DNA and Protein Synthesis » Translation Process

What you'll learn this session

Study time: 30 minutes

The stages of translation in protein synthesis
The role of mRNA, tRNA and ribosomes in translation
How the genetic code is read during translation
The importance of amino acids in protein formation
How polypeptide chains fold to form functional proteins

Introduction to Translation

Translation is the second major step in protein synthesis, where the genetic information carried by mRNA is decoded to produce a specific protein. This process happens in the cytoplasm at ribosomes, where amino acids are joined together in a specific sequence to form polypeptides (which will eventually fold into proteins).

Key Definitions:

Translation: The process where the genetic code in mRNA is used to make proteins.
Ribosome: Cellular structure made of protein and RNA where translation occurs.
tRNA (transfer RNA): Small RNA molecules that bring amino acids to the ribosome.
Codon: A sequence of three nucleotides in mRNA that codes for a specific amino acid.
Anticodon: A sequence of three nucleotides on tRNA that pairs with a complementary codon on mRNA.

📖 From DNA to Protein: The Big Picture

Remember that protein synthesis involves two main steps:

Transcription: DNA → mRNA (happens in the nucleus)
Translation: mRNA → Protein (happens in the cytoplasm)

Translation is where the actual protein is made, based on the instructions carried by the mRNA from the DNA.

🎯 Why Translation Matters

Translation is crucial because proteins do most of the work in your cells. They:

Form structural components (like muscle fibres)
Act as enzymes to speed up reactions
Transport substances around your body (like haemoglobin)
Help fight disease (like antibodies)

Without accurate translation, these proteins wouldn't function properly!

The Genetic Code

Before we dive into the translation process, let's understand the genetic code that makes it all possible.

Understanding the Genetic Code

The genetic code is like a dictionary that translates the language of nucleic acids (DNA and RNA) into the language of proteins (amino acids).

The genetic code is based on codons - three-letter "words" made of nucleotides.
There are 64 possible codons (4³ combinations of the four RNA bases: A, U, G and C).
Most amino acids are coded for by more than one codon (this is called the degeneracy of the genetic code).
The code is universal - it works the same way in almost all organisms on Earth.

Interesting Fact: Special Codons

Not all codons code for amino acids:

Start codon (AUG): This codon signals where translation should begin and also codes for the amino acid methionine.
Stop codons (UAA, UAG, UGA): These signal the end of translation. They don't code for any amino acid.

The Translation Process

Translation happens in three main stages: initiation, elongation and termination. Let's look at each step in detail.

🏃 Initiation

1. mRNA binds to the small subunit of a ribosome

2. The start codon (AUG) is recognised

3. The first tRNA carrying methionine attaches to the start codon

4. The large ribosomal subunit joins to form the complete ribosome

📝 Elongation

1. The ribosome has two sites: P-site (peptidyl) and A-site (aminoacyl)

2. A new tRNA brings an amino acid to the A-site

3. A peptide bond forms between amino acids

4. The ribosome moves along the mRNA (translocation)

5. This process repeats, adding amino acids one by one

🛑 Termination

1. A stop codon (UAA, UAG, or UGA) enters the A-site

2. Release factors recognise the stop codon

3. The completed polypeptide chain is released

4. The ribosome subunits separate

5. The polypeptide folds to form a functional protein

The Role of tRNA in Translation

Transfer RNA (tRNA) molecules are the unsung heroes of translation. They act as adapters that convert the language of nucleic acids to the language of proteins.

🎲 Structure of tRNA

tRNA has a distinctive cloverleaf shape with:

An anticodon loop that pairs with mRNA codons
An amino acid attachment site at the 3' end
Several loops that give it its 3D structure

Each tRNA is specific to one amino acid but may recognise multiple codons due to "wobble" pairing.

🚀 tRNA in Action

Here's how tRNA works during translation:

Each tRNA is charged with its specific amino acid by an enzyme called aminoacyl-tRNA synthetase
The tRNA's anticodon pairs with the complementary codon on the mRNA
The amino acid is transferred to the growing polypeptide chain
The empty tRNA is released and can be recharged with another amino acid

Ribosomes: The Protein Factories

Ribosomes are complex molecular machines made of proteins and ribosomal RNA (rRNA). They provide the site where translation occurs.

Ribosome Structure and Function

Ribosomes consist of two subunits that come together during translation:

Small subunit: Binds to mRNA and helps decode the genetic message
Large subunit: Contains the peptidyl transferase centre where peptide bonds form between amino acids

Ribosomes have three important sites:

A-site (Aminoacyl): Where incoming tRNAs enter with their amino acids
P-site (Peptidyl): Where the growing polypeptide chain is held
E-site (Exit): Where empty tRNAs leave the ribosome

Case Study Focus: Antibiotics and Translation

Many antibiotics work by targeting bacterial ribosomes and disrupting translation. For example:

Tetracyclines block tRNAs from entering the A-site of bacterial ribosomes
Erythromycin prevents the movement of ribosomes along mRNA
Chloramphenicol inhibits the formation of peptide bonds

These antibiotics can kill bacteria without harming human cells because bacterial ribosomes (70S) differ structurally from human ribosomes (80S).

From Polypeptide to Functional Protein

Once translation is complete, the polypeptide chain isn't yet a functional protein. It needs to fold into its correct three-dimensional shape.

📐 Protein Folding

Proteins fold into specific shapes based on:

Primary structure: The sequence of amino acids
Secondary structure: Regular patterns like alpha helices and beta sheets
Tertiary structure: The overall 3D shape of a single polypeptide
Quaternary structure: How multiple polypeptides arrange together (if applicable)

The shape of a protein is crucial for its function!

⚠ When Translation Goes Wrong

Errors in translation can lead to:

Missense mutations: Wrong amino acid is inserted
Nonsense mutations: Premature stop codon causes truncated proteins
Frameshift mutations: Reading frame shifts, changing all subsequent amino acids

These errors can cause proteins to misfold or lose function, potentially leading to diseases like cystic fibrosis or sickle cell anaemia.

Summary: The Translation Process

Let's recap the key points about translation:

Translation is the process of making proteins using the genetic information in mRNA.
It occurs at ribosomes in the cytoplasm.
tRNA molecules bring amino acids to the ribosome based on the codons in mRNA.
The process has three stages: initiation, elongation and termination.
After translation, the polypeptide chain folds to form a functional protein.

Real-World Application: Insulin Production

Understanding translation has allowed scientists to produce human insulin for diabetic patients using bacteria:

The human insulin gene is inserted into bacterial DNA
The bacteria transcribe and translate this gene
The bacteria produce human insulin protein
The insulin is harvested, purified and used to treat diabetes

This is possible because the genetic code is universal - bacterial ribosomes can read human mRNA and produce human proteins!

🧠 Test Your Knowledge!