# Amino acids, proteins and DNA

> CIE A-Level Chemistry · CIE 9701 (2022-2024)
> Source: https://www.owlsprep.com/study/cie-9701-u25-overview/
> Weight: n/a

This unit explores the structure and properties of key biological organic molecules: amino acids, proteins, and DNA. You will connect organic chemistry concepts to biological function.

**Prerequisites:** Core organic functional group chemistry; Intermolecular forces including hydrogen bonding; Acid-base behaviour of carboxylic acids and amines

## Learning objectives

- Identify the structure of α-amino acids and describe their zwitterionic and acid-base behaviour
- Explain the four levels of protein structure and the forces that stabilise each level
- Describe the structure of DNA nucleotides and the double-helix model of DNA
- Relate the molecular structure of these biomolecules to their biological functions

## Unit at a Glance

This unit follows a logical building sequence: we start with the monomer (amino acids), build up to the polymer (proteins), then examine the nucleic acid polymer DNA that codes for protein synthesis. All topics build on core organic chemistry concepts to highlight the core biological chemistry principle: structure determines function.

The unit is split into three core sub-topics:
- [Amino acids](https://www.owlsprep.com/study/cie-9701-u25-amino-acids/) — Learn the structure of α-amino acids, zwitterion formation, acid-base behaviour, and peptide-forming condensation reactions.
- [Proteins](https://www.owlsprep.com/study/cie-9701-u25-proteins/) — Explore the four levels of protein structure, the bonding that stabilises each level, and denaturation by temperature and pH.
- [Structure of DNA](https://www.owlsprep.com/study/cie-9701-u25-structure-of-dna/) — Understand nucleotide structure, the double-helix model of DNA, and complementary base pairing rules.

## Common pitfalls

- **Wrong:** Confusing the types of bonding in different levels of protein structure
  - Why it fails: Students often mix up which bonds stabilise each structural level of proteins
  - Correct: Primary = peptide bonds, secondary = hydrogen bonding between backbone groups, tertiary = multiple interactions including disulfide bonds, quaternary = same interactions between multiple polypeptide chains
- **Wrong:** Drawing uncharged amino acid groups at the isoelectric point
  - Why it fails: Many students forget that amino acids exist as zwitterions near neutral pH
  - Correct: At the isoelectric point: the amine group is protonated ($-NH_3^+$) and the carboxyl group is deprotonated ($-COO^-$) for a net charge of zero
- **Wrong:** Misidentifying complementary base pairs in DNA
  - Why it fails: Students often incorrectly pair adenine with guanine and cytosine with thymine
  - Correct: Adenine (A) only pairs with thymine (T), cytosine (C) only pairs with guanine (G)

## Cheatsheet

| Concept | Key Fact |
| --- | --- |
| Zwitterion at isoelectric point | $H_3N^+CH(R)COO^-$ |
| Peptide bond structure | $-CONH-$ linkage between amino acids |
| Secondary protein structure stabilisation | Hydrogen bonds between backbone $C=O$ and $N-H$ groups |
| DNA nucleotide composition | Phosphate + deoxyribose sugar + nitrogenous base |
| Complementary base pairing | A-T (2 H-bonds), C-G (3 H-bonds) |

## What's next

Begin this unit by exploring the core monomer structure: amino acids, which lays the foundation for understanding proteins and DNA. After completing all three sub-topics in this unit, you will progress to the next unit covering organic polymerisation.

- [Amino acids](https://www.owlsprep.com/study/cie-9701-u25-amino-acids/)

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