# Nucleotides

> CIE A-Level Biology · 9700
> Source: https://www.owlsprep.com/study/cie-9700-u2-nucleotides/

This sub-topic covers the basic monomer structure of nucleic acids (DNA and RNA), chemical differences between DNA and RNA nucleotides, and how nucleotides polymerise to form genetic material.

**Prerequisites:** [Condensation and hydrolysis reactions](https://www.owlsprep.com/study/cie-9700-u2-condensation-hydrolysis/)

## Learning objectives

- Describe the general structure of a nucleotide
- Distinguish between DNA and RNA nucleotides
- Classify nitrogenous bases into purines and pyrimidines
- Explain how nucleotides form polynucleotide chains

## General Structure of a Nucleotide

A nucleotide is the monomer subunit that builds all nucleic acids, the information-carrying molecules of all living cells. Every nucleotide has three key covalently bonded components arranged in a consistent structure.

**Nucleotide** — A nucleic acid monomer consisting of a pentose sugar, a nitrogen-containing organic base, and a phosphate group covalently bonded together.

*Example:* A deoxyadenosine nucleotide, a monomer of DNA.

- **Pentose sugar**: A 5-carbon sugar, either ribose (RNA) or deoxyribose (DNA)
- **Nitrogenous base**: Nitrogen-containing ring structure that encodes genetic information
- **Phosphate group**: Negatively charged acidic group derived from phosphoric acid

**Worked example:** State which carbon atom on the pentose sugar of a nucleotide is bonded to (a) the nitrogenous base, (b) the phosphate group

1. Recall the standard numbering system for pentose carbons: carbons are numbered clockwise starting from the oxygen in the sugar ring.
2. For (a): The nitrogenous base always forms a covalent bond to carbon 1 (C1) of the pentose sugar.
3. For (b): The phosphate group is always covalently bonded to carbon 5 (C5), which extends outside the sugar ring.

> **Exam tip:** Examiners expect you to reference specific carbon numbering when describing nucleotide structure, always include this detail.

## Classification of Nitrogenous Bases

All nitrogenous bases fall into two groups based on their chemical structure. This size difference is critical for the stable double helix structure of DNA, where a large base always pairs with a small base.

**Purines** — Large nitrogenous bases with a double-ring structure: a six-membered ring fused to a five-membered ring.

*Example:* Adenine (A) and Guanine (G)

**Pyrimidines** — Small nitrogenous bases with a single six-membered ring structure.

*Example:* Cytosine (C), Thymine (T), Uracil (U)

> **mnemonic**
>
> Purines = **Pure As Gold**: Purine = Adenine, Guanine. Pyrimidines = **CUT the Py**: C = Cytosine, U = Uracil, T = Thymine are Pyrimidines.

**Worked example:** Guanine is a base found in DNA. State its classification, number of rings, and whether it is larger or smaller than cytosine.

1. Use the classification rule: bases with two rings are purines, single rings are pyrimidines.
2. From the mnemonic, guanine is a purine, so it has two rings.
3. Cytosine is a pyrimidine, so guanine is larger than cytosine.

## Differences Between DNA and RNA Nucleotides

DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are the two main nucleic acids in cells. Their nucleotides differ in two consistent, key structural features.

| Feature | DNA Nucleotide | RNA Nucleotide |
| --- | --- | --- |
| Pentose sugar | Deoxyribose (no oxygen on C2) | Ribose (has hydroxyl group on C2) |

**Worked example:** A nucleotide contains uracil. Is this a DNA or RNA nucleotide? Explain your answer.

1. Recall that the base composition differs between DNA and RNA nucleotides.
2. DNA nucleotides always contain thymine, not uracil. Uracil is only found in RNA nucleotides in place of thymine.
3. Conclusion: This must be an RNA nucleotide.

**Check your understanding**

Test your understanding of nucleotide differences:

1. Which feature is unique to DNA nucleotides?

   - Phosphate group
   - Deoxyribose sugar
   - Purine base
   - Uracil

   *Why:* Correct! Phosphate and purine bases are found in both DNA and RNA, uracil is only in RNA.

## Formation of Polynucleotide Chains

Nucleotides join via condensation reactions to form long polynucleotide chains, the backbone of all nucleic acids. The chain has a free phosphate group at the 5' end and a free hydroxyl group at the 3' end, giving the strand directionality.

**Worked example:** Describe how two nucleotides join to form a dinucleotide, name the bond formed, and state the type of reaction that occurs.

1. The reaction occurs between the phosphate group attached to the 5' carbon of one nucleotide and the hydroxyl (-OH) group on the 3' carbon of the second nucleotide's pentose sugar.
2. A condensation reaction takes place, which eliminates one molecule of water as the new covalent bond forms.
3. The covalent bond formed between the two nucleotides is called a phosphodiester bond.
4. Repeating this reaction across many nucleotides produces a polynucleotide chain with a sugar-phosphate backbone.

> **Exam tip:** Always name the bond as a phosphodiester bond — generic terms like 'covalent bond' will not get full marks in CIE exams.

## Common pitfalls

- **Wrong:** Calling the sugar in DNA 'dideoxyribose'
  - Why it fails: Confusion with modified sugars used in sequencing leads to lost marks
  - Correct: The sugar in DNA is correctly called deoxyribose, with one less oxygen atom than ribose
- **Wrong:** Calling the bond between nucleotides a phosphodiester bridge
  - Why it fails: CIE exam markers only accept the term 'phosphodiester bond'
  - Correct: Always refer to the bond joining adjacent nucleotides as a phosphodiester bond
- **Wrong:** Stating all nucleotides contain ribose sugar
  - Why it fails: Forgets DNA uses deoxyribose, a common 1-mark multiple choice trap
  - Correct: Only RNA nucleotides contain ribose; DNA nucleotides contain deoxyribose
- **Wrong:** Mixing up purine and pyrimidine ring counts
  - Why it fails: Wrong classification leads to incorrect answers about complementary base pairing
  - Correct: Use the mnemonic: purines are double-ring, pyrimidines are single-ring
- **Wrong:** Claims RNA is never base-paired
  - Why it fails: Ignores internal base pairing in tRNA and rRNA, which CIE can test
  - Correct: Correctly state RNA is typically single-stranded, not that it never has base pairing

## Cheatsheet

| Component | DNA Nucleotide | RNA Nucleotide | Base Class (Rings) |
| --- | --- | --- | --- |
| Pentose sugar | Deoxyribose | Ribose | - |
| Bases present | A, C, G, T | A, C, G, U | - |
| Adenine | - | - | Purine (2) |
| Guanine | - | - | Purine (2) |
| Cytosine | - | - | Pyrimidine (1) |
| Thymine | - | - | Pyrimidine (1) |
| Uracil | - | - | Pyrimidine (1) |
| Bond between nucleotides | Phosphodiester | Phosphodiester | - |

## What's next

Nucleotides are the fundamental building blocks of all nucleic acids, so mastery of this topic is essential for all molecular biology and genetics topics in CIE A-Level Biology. The structure of nucleotides directly determines the structure of DNA and how genetic information is stored, replicated and expressed. All subsequent topics on the central dogma of molecular biology depend on understanding the basic structure and properties of nucleotides.

- [Biochemical tests for biological molecules](https://www.owlsprep.com/study/cie-9700-u2-biochemical-tests-for-biological-molecules/)
- [Enzymes](https://www.owlsprep.com/study/cie-9700-u3-overview/)
- [Enzyme properties and mode of action](https://www.owlsprep.com/study/cie-9700-u3-enzyme-properties-and-mode-of/)

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