Test Your Knowledge Flash Cards 🔒Play Lemonaire 🔒Play Last Stand

Reproduction » Protein Synthesis and Mutations

What you'll learn this session

Study time: 30 minutes

AQA spec: 4.6.1.5 (Higher tier only)

How a protein is made from a gene: a simple description of protein synthesis
Why a protein folds into a unique shape, and why that shape matters
How a change in DNA can change the protein that is made
Why most mutations do no harm, and what happens when a few do
What non-coding DNA does, and how genetic variants can influence phenotype

🔒 Unlock Full Course Content

Unlock This Course

Making a protein: protein synthesis

You already know that a gene is a section of DNA, and that a sequence of three bases is the code for one amino acid. Now we put that code to work. Protein synthesis is how a cell builds a protein from the instructions in a gene.

Here is the simple description you need to recall:

Proteins are made on ribosomes.
The ribosome builds the protein according to a template. The template carries the code from the gene.
Carrier molecules bring specific amino acids to the ribosome. Each carrier brings one particular amino acid.
The amino acids are added to the growing protein chain in the correct order, set by the order of bases in the gene.

You do not need to know the structure of the template or the carrier molecules, or the detailed structure of amino acids or proteins. You just need the idea: a template, carriers, ribosomes and a chain of amino acids built in order.

Key terms:

Protein synthesis: making a protein on a ribosome by adding amino acids in the order set by a gene.
Carrier molecule: a molecule that brings a specific amino acid to add to the growing protein chain.

How the order of bases decides the protein

The order of bases in a gene controls the order of amino acids in the chain. So the structure of DNA decides which protein is made.

📝 Different genes

Different genes have a different order of bases. They give a different order of amino acids, so they make different proteins.

🔁 Same gene

The same gene always gives the same order of amino acids, so the cell makes the same protein each time.

Think of a recipe card. If the steps are in the same order, you make the same dish every time. Change the steps, and you may make something different.

Folding into a unique shape

When the protein chain is complete, it folds up. The chain folds into a unique shape. Different proteins fold into different shapes because they have different orders of amino acids.

This unique shape is what lets the protein do its job. The spec gives three types of job:

⚡ Enzymes

The shape lets the enzyme fit its substrate at the active site.

💌 Hormones

The shape lets the hormone do its job as a chemical messenger in the body.

🧱 Structures

Proteins such as collagen form structures in the body. Their shape gives them strength.

Link it together

Order of bases in the gene → order of amino acids in the chain → unique folded shape → the protein can do its job.

Mutations: changes to DNA

A mutation is a change in the structure of DNA, for example a change to the order of bases in a gene. Mutations occur continuously, all the time.

If the order of bases in a gene changes, the order of amino acids it codes for may change. That can change the protein that is made. This is how a change in DNA structure may result in a change in the protein synthesised by a gene.

But most mutations are not a problem:

Most mutations do not alter the protein, or only alter it slightly.
In these cases the protein's appearance or function is not changed.

A few mutations are different. A few mutations code for an altered protein with a different shape. Because the shape is what lets a protein do its job, a different shape can mean the protein no longer works properly.

🔒 An enzyme

The altered enzyme may no longer fit the substrate binding site (the active site). The substrate cannot bind, so the reaction is not catalysed.

🧱 A structural protein

An altered structural protein, such as collagen, may lose its strength. The structure it forms is then weaker.

Worked example

A gene codes for an enzyme. A mutation changes one base in the gene.

Step 1: the order of bases has changed, so the order of amino acids may change.

Step 2: the chain may fold into a different shape.

Step 3: the active site has a different shape, so the substrate may no longer fit.

Step 4: the enzyme cannot catalyse its reaction properly.

If the mutation does not change the protein, none of this happens and the enzyme works as before.

Non-coding DNA and genetic variants

Not all parts of DNA code for proteins. The parts that do are coding DNA. The other parts are non-coding DNA.

Non-coding parts of DNA can switch genes on and off. Whether a gene is switched on decides whether its protein is made. So a variation in non-coding DNA may affect how genes are expressed.

Key terms:

Mutation: a change in the structure of DNA.
Non-coding DNA: parts of DNA that do not code for proteins but can switch genes on and off.
Gene expression: whether and how much a gene is switched on to make its protein.
Genetic variant: a different version of a section of DNA, for example one that has a mutation.
Phenotype: the characteristics an organism shows.

So genetic variants may influence phenotype in two ways:

🧬 In coding DNA

The variant alters the activity of a protein. For example, an enzyme works less well or a structural protein is weaker.

💡 In non-coding DNA

The variant alters how genes are expressed, because the non-coding DNA switches genes on and off.

Common mistakes

Do not say that all mutations are harmful. Most do not alter the protein, or only alter it slightly.

Do not say that the mutation changes the protein's job directly. The mutation changes the order of amino acids, which may change the shape, and the shape decides the job.

Do not say that non-coding DNA is useless. It can switch genes on and off.

Do not write about mRNA or tRNA. You only need to say a template and carrier molecules.

Exam-style question

(a) Where are proteins made in a cell? [1 mark]

(b) Describe how a protein chain is built during protein synthesis. [2 marks]

(d) Some DNA does not code for proteins. Suggest how a change in this DNA could affect an organism. [1 mark]

Model answer

(a) On ribosomes (1).

(b) Carrier molecules bring specific amino acids (1) which are added to the growing chain in the correct order, according to a template (1).

(c) The mutation changes the order of bases, so the order of amino acids changes (1). The protein folds into a different shape (1), so the active site no longer fits the substrate (1).

(d) Non-coding DNA can switch genes on and off, so a change could affect how genes are expressed (1).

Exam tip

In part (c), link three steps in order: DNA changes, so shape changes, so the job cannot be done. Naming the shape in your answer is what earns the marks.

Test Your Knowledge