GCSE · Biology · Edexcel · Spec 1BI0

Genetic variants in coding DNA and phenotype

Change one letter in a word and the meaning can flip. DNA is written in three-letter words, so what does one changed letter do? Sometimes a lot, sometimes nothing.

Biology · Genetic variants

Follow one changed base down every route

A single base in DNA has changed. Pick where it landed, then keep choosing. Each route ends somewhere different.

Where the change lands → What the changed triplet codes for → The protein's 3D shape → What the protein does

Every route starts here. Where the change lands decides what happens next.

3 routes a mutation can take.

On One base in the DNA changes. 2 branches to choose from.

One letter of DNA changes. Does the organism? It depends which way you go.

Watch out: Not every route ends with a changed protein, and not every change in DNA is in a gene.

Biology · The parts of the code

A stretch of DNA, before anything goes wrong

View: Read in threes. Showing 3 layers: The bases, Gene and non-coding DNA, Triplets

View

Layers

Explore

Switch the view, then tap a part to explore it.

The gene is read as triplets. Each triplet codes for a specific amino acid.

These bases are invented to show the idea. Switch the view, then tap a part.

Biology · Kinds of mutation

What happened to the strand?

Compare the DNA before and after. Which kind of mutation is it?

Still to sort

Substitution (0)

A nucleotide is swapped for a different one.

Where the line is: Same number of bases before and after, with one base different. Nothing has been added or taken away.

Insertion (0)

A nucleotide is added.

Where the line is: The strand ends up one base longer than it started.

Deletion (0)

A nucleotide is removed.

Where the line is: The strand ends up one base shorter than it started.

6 of 6 still to sort.

These sequences are invented to show the idea. Compare each strand before and after the mutation.

Biology · What do you think?

Which of these sounds right to you?

A base in some DNA has changed, and a student is trying to work out what that means.

Which idea is closest to what you think right now?

Any change in DNA creates an allele.If a change is outside a gene, it can't matter, because only genes make proteins.If a base changes, the phenotype changes.A variant in a gene is an allele. A variant in non-coding DNA is not an allele, but it can still affect proteins.

How sure are you?

I'm sureI think soI'm guessing

Any change in DNA creates an allele.

This is a common misconception

Close, but not quite. An allele is a different version of a gene. A variant in a gene is an allele. A variant in non-coding DNA is a genetic variant, but it is not an allele, because it is not a version of a gene.

If a change is outside a gene, it can't matter, because only genes make proteins.

This is a common misconception

It can still matter. Some non-coding sections of DNA help to control when proteins are made by the genes, so a variant there can affect whether or when a protein is made.

If a base changes, the phenotype changes.

This is a common misconception

Not necessarily. A changed triplet may still code for the same amino acid, because there are more triplet codes than amino acids. Then the amino acid sequence doesn't change, and there is nothing to change the protein's shape or what it does.

A variant in a gene is an allele. A variant in non-coding DNA is not an allele, but it can still affect proteins.

This matches the evidence

Spot on. Alleles are different versions of genes, so only a variant in a gene is an allele. A variant in non-coding DNA can still matter if that stretch helps control whether or when a protein is made.

Allele means a variant in a gene. And a changed triplet doesn't always change the amino acid, so a mutation doesn't always change the organism.

Biology · Write it out

Now you explain it

Describe two causes of mutation. Then explain how a mutation in a gene could change an organism's phenotype. [6 marks]

Your answer

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

The mark scheme

One cause: errors when DNA is copied, when cells divide to make new cells. (1 mark)
A second cause: mutagens (for example chemicals in tobacco smoke) or ionising radiation (such as X-rays, gamma rays or high-energy ultraviolet). (1 mark)
The mutation changes the base sequence of the gene, which can change the amino acid sequence of the protein. (1 mark)
A changed amino acid sequence can change the 3D shape of the protein. (1 mark)
A changed shape can change what the protein does (for example the shape of an enzyme's active site). (1 mark)
This change in the protein can have an impact on the organism's phenotype. (1 mark)

Model answer

Most mutations are caused by errors when DNA is copied as cells divide. They can also be caused by mutagens, such as chemicals in tobacco smoke, or by ionising radiation. If a mutation changes the base sequence of a gene, it can change the amino acid sequence of the protein. That can change the protein's 3D shape, and a different shape can change what the protein does, for example the shape of an enzyme's active site. A change in the protein can then have an impact on the organism's phenotype.

Two causes, then a chain: base sequence, amino acid sequence, 3D shape, function, phenotype. Each link is a mark.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Genetic variants, alleles, proteins and phenotype

What you need to know

DNA is a polymer made of repeating nucleotides, coded A, T, C and G, and it carries the genetic code.
A gene is a section of DNA holding the code for a protein; the genome is genes plus non-coding DNA.
The code is read in threes, called the triplet code, and each triplet codes for a specific amino acid.
Have a goHere is a stretch of a gene: GCATTGCAT. Read it the way the code is read. How many triplets is that, and what are they?
Three: GCA, TTG and CAT.
The code is read in groups of three bases, so nine bases make three triplets.
The base order in a gene sets the amino acid order, and the right order gives the protein its structure.
A mutation is a change in the base sequence: a nucleotide can be substituted, inserted or deleted, creating a genetic variant.
Have a goMaya says, "A mutation just means one base gets swapped for another." She has one of three right. What are the other two?
A nucleotide can also be inserted or deleted.
Substitution is only one way to change the base sequence. A nucleotide can be substituted, inserted or deleted.
Most mutations are errors when DNA is copied as cells divide; mutagens and ionising radiation can cause them too.
A variant in a gene is an allele, a different version of that gene, and it can produce a different phenotype.
Variants in non-coding DNA are not alleles, but some non-coding sequences control whether or when a protein is made.
A changed triplet may or may not change the amino acid, because there are more triplet codes than amino acids.
A changed amino acid sequence can change the protein's 3D shape, and a changed shape can change what it does.
Have a goA gene's amino acid sequence changes. Put these in the order they can follow: protein does a different job, phenotype affected, 3D shape changes.
3D shape changes, then the protein does a different job, then the phenotype is affected.
A changed amino acid sequence can change the shape, a changed shape can change what the protein does, and that can have an impact on the phenotype.

The big picture

A mutation changes the order of bases in DNA and creates a genetic variant. A variant in a gene is an allele, and it may change the amino acid sequence, the protein's 3D shape and function, and so the phenotype. Or it may change nothing, because a changed triplet can still code for the same amino acid. A variant in non-coding DNA is not an allele, but it can still affect whether or when a protein is made.

Key points

1A mutation changes the order of bases. A nucleotide can be substituted, inserted or deleted.

2Variants in genes are alleles. Variants in non-coding DNA are not, though they can still affect proteins.

3A changed triplet may still code for the same amino acid, so a mutation may not change the phenotype.

4When the amino acid sequence does change, the 3D shape can change, then what the protein does, then the phenotype.

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Worked example

Problem

A gene contains the triplets TGC AAT. After a mutation it reads TGC GAT. Work out what kind of mutation this is, and say what you can and can't tell about the protein.

⚠ Watch out

Thinking every mutation changes the organism, or that every genetic variant is an allele. A changed triplet may still code for the same amino acid, and alleles are variants in genes only.

🧠

Memory hook

Letter, word, meaning. A changed base changes a triplet, but the amino acid it codes for, and so the protein, might not follow.

✓

Check yourself

Cover the page. A triplet in a gene changes, but the protein ends up with exactly the same amino acid sequence. In one or two sentences, explain how that can happen.

Flashcards

(14)

What is DNA?

A large chemical molecule, a polymer made of repeating groups called nucleotides, that carries the genetic code of all living organisms.

What are the four types of nucleotide coded as?

A, T, C and G.

What is the genetic code, and what is it for?

The order of the bases in DNA. It is the instructions for the cell to make proteins.

What is a gene, and what makes up the genome?

A gene is a section of DNA that holds the genetic code for a protein. The genome is made up of genes and non-coding DNA.

What is the triplet code?

The genetic code is read in groups of three bases, and each triplet codes for a specific amino acid.

What does the order of bases in a gene decide?

The order of amino acids in the protein. Putting them together in the right order gives the protein its structure.

What is a mutation, and what are the three ways a nucleotide can change?

A change in the base sequence of the DNA in the genome. A nucleotide can be substituted, inserted or deleted.

What causes mutations?

Most are errors when DNA is copied as cells divide. Mutagens (such as chemicals in tobacco smoke) and ionising radiation (X-rays, gamma rays, high-energy ultraviolet) can cause them too.

What is a genetic variant?

A region of DNA in which the sequence of nucleotide bases has been changed. It can be in a gene or in non-coding DNA.

What is an allele?

A different version of a gene, made when a mutation changes the base sequence of the gene. It can produce a different phenotype.

Is a variant in non-coding DNA an allele? Can it matter?

It is not an allele, because alleles are versions of genes. It can still affect proteins, because some non-coding sequences control whether or when a protein is made.

Why might a changed triplet not change the amino acid?

There are more triplet codes than amino acids, so sometimes a changed code still codes for the same amino acid.

How can a changed amino acid sequence affect the protein?

It can change the protein's 3D shape, and a changed shape can change what it does, for example the shape of an enzyme's active site.

What is a phenotype?

The physical characteristics of an organism. Changes in proteins can have impacts on it.

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Follow one changed base down every route

What happened to the strand?

Which of these sounds right to you?

Now you explain it

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