GCSE · Biology · AQA · Spec 8461

Mutations and gene expression (HT, biology only)

Changes to your DNA, called mutations, happen all the time. So why isn't your body constantly going wrong? Because most of them don't matter. So which ones do?

Higher

Biology · Follow one mutation

Where does this mutation end up?

One base in your DNA has just changed. At each fork, choose a condition and see where that route ends.

Where it lands → What it does to the protein

  • This isn't a rare accident. Mutations occur continuously. The first question is where in the DNA the change has landed.

3 possible routes for one mutation.

On A mutation: one base in the DNA changes. 2 branches to choose from.

Mutations are changes to DNA, and they happen continuously. This tree shows the routes one change can take. Walk every route, then look back at which endings are common and which are rare.

Watch out: 'Mutation' doesn't mean 'harmful'. Most mutations don't alter the protein at all, or alter it so slightly that its appearance and function stay the same.
UK note

AQA GCSE Biology 8461, section 4.6.1.5: this is Higher tier only, and it is separate-Biology content. You need to be able to describe how genetic variants may influence phenotype through coding and non-coding DNA.

Higher

Predict, then check

Commit to an answer before you look. Think about how the bases are grouped.

Here is a short made-up stretch of the coding DNA of a gene, written in groups of three: TAC GGA TTC CAA. The 8th base (the second T in TTC) is swapped for a C, and nothing is added or removed, so the stretch now reads TAC GGA TCC CAA. Which amino acids in the protein chain could be different?

Higher

Biology · From base to broken protein

Build the chain yourself

Start from one changed base. Link each change to the next change it can cause, all the way to a protein that no longer does its job. There are two possible endings.

  1. The order of amino acids in the chain changes
  2. The substrate no longer fits the enzyme's active site
  3. One base in a gene's coding DNA changes
  4. The finished chain folds into a different shape
  5. A triplet now codes for a different amino acid
  6. A structural protein, such as collagen, loses strength
Add a link

No links yet.

Higher

Two routes to a different phenotype

Variant in coding DNAvsVariant in non-coding DNA

Your phenotype is the set of characteristics you actually have, the ones that show up. Not all DNA codes for proteins, so a variant can reach your phenotype by two different routes.

Focus

Does a protein's shape change?

Variant in coding DNA

It may. A changed triplet can change the amino-acid order, and so the shape the chain folds into.

Variant in non-coding DNA

No. This DNA doesn't code for a protein, so no protein's amino acids change.

The insight

This is the difference people miss. A non-coding variant can change the phenotype without changing the shape of a single protein.

What the DNA here does

Variant in coding DNA

Its bases code for a protein. The order of bases sets the order of amino acids.

Variant in non-coding DNA

It doesn't code for a protein, but some of it switches genes on and off.

What the variant can alter

Variant in coding DNA

The activity of a protein, meaning how well it does its job.

Variant in non-coding DNA

How genes are expressed, meaning whether they are switched on or off.

Can it change the phenotype?

Variant in coding DNA

Yes, if the protein's activity changes.

Variant in non-coding DNA

Yes, if it changes how a gene is expressed.

Higher

Biology · Your turn to explain

Write it, then check it

Collagen is a structural protein. A mutation changes one base in the part of a gene that codes for collagen. Explain how this mutation could make the collagen weaker. [4 marks]

0 words · your answer stays on this page and is not sent anywhere.

UK note

In AQA GCSE Biology, Inheritance, variation and evolution is assessed on Paper 2. The command word 'explain' means give the reasons, which here means every link of the chain.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Whether a change matters depends on where it lands, and what it does once it's there.

What you need to know

  • Mutations occur continuously.
  • Three bases code for one amino acid. The order of bases controls the order in which amino acids are assembled into a protein.
  • A completed protein chain folds into a unique shape. That shape lets it do its job as an enzyme, a hormone or a structure such as collagen.
  • Most mutations don't alter the protein, or alter it so slightly that its appearance and function are unchanged. A few code for a protein with a different shape.
  • Not all DNA codes for proteins. Non-coding DNA can switch genes on and off, so variants there may alter how genes are expressed.

The big picture

Mutations happen continuously, but most don't alter the protein, or alter it only slightly, so appearance and function stay the same. Three bases code for one amino acid, and the order of bases controls the order of amino acids. A few mutations give a protein a different shape, so the substrate may no longer fit an enzyme's active site, or a structural protein may lose strength. Not all DNA codes for proteins. Variants in non-coding DNA can change how genes are switched on and off, and so can change the phenotype.

Key points

1Where a mutation lands decides its route: coding DNA affects a protein, and non-coding DNA affects whether genes are switched on or off.
2Swapping one base for another changes one triplet, which may change one amino acid in the chain's order.
3A different amino-acid order can make the chain fold into a different shape. The substrate may then no longer fit an enzyme's active site, and a structural protein may lose strength.
4A genetic variant may influence phenotype in two ways: in coding DNA by altering a protein's activity, and in non-coding DNA by altering how genes are expressed.

Worked example

Problem

A person has a variant where one base is different from most people's DNA. Every one of their proteins has its normal shape, but one gene that is normally switched on is now switched off. Where in the DNA is the variant most likely to be, and how has it affected their phenotype?

⚠ Watch out

Thinking every mutation changes a protein and causes harm. Most mutations leave the protein unchanged or only slightly changed. A variant in non-coding DNA changes no protein at all, and it can still change the phenotype by switching a gene on or off.

🧠

Memory hook

Three bases, one amino acid. Order sets order, and shape does the job. Coding changes the protein. Non-coding flips the switch.

✓

Check yourself

Close the page and say the chain out loud: from one changed base to a substrate that can't fit the active site, in four linked steps. Then name the ending that most mutations reach instead.

Flashcards

(13)
How often do mutations occur?
Continuously. They are happening all the time.
How many bases code for one amino acid?
Three, which is called a triplet.
What controls the order of amino acids in a protein?
The order of bases in the DNA.
What happens to a protein chain once it is complete?
It folds up into its own unique shape.
Why does a protein's unique shape matter?
It is what lets the protein do its job, as an enzyme, a hormone or a structure such as collagen.
What do most mutations do to the protein?
Nothing, or only a slight change. The protein's appearance and function stay the same.
What do a few mutations do to the protein?
They code for an altered protein with a different shape.
How can a change in shape affect an enzyme?
The substrate may no longer fit the enzyme's active site, where it binds.
How can a change in shape affect a structural protein like collagen?
It may lose its strength.
Does all DNA code for proteins?
No. Some DNA is non-coding.
What can non-coding DNA do to genes?
It can switch them on and off.
How can a variant in coding DNA influence phenotype?
By altering the activity of a protein.
How can a variant in non-coding DNA influence phenotype?
By altering how genes are expressed, meaning whether they are switched on or off.

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