Genetic Mutations (A Level Only)

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レッスンノート

What is a Gene Mutation?

A gene mutation is a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide.
Mutations occur continuously and spontaneously, most often as errors during DNA replication.
Because the DNA base sequence determines the amino acid sequence, a mutation can change the polypeptide a gene codes for.
Most mutations do not alter the polypeptide, or only alter it slightly, so its structure or function is unchanged.
This is often because the genetic code is degenerate — more than one triplet can code for the same amino acid.

Insertion Mutations

An insertion mutation occurs when a nucleotide with a new base is randomly inserted into the DNA sequence.
It changes the amino acid coded for by the original base triplet because it creates a new, different triplet.
It also has a knock-on effect, changing all the triplets further on in the sequence — this is a frameshift mutation.
This may dramatically change the amino acid sequence produced and therefore the ability of the polypeptide to function.

Deletion Mutations

A deletion mutation occurs when a nucleotide (and therefore its base) is randomly deleted from the DNA sequence.
Like an insertion, it changes the amino acid that would have been coded for.
It also causes a frameshift by changing the groups of three bases further on in the sequence.
This may dramatically change the amino acid sequence and therefore the ability of the polypeptide to function.

Substitution Mutations

A substitution mutation occurs when a base in the DNA sequence is randomly swapped for a different base.
Unlike insertion or deletion, it only changes the amino acid for the triplet in which the mutation occurs — there is no knock-on effect.
Silent mutations do not alter the amino acid sequence because the genetic code is degenerate.
Missense mutations alter a single amino acid in the polypeptide chain.
Nonsense mutations create a premature stop codon, so the polypeptide chain is incomplete, affecting final protein structure and function.

Inversion Mutations

An inversion mutation usually occurs during crossing-over in meiosis.
The DNA of a single gene is cut in two places; the cut portion is inverted 180° and rejoined in the same place within the gene.
A large section of the gene is 'backwards', so multiple amino acids are affected.
Inversion mutations frequently result in a non-functional protein; in some cases an entirely different protein is produced.
The mutation is often harmful because the original gene can no longer be expressed from that chromosome.
If the other chromosome in the pair carries a working gene, the effect may be lessened.

Duplication Mutations

A duplication mutation occurs when a whole gene or section of a gene is duplicated, so two copies appear on the same chromosome.
The original version of the gene remains intact, so the mutation is not harmful.
Over time, the second copy can undergo mutations that enable it to develop new functions.
Duplication mutations are an important source of evolutionary change.
α, β and gamma haemoglobin genes evolved due to duplication mutations.

Translocation Mutations

A translocation mutation occurs when a gene is cut in two places and the cut section attaches to a separate gene.
The cut gene becomes non-functional because it has a section missing.
The gene that has gained the translocated section is also likely to be non-functional.
If a section of a proto-oncogene is translocated onto a gene controlling cell division, it could boost expression and lead to tumours.
If a tumour suppressor gene is translocated and becomes faulty, the cell could continue to replicate when it contains faulty DNA.

Effects of Mutations on Polypeptides and Phenotype

Most mutations do not alter the polypeptide, or only alter it slightly, so its appearance or function is unchanged.
A small number of mutations code for a significantly altered polypeptide with a different shape.
This may affect the protein's function — for example, if an enzyme's active site changes shape, the substrate may no longer bind.
A structural protein such as collagen may lose its strength if its shape changes.
If a mutation causes a major alteration in a polypeptide, cellular mechanisms may be affected, impacting the phenotype.
For example, a mutation in the TYR gene affects an enzyme needed for melanin production, causing albinism (pale skin, often white hair).

Causes and Rates of Mutation

The rate of mutation can be estimated as around one mutation per 100,000 genes per generation.
Mutagenic agents are factors that increase the frequency of mutations in DNA.
Physical mutagens cause mutations through physical means, typically radiation such as ionising radiation (e.g. X-rays, gamma rays).
Chemical mutagens interfere with DNA molecules chemically.
Biological mutagens are organisms or biological molecules that interfere with DNA, such as viruses (e.g. HPV can lead to cervical cancer).
Exposure to mutagenic agents increases the rate of mutation.
Do not confuse mutagenic agents with carcinogens (agents that cause cancer): a mutagen might cause DNA mutations without leading to cancer.

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練習問題

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1.Which of the following are examples of mutagenic agents? (select all that apply)
Medium
- AX-rays
- BGamma rays
- CViruses such as HPV
- DWater
- EOxygen
2.Most mutations do not alter the polypeptide or only alter it slightly so that its structure or function is not changed.
Easy
True or false?
3.Which type of mutation involves a nucleotide being randomly inserted into the DNA sequence, creating a new triplet of bases and causing a frameshift?
Easy
- ASubstitution
- BDeletion
- CInsertion
- DInversion
4.Which type of mutation involves a base in the DNA sequence being randomly swapped for a different base, affecting only the triplet in which the mutation occurs?
Easy
- AInsertion
- BDeletion
- CSubstitution
- DTranslocation
5.Which type of substitution mutation creates a premature stop codon, causing the polypeptide chain to be incomplete?
Medium
- ASilent mutation
- BMissense mutation
- CNonsense mutation
- DFrameshift mutation
6.Which type of substitution mutation alters a single amino acid in the polypeptide chain?
Medium
- ASilent mutation
- BMissense mutation
- CNonsense mutation
- DDeletion mutation
7.Which feature of the genetic code allows a silent mutation to occur?
Medium
- AThe genetic code is universal
- BThe genetic code is degenerate
- CThe genetic code is non-overlapping
- DThe genetic code is triplet-based
8.Which type of mutation involves a section of a gene being cut in two places, inverted 180°, and then rejoined, affecting multiple amino acids?
Medium
- AInsertion
- BDeletion
- CInversion
- DDuplication

もっと問題

9.Which type of mutation involves a whole gene or section of a gene being duplicated so that two copies appear on the same chromosome?
Medium
- ATranslocation
- BInversion
- CDuplication
- DSubstitution
10.Which type of mutation involves a section of a gene being cut and attaching to a separate gene, potentially making both genes non-functional?
Hard
- ADuplication
- BInversion
- CTranslocation
- DDeletion
11.Which of the following are types of substitution mutations? (select all that apply)
Medium
- ASilent mutation
- BMissense mutation
- CNonsense mutation
- DFrameshift mutation
- EDeletion mutation
12.Insertion and deletion mutations cause a frameshift that changes all base triplets downstream from the mutation.
Easy
True or false?
13.A substitution mutation always causes a frameshift that changes all downstream triplets.
Easy
True or false?
14.Mutagenic agents are the same as carcinogens.
Easy
True or false?
15.Match each mutation type to its description.
Medium
- Insertion
- Deletion
- Substitution
- Inversion
- A base is randomly swapped for a different base, affecting only one triplet
- A nucleotide is randomly inserted, creating a new triplet and causing a frameshift
- A nucleotide is randomly deleted, causing a frameshift
- A section of a gene is cut, inverted 180°, and rejoined
16.Place the steps of an inversion mutation in the correct order.
Medium
- The DNA of a single gene is cut in two places
- The cut portion is inverted 180°
- The inverted portion is rejoined to the same place within the gene
- A large section of the gene is 'backwards', affecting multiple amino acids
17.A mutation in the TYR gene affects the structure of an enzyme needed for melanin production. What is the resulting phenotype in humans?
Medium
- AAlbinism
- BSickle cell anaemia
- CCystic fibrosis
- DHaemophilia
18.Which of the following best describes a gene mutation?
Medium
- AA change in the number of chromosomes in a cell
- BA change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide
- CA change in the structure of a protein after it is synthesised
- DA change in the rate of DNA replication
19.Which statement about the effect of a mutation on a polypeptide is correct?
Medium
- AAll mutations change the amino acid sequence of the polypeptide
- BMost mutations do not alter the polypeptide or only alter it slightly
- CMutations always produce a non-functional protein
- DMutations only occur in exons
20.Which of the following statements about the genetic code and mutations are correct? (select all that apply)
Medium
- AThe genetic code is degenerate, so some mutations do not change the amino acid sequence
- BA substitution mutation usually has a smaller effect on the polypeptide than an insertion mutation
- CInsertion and deletion mutations cause a frameshift
- DA silent mutation always changes the amino acid sequence
- EA nonsense mutation creates a premature stop codon
21.A duplication mutation is harmful because the original version of the gene is destroyed.
Easy
True or false?
22.α, β, and gamma haemoglobin genes evolved due to duplication mutations.
Easy
True or false?
23.Which of the following is a biological mutagenic agent?
Medium
- AX-rays
- BGamma rays
- CHPV virus
- DIonising radiation
24.Which of the following correctly describes the effect of a nonsense mutation?
Medium
- AIt creates a premature stop codon, producing an incomplete polypeptide
- BIt alters a single amino acid in the polypeptide chain
- CIt does not alter the amino acid sequence of the polypeptide
- DIt causes a frameshift that changes all downstream triplets
25.Which of the following correctly describes the effect of a missense mutation?
Medium
- AIt creates a premature stop codon
- BIt alters a single amino acid in the polypeptide chain
- CIt does not alter the amino acid sequence
- DIt deletes a nucleotide and causes a frameshift
26.Which of the following correctly describes the effect of a silent mutation?
Medium
- AIt creates a premature stop codon
- BIt alters a single amino acid in the polypeptide chain
- CIt does not alter the amino acid sequence of the polypeptide
- DIt causes a frameshift that changes all downstream triplets
27.Which of the following are physical mutagens? (select all that apply)
Medium
- AX-rays
- BGamma rays
- CIonising radiation
- DViruses
- EChemical mutagens
28.A silent mutation occurs because the genetic code is degenerate.
Easy
True or false?
29.A substitution mutation will only change the amino acid for the triplet in which the mutation occurs and will not have a knock-on effect.
Easy
True or false?
30.Which of the following are ways in which mutations can change the DNA base sequence of a gene? (select all that apply)
Medium
- AInsertion of a nucleotide
- BDeletion of a nucleotide
- CSubstitution of a base
- DTranscription of a gene
- ETranslation of mRNA
31.Which of the following statements about insertion and deletion mutations are correct? (select all that apply)
Medium
- AThey both change the amino acid coded for by the triplet where the mutation occurs
- BThey both cause a frameshift that changes base triplets further along the sequence
- CThey only affect the single triplet in which the mutation occurs
- DThey may dramatically change the amino acid sequence and the polypeptide's ability to function
- EThey are types of substitution mutation
32.Which of the following are possible effects of a gene mutation on the polypeptide it codes for? (select all that apply)
Medium
- ANo change to the polypeptide at all
- BA slight change that does not affect structure or function
- CA significantly altered polypeptide with a different shape
- DA change in the active site shape so substrate can no longer bind
- EA guaranteed improvement in protein function
33.A translocation mutation cuts a section of a proto-oncogene and attaches it to a gene controlling cell division. Which outcomes are possible? (select all that apply)
Hard
- AExpression of the gene controlling cell division may be boosted
- BTumours may develop
- CThe cut gene becomes non-functional because it has a section missing
- DThe gene that gained the section is likely to be non-functional
- EThe original gene is duplicated so two working copies remain
34.Which of the following are types of mutagenic agent? (select all that apply)
Easy
- APhysical mutagens
- BChemical mutagens
- CBiological mutagens
- DCarcinogens
- EEnzymes
35.Which of the following statements about the rate of mutation are correct? (select all that apply)
Medium
- AThe rate can be estimated as around one mutation per 100,000 genes per generation
- BMutations occur spontaneously and randomly during DNA replication
- CExposure to mutagenic agents can increase the rate of mutation
- DThe rate of mutation is fixed and cannot be changed by the environment
- EMutations only occur when mutagenic agents are present
36.Match each mutation type to the correct description.
Medium
- Inversion
- Duplication
- Translocation
- Substitution
- A base is swapped for a different base, affecting only one triplet
- A section of a gene is cut in two places, inverted 180° and rejoined
- A whole gene or section is copied so two copies appear on the same chromosome
- A section of a gene is cut and attaches to a separate gene
37.Match each substitution mutation type to its effect on the polypeptide.
Medium
- Silent mutation
- Missense mutation
- Nonsense mutation
- Does not alter the amino acid sequence
- Alters a single amino acid in the chain
- Creates a premature stop codon so the chain is incomplete
38.Match each mutagenic agent to the correct category.
Medium
- X-rays and gamma rays
- Substances that interfere chemically with DNA
- Viruses such as HPV
- Physical mutagen
- Chemical mutagen
- Biological mutagen
39.Place the steps of a translocation mutation in the correct order.
Medium
- The DNA of a gene is cut in two places
- The cut section of the gene is removed
- The section attaches to a separate gene
- The cut gene is left non-functional
40.Order the events that occur when a nonsense mutation affects a polypeptide.
Medium
- A base substitution creates a premature stop codon
- Translation stops early at the premature stop codon
- The polypeptide chain produced is incomplete
- The final protein structure and function are affected
41.Complete the sentence about the rate of mutation.
Easy
The rate at which mutations occur can be estimated as around one mutation per ____ genes per generation.
42.Complete the sentence about inversion mutations.
Easy
Inversion mutations frequently result in a ____ protein.
43.Name the three types of mutagenic agents described.
Medium
The three types of mutagenic agents are ____, ____ and ____ mutagens.
44.Complete the sentence about the effect of mutations on enzymes.
Easy
If the shape of the active site on an enzyme changes, the ____ may no longer be able to bind to the active site.
45.Complete the sentence about the effect of a mutation on a structural protein.
Medium
A structural protein such as ____ may lose its strength if its shape changes.
46.Name the type of mutation that occurs when a section of a gene is cut in two places and the cut portion is inverted 180° before being rejoined.
Easy
47.Name the type of mutation that occurs when a whole gene or section of a gene is duplicated so that two copies appear on the same chromosome.
Easy
48.Name the type of mutation that occurs when a section of a gene is cut and attaches to a separate gene.
Medium
49.Complete the sentence about the effect of a mutation on a protein's shape.
Medium
A small number of mutations code for a significantly altered polypeptide with a different ____, which may affect the ability of the protein to perform its function.
50.Complete the sentence about the effect of mutations on the genetic code.
Medium
Most mutations do not alter the polypeptide, or alter it only slightly, because the genetic code is ____.
51.Complete the sentence about the effect of a mutation on an enzyme.
Medium
If a mutation changes the shape of an enzyme's ____, the substrate may no longer be able to bind to it.
52.A DNA coding sequence contains 438 coding nucleotides. Calculate the number of amino acids that make up the polypeptide it codes for.
Hard
53.Define the term mutation.
Easy
54.Compare the DNA changes that occur in deletion and substitution mutations.
Easy
55.Explain how a mutation may result in the synthesis of a non-functional protein.
Medium
56.Define the term silent mutation.
Easy
57.State why it is not possible to predict where or when a mutation is likely to occur during DNA replication.
Medium
58.A DNA coding sequence contains 438 coding nucleotides. Calculate the number of amino acids that make up the polypeptide it codes for.
Hard
- A131 amino acids
- B176 amino acids
- C161 amino acids
- D146 amino acids

復習ノート

## Genetic Mutations **Gene mutation** = change in the base sequence of DNA that may alter the polypeptide produced. - Mutations occur **spontaneously and randomly**, most often during **DNA replication**. - The genetic code is **degenerate**, so many mutations do not change the amino acid sequence. - Mutations can involve **insertion, deletion, substitution** of a nucleotide, or **inversion, duplication, translocation** of a section of a gene. ## Types of mutation - **Insertion** – a nucleotide is randomly inserted; creates a new triplet and causes a **frameshift**, changing all downstream triplets. - **Deletion** – a nucleotide is randomly deleted; also causes a **frameshift**. - **Substitution** – a base is swapped for another; only affects the triplet where it occurs (no knock-on effect). Three forms: - **Silent** – no change to amino acid sequence (code is degenerate). - **Missense** – alters a single amino acid. - **Nonsense** – creates a **premature stop codon**, so the polypeptide is incomplete. - **Inversion** – a section of a gene is cut, inverted 180°, and rejoined; multiple amino acids affected; often non-functional protein. - **Duplication** – a gene or section is duplicated; original remains intact, so not harmful; important source of evolutionary change (e.g. alpha, beta, gamma haemoglobin genes). - **Translocation** – a section of a gene is cut and attaches to a separate gene; both genes likely non-functional. Can affect proto-oncogenes or tumour suppressor genes, leading to tumours. ## Effects of mutations - Most mutations do not alter the polypeptide or only slightly, so structure/function is unchanged. - Some change all downstream triplets → **non-functional polypeptide**. - A significantly altered polypeptide may have a different shape, affecting function: - Enzyme active site may change shape → substrate cannot bind. - Structural protein (e.g. collagen) may lose strength. - Mutations can affect **phenotype** via cellular mechanisms, e.g. mutation in TYR gene → no melanin → **albinism**. ## Causes of mutations - Rate ≈ **one mutation per 100,000 genes per generation**. - **Mutagenic agents** increase mutation frequency: - **Physical** – ionising radiation (X-rays, gamma rays). - **Chemical** – interfere with DNA chemically. - **Biological** – viruses (e.g. HPV → cervical cancer). - Do not confuse mutagens with **carcinogens** (cause cancer); a mutagen may cause DNA mutations without leading to cancer.

Gene mutation: A change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide.
Insertion mutation: A mutation that occurs when a nucleotide (with a new base) is randomly inserted into the DNA sequence, causing a frameshift.
Deletion mutation: A mutation that occurs when a nucleotide (and therefore its base) is randomly deleted from the DNA sequence, causing a frameshift.
Substitution mutation: A mutation that occurs when a base in the DNA sequence is randomly swapped for a different base; only affects the triplet where it occurs.
Silent mutation: A substitution mutation that does not alter the amino acid sequence of the polypeptide because the genetic code is degenerate.
Missense mutation: A substitution mutation that alters a single amino acid in the polypeptide chain.
Nonsense mutation: A substitution mutation that creates a premature stop codon, causing the polypeptide chain produced to be incomplete.
Inversion mutation: A mutation where a section of a gene is cut, inverted 180°, and rejoined, affecting multiple amino acids and often resulting in a non-functional protein.
Duplication mutation: A mutation where a whole gene or section of a gene is duplicated so that two copies appear on the same chromosome; the original remains intact.
Translocation mutation: A mutation where a section of a gene is cut and attaches to a separate gene, making both genes likely non-functional.
Mutagenic agents: Factors that increase the frequency of mutations in DNA, including physical, chemical, and biological mutagens.
Carcinogens: Agents that cause cancer; some also cause mutations, but not all mutagens lead to cancer.

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