13.6 Gene Expression and Mutation

A free GCSE Biology lesson for AQA (8461) from OBA School: coding and non-coding DNA, genetic variants, and how mutations can change proteins (Higher tier). Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain that non-coding DNA can switch genes on and off (Higher tier)
• Describe how genetic variants in coding and non-coding DNA can affect the phenotype (Higher tier)
• Describe mutations as changes in DNA that happen continuously
• Explain why most mutations have little effect but a few change a protein's shape (Higher tier)
• Work out the effect of substitution, insertion and deletion on a base sequence

Key words

• Gene expression: Whether a gene is switched on and used to make its protein, and how much protein is made.
• Coding DNA: DNA whose base sequence codes for the order of amino acids in a protein.
• Non-coding DNA: DNA that does not code for a protein. Some of it switches genes on and off.
• Genetic variant: A different version of a DNA sequence (an allele) found in some individuals.
• Mutation: A change in the DNA base sequence. New variants arise by mutation.
• Phenotype: The characteristics an organism shows, e.g. eye colour or enzyme activity.

Switching genes on and off (Higher tier)

Not all parts of DNA code for proteins. Non-coding parts of DNA can switch genes on and off. Every body cell has the same genes, but a liver cell switches on different genes from a muscle cell. That is how cells become specialised.

So variations in these non-coding areas of DNA may affect how genes are expressed: a gene might be switched on at the wrong time, switched off, or make too much or too little protein.

Key fact: Genetic variants can change the phenotype in two ways: in coding DNA by altering the activity of a protein; in non-coding DNA by altering how genes are expressed.

Mutations

A mutation is a change in the DNA. Mutations occur continuously (all the time), usually when DNA is copied before a cell divides. The chance is increased by ionising radiation (e.g. X-rays, UV light) and some chemicals, such as those in tobacco smoke.

• Substitution; What happens one base is swapped for another; Example (original: TAC GGA TCA) TAC GCA TCA (one triplet changed)
• Insertion; What happens an extra base is added; Example (original: TAC GGA TCA) TAC GGG ATC A… (every triplet after it shifts)
• Deletion; What happens a base is lost; Example (original: TAC GGA TCA) TAC GAT CA… (every triplet after it shifts)

Worked example: counting changed triplets

A gene has 300 triplets. A base in triplet 91 is deleted. Roughly how many triplets could be read differently?
Every triplet from 91 onwards is shifted: 300 − 90 = 210 triplets.
A substitution in triplet 91 would change at most 1 triplet.

Answer: about 210 triplets (deletion) compared with 1 (substitution)

What a mutation does to the protein (Higher tier)

• Most mutations do not alter the protein, or only alter it slightly so that its appearance or function is not changed. (The new triplet may code for the same amino acid, or the change may be in DNA that is not used.)
• A few mutations code for an altered protein with a different shape.
• An enzyme may no longer fit the substrate binding site (active site), so it cannot work.
• A structural protein such as collagen may lose its strength.

Exam tip: Use the chain of reasoning: change in base sequence → change in amino acid order → protein folds differently → different shape → active site no longer fits / protein cannot do its job.

Are mutations always harmful?

Rarely, a mutation gives an advantage. For example, a mutation in bacteria may make them resistant to an antibiotic, and in some human populations a variant lets adults keep digesting lactose (milk sugar). Mutations are the original source of all genetic variation, and that variation is what natural selection acts on.

Worked example: data on mutation effects

In a study of 400 mutations in one bacterial gene: 296 had no effect, 88 reduced the enzyme's activity and 16 increased it.
Percentage with no effect = 296 ÷ 400 × 100 = 74%. Increased activity = 16 ÷ 400 × 100 = 4%.

Answer: 74% no effect; only 4% helpful to the enzyme

Common mistakes

• Saying all mutations are harmful. Most have no effect, and a few are useful.
• Thinking non-coding DNA is useless. Some of it controls when genes are switched on.
• Writing "the mutation changes the shape of the DNA so the enzyme is denatured". The amino acid order changes, so the protein folds into a different shape.
• Forgetting that insertions and deletions shift every triplet after the change, so they usually have a bigger effect than a substitution.

Revision questions

Try these, then check your answers below each one.

Question 1. What is a mutation? (Choose from: A change in the DNA base sequence, A change in phenotype caused by the environment, The joining of two gametes, Switching a gene off.)

Answer: A change in the DNA base sequence

Working: A mutation is a change in DNA. Mutations occur continuously.

Question 2. (Higher) True or false? Most mutations change a protein so much that it cannot work.

Answer: False

Working: Most mutations do not alter the protein, or only alter it slightly so its function is not changed.

Question 3. (Higher) A mutation changes the shape of an enzyme's active site. What is the most likely effect? (Choose from: The substrate no longer fits, so the enzyme cannot catalyse the reaction, The enzyme works faster at all temperatures, The gene is removed from the chromosome, The substrate changes shape to fit.)

Answer: The substrate no longer fits, so the enzyme cannot catalyse the reaction

Working: A changed active site no longer fits the substrate binding site, so the enzyme does not work.

Related lessons

• Previous lesson: 13.5 DNA Structure and Protein Synthesis
• Next lesson: 13.7 Inheritance in Action
• In this chapter: 13.1 Types of Reproduction
• In this chapter: 13.2 Cell Division in Sexual Reproduction
• In this chapter: 13.3 The Best of Both Worlds

Continue in the full course

This lesson is section 13.6 (AGCBH1306) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.