Protein Synthesis

This lesson covers protein synthesis — the process by which cells use the genetic code in DNA to produce proteins — as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand transcription, translation and the roles of mRNA, codons, ribosomes and amino acids.

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Why Proteins Matter

Proteins are essential molecules that carry out a huge range of functions in the body:

Protein type	Example	Function
Enzymes	Amylase, protease	Speed up chemical reactions (biological catalysts)
Structural	Collagen, keratin	Provide support and strength in tissues
Hormones	Insulin, growth hormone	Chemical messengers that control body processes
Antibodies	Immunoglobulins	Defend the body against pathogens
Transport	Haemoglobin	Carry substances around the body

Each protein is made of a specific sequence of amino acids joined together. There are 20 different amino acids used to make proteins in the human body.

Exam Tip: Enzymes, hormones and antibodies are all proteins. If the exam asks for examples of proteins, these are safe choices.

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The Genetic Code

The sequence of bases in a gene provides the code for assembling amino acids into a protein:

• Each set of three bases in DNA is called a triplet (or codon on mRNA).
• Each triplet codes for one specific amino acid.
• The order of triplets determines the order of amino acids in the protein.

Since there are 4 bases and each codon is 3 bases long, there are 4 x 4 x 4 = 64 possible codons — more than enough to code for the 20 amino acids (with some redundancy).

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Overview of Protein Synthesis

Protein synthesis occurs in two stages:

1. Transcription — takes place in the nucleus
2. Translation — takes place at the ribosomes in the cytoplasm

graph LR
    A["DNA in nucleus"] -->|"Transcription"| B["mRNA strand"]
    B -->|"Moves to cytoplasm"| C["Ribosome"]
    C -->|"Translation"| D["Amino acid chain (protein)"]

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Stage 1: Transcription

Transcription is the process of copying the genetic code from DNA onto a molecule of messenger RNA (mRNA).

Steps of Transcription

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

DNA base	mRNA base
A	U
T	A
C	G
G	C

Exam Tip: Remember that RNA uses uracil (U) instead of thymine (T). This is a very common exam question. DNA has T; RNA has U.

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Stage 2: Translation

Translation is the process of reading the mRNA code and assembling amino acids into a polypeptide (protein chain).

Steps of Translation

1. The mRNA strand attaches to a ribosome in the cytoplasm.
2. The ribosome reads the mRNA code three bases at a time — each group of three bases on mRNA is called a codon.
3. Each codon codes for a specific amino acid.
4. Transfer RNA (tRNA) molecules carry the correct amino acids to the ribosome. Each tRNA has an anticodon that is complementary to the mRNA codon.
5. The amino acids are joined together by peptide bonds in the order dictated by the mRNA codons.
6. The chain of amino acids folds into a specific 3D shape, forming the final protein.

graph TD
    A["mRNA arrives at ribosome"] --> B["Ribosome reads codons (3 bases)"]
    B --> C["tRNA brings matching amino acid"]
    C --> D["Amino acids joined by peptide bonds"]
    D --> E["Polypeptide chain grows"]
    E --> F["Chain folds into functional protein"]

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The Role of Each Molecule

Molecule	Role in protein synthesis
DNA	Holds the genetic code in the nucleus; template for mRNA
mRNA	Carries the copied code from the nucleus to the ribosome
tRNA	Brings the correct amino acid to the ribosome
Ribosome	Reads the mRNA and assembles the amino acid chain
Amino acids	The building blocks that are joined to form proteins

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From Gene to Protein — A Worked Example

If a section of DNA template strand reads: T A C G G A T T C

1. The mRNA produced by transcription: A U G C C U A A G
2. This mRNA is read in codons: AUG | CCU | AAG
3. Each codon codes for an amino acid:
   • AUG → Methionine (start codon)
   • CCU → Proline
   • AAG → Lysine

The resulting short polypeptide: Met – Pro – Lys

Exam Tip: In the exam, you may be asked to work out the mRNA sequence from a DNA strand or identify amino acids from codons. Always remember: DNA → mRNA uses complementary base pairing with U replacing T.

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Summary

• Proteins are made of amino acids joined in a specific order determined by the DNA base sequence.
• Protein synthesis has two stages: transcription (DNA → mRNA in the nucleus) and translation (mRNA → protein at ribosomes).
• Each group of three bases (codon) on mRNA codes for one amino acid.
• mRNA carries the code from the nucleus to the ribosome; tRNA delivers amino acids.
• RNA uses uracil (U) instead of thymine (T).
• The sequence of amino acids determines the shape and function of the protein.

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Worked Example: Transcription and Translation