DNA and Protein Synthesis [H]

This lesson is Higher Tier only content for AQA GCSE Biology. It covers how the genetic code in DNA is used to build proteins through the processes of transcription and translation. Proteins are essential molecules that carry out almost every function in the body, from enzymes to structural components.

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Why Proteins Matter

Proteins are vital for life. They perform a huge range of functions in the body:

Protein type	Function	Example
Structural proteins	Provide support and shape to cells and tissues	Collagen in skin and bone, keratin in hair
Enzymes	Act as biological catalysts to speed up chemical reactions	Amylase, lipase, protease
Hormones	Chemical messengers that coordinate body processes	Insulin, growth hormone
Antibodies	Defend the body against pathogens	Immunoglobulins
Transport proteins	Carry substances around the body	Haemoglobin carries oxygen in red blood cells

Exam Tip: A common Higher Tier question asks you to explain why genes are important. The key answer is: genes code for the sequence of amino acids in a protein, and proteins control most cell functions. Gene -> amino acid sequence -> protein -> function.

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From Gene to Protein: The Overview

The process of making a protein from a gene involves two main stages:

1. Transcription — making an mRNA copy of the gene (occurs in the nucleus)
2. Translation — using the mRNA to assemble a chain of amino acids at a ribosome (occurs in the cytoplasm)

graph LR
    A[DNA in nucleus] -->|Transcription| B[mRNA]
    B -->|mRNA leaves nucleus| C[Ribosome in cytoplasm]
    C -->|Translation| D[Protein amino acid chain]
    D --> E[Protein folds into 3D shape]
    E --> F[Functional protein]

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The Genetic Code

The sequence of bases in DNA forms a code. Each set of three bases is called a codon or triplet code. Each triplet codes for a specific amino acid.

Key term	Definition
Triplet code	A sequence of three bases on DNA that codes for one amino acid
Codon	A sequence of three bases on mRNA that codes for one amino acid
Amino acid	The building blocks of proteins; 20 different amino acids exist
Non-coding DNA	Sections of DNA that do not code for amino acids (previously called "junk DNA")

There are only 4 bases but 20 amino acids. Using groups of three bases gives 64 possible combinations (4 x 4 x 4 = 64), which is more than enough to code for all 20 amino acids plus stop signals.

Exam Tip: You do NOT need to memorise which triplet codes for which amino acid. However, you must understand the principle that every three bases code for one amino acid.

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Step 1: Transcription

Transcription occurs in the nucleus and produces a molecule of messenger RNA (mRNA).

How transcription works:

1. The DNA double helix unwinds and the two strands separate (the hydrogen bonds between base pairs break)
2. One strand of DNA acts as a template
3. Free RNA nucleotides line up against the template strand following complementary base pairing rules
4. In RNA, uracil (U) replaces thymine (T), so the pairing rules become: A-U and C-G
5. The RNA nucleotides are joined together to form a strand of mRNA
6. The mRNA strand detaches from the DNA and moves out of the nucleus through a nuclear pore into the cytoplasm
7. The DNA strands rejoin

DNA base	RNA base it pairs with
A	U (uracil)
T	A
C	G
G	C

graph TD
    A[DNA unwinds in nucleus] --> B[Template strand exposed]
    B --> C[RNA nucleotides line up by complementary base pairing]
    C --> D[mRNA strand formed]
    D --> E[mRNA leaves nucleus through nuclear pore]
    E --> F[mRNA travels to ribosome in cytoplasm]

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Step 2: Translation

Translation occurs at a ribosome in the cytoplasm. The mRNA code is read and used to assemble a chain of amino acids.

How translation works:

1. The mRNA attaches to a ribosome
2. The ribosome reads the mRNA code three bases (one codon) at a time
3. Transfer RNA (tRNA) molecules carry specific amino acids to the ribosome
4. Each tRNA has an anticodon (three bases) that is complementary to a codon on the mRNA
5. When a tRNA anticodon matches an mRNA codon, the amino acid it carries is added to the growing chain
6. The ribosome moves along the mRNA, reading each codon in sequence
7. Amino acids are joined together by peptide bonds to form a polypeptide chain
8. When a stop codon is reached, the polypeptide chain is released
9. The polypeptide chain then folds into a specific 3D shape to become a functional protein

graph TD
    A[mRNA attaches to ribosome] --> B[Ribosome reads codons three bases at a time]
    B --> C[tRNA brings matching amino acid]
    C --> D[Amino acid added to polypeptide chain]
    D --> E[Ribosome moves to next codon]
    E -->|Repeat| B
    E -->|Stop codon reached| F[Polypeptide chain released]
    F --> G[Chain folds into functional protein]

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Non-Coding DNA [H]

Not all DNA codes for proteins. Large sections of DNA are non-coding sequences that do not code for amino acids. These regions were once dismissed as "junk DNA" but scientists now know they have important functions:

• Regulating gene expression — turning genes on and off
• Producing RNA molecules that help control cell processes
• Structural roles — helping to organise the chromosome

It is estimated that only about 1.5% of human DNA actually codes for proteins.

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Mutations [H]

A mutation is a change in the base sequence of DNA. Mutations can occur spontaneously during DNA replication or can be caused by mutagens (e.g. UV radiation, certain chemicals, X-rays).

Types of Mutation