Genetic Mutations (A Level Only)

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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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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

複習筆記

## 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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