A, U, C, G
Stability	High	Low
Typical length	Millions–billions of nucleotides	Tens–tens of thousands
Typical strands	Double (double helix)	Usually single

8. Common Exam Mistakes

Saying a nucleotide is just "a base and a sugar" — it must include all three components (sugar, base, phosphate).
Confusing "purine" with "pyrimidine". Remember: Purines are Pure As Gold (A, G); Pyrimidines are CUT (C, U, T).
Saying that thymine is "in RNA". Thymine is in DNA only; uracil replaces it in RNA.
Writing "phosphate bond" instead of phosphodiester bond.
Forgetting to say "condensation reaction, releasing water" when describing nucleotide polymerisation.
Confusing the 3' and 5' ends. The 5' end has a phosphate; the 3' end has a hydroxyl.
Forgetting that the base is attached at the 1' carbon via a glycosidic bond.

9. Exam-Style Questions

Draw and label a diagram of a DNA nucleotide. (4)
State the differences between a DNA nucleotide and an RNA nucleotide. (3)
Describe how two nucleotides join to form a dinucleotide. (3)
Explain why DNA is chemically more stable than RNA. (2)

Model answer for (3): "A condensation reaction takes place between the 3'-hydroxyl group of one nucleotide and the 5'-phosphate of the second nucleotide. A phosphodiester bond forms between them. One molecule of water is released."

Summary

A nucleotide = pentose sugar + nitrogenous base + phosphate.
Pentose sugars are ribose (RNA) and deoxyribose (DNA).
Bases are divided into purines (A, G — double ring) and pyrimidines (C, T, U — single ring).
Nucleotides join by condensation, forming phosphodiester bonds between the 3' of one nucleotide and the 5' of the next.
Polynucleotides are directional, with 5' and 3' ends, and carry information in their base sequence.

Lesson 1: Nucleotide Structure — The Building Blocks of Nucleic Acids