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Gene Mutations

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Biology

Changes to a DNA base sequence are called gene mutations. These usually occur during DNA replication. Genes code for polypeptides, proteins or functional RNA products. Each gene is read in sequences of three bases or triplets, meaning every three nucleotides correspond to one amino acid. These triplets are called codons. Changes to a codon may have a knock-on effect on the products the gene codes for. For example, this could mean the protein produced will be altered as a result. Sometimes, there is no effect, as we will learn later. Regardless, any change to our DNA can potentially change the proteins in our cells.

Read our article on Protein Synthesis to learn more about genes and proteins.

Types of gene mutations

Gene mutations fall into three categories depending on the type of change to the DNA sequence:

  • Substitution
  • Insertion
  • Deletion

If only one nucleotide is substituted, deleted or inserted, it is called a point mutation. We will explore each type of gene mutation here.

Substitution

Substitutions occur when an incorrect nucleotide replaces the original nucleotide.

The codon ACC codes for the amino acid threonine. If a substitution event occurs during DNA replication that changes ACC to CCC, the protein will have a proline instead of a threonine in that position. Since different amino acids have different physical and chemical properties, a simple change can lead to significant functional changes for the protein.

It is important to note that some substitutions do not always lead to a change in the amino acid sequence due to the degenerate nature of the genetic code. "Degenerate" in this context means that more than one codon can code for the same amino acid.

The four nucleotides in DNA are adenine (A), thymine (T), cytosine (C) and guanine (G).

Insertion

Insertions occur when a new nucleotide is added to the DNA base sequence. As a result, adding just one nucleotide can change every successive triplet after the mutation. This addition is called a frameshift mutation, which we will explain in the following section.

Deletion

Deletions describe the removal of a nucleotide from the DNA base sequence. Again, this is also a frameshift mutation, as every successive triplet is changed.

Effects of gene mutations

Many students think all mutations cause detrimental effects on the organism, but that is not always the case. Mutations can be categorized into four different types depending on their functional effects:

  • Silent mutation
  • Nonsense mutation
  • Missense mutation
  • Frameshift mutation

We will explore each effect in detail here.

Silent mutations

Silent mutations occur when a substitution event still codes for the original amino acid.

The triplet GGC codes for glycine. If a substitution occurs and the triplet is mutated into GGA, glycine is still the corresponding amino acid.

This is due to the degeneracy of the genetic code, which means that more than one codon can code for the same amino acid. The mutations are neutral and do not alter the function of the protein.

Nonsense mutations

Nonsense mutations occur when a substitution, deletion or insertion results in a codon becoming a stop codon. A stop codon terminates a polypeptide's elongation and does not code for an amino acid.

The three stop codons are TAG, TAA and TGA (in DNA). The polypeptide is terminated prematurely when this occurs, producing a non-functional protein. These mutations are largely detrimental to the organism.

The DNA stop codons are TAG, TAA and TGA. Thus, in mRNA, the stop codons are UAG, UAA and UGA.

Missense mutations

Missense mutations occur when an incorrect amino acid is coded for, usually caused by substitution events. The effects of this type of mutation depend heavily on the differences in chemical properties between the original amino acid and the incorrect amino acid.

Alanine is a non-polar hydrophobic amino acid, and lysine is a basic positively charged amino acid. Alanine and proline have very different chemical properties. If alanine replaces lysine in a protein, this could lead to large changes in amino acid interactions. In this context, this could lead to detrimental effects such as defects in vital enzymes. For example, mutations in enzymes involved in metabolic processes, such as cellular respiration, could affect the rate at which ATP is produced.

These mutations can also lead to beneficial effects if the new protein product helps the organism's survival.

Antibiotic resistance in bacteria is a beneficial result of mutation events (for bacteria). The accumulation of mutations in some bacterial strains have enabled them to survive against antibiotic exposure, and while this mutation may be harmful to humans, it benefits the bacterial population.

Frameshift mutations

Frameshift mutations describe the changes in every successive triplet code following a deletion or insertion event. This event causes a change in every amino acid following the mutation. The ‘reading frame’ describes the codon sequence from which the ribosomes translate the RNA triplets into amino acids. These mutations "shift" this reading frame.

The effect of this mutation is very pronounced as every amino acid could be completely different, meaning the chemical interactions within the protein will be substantially affected. The protein will most likely be non-functional.

Cystic fibrosis is a genetic condition caused by a frameshift mutation in the CF transmembrane conductance regulator (CFTR) gene. This receptor is necessary to maintain the salt-water balance in the lungs.

Causes of gene mutations

Gene mutations occur spontaneously due to errors in DNA replication. This includes defects in DNA polymerase, the enzymatic driver of semi-conservative replication. The basal mutation rate for point mutations is approximately 7 x 10-9 per base pair per generation. However, this rate can increase as a result of mutagen exposure. Scientists can also modify DNA sequences and insert point mutations in specific regions.

Semi-conservative replication describes the generation of a new DNA strand by copying one of the old strands that are part of the double helix.

The basal mutation rate refers to the speed at which mutations occur in DNA without other accelerating factors, such as exposure to certain chemical agents.

Mutagens are agents that induce DNA mutations. They can be physical, chemical or biological.

Physical mutagens

Physical mutagens include ionising radiation, such as X-rays, and non-ionising radiation, such as ultraviolet (UV) light. These can alter the structure of nucleotides and break DNA strands which increases the mutation rate.

X-rays can produce hydroxyl radicals as a result of ionisation. These hydroxyl radicals can disrupt the bonds in the sugar-phosphate backbone of DNA and cause strand breaks.

Meanwhile, UV photons can be absorbed by nucleotides to form irregular structures called dimers. These dimers cause incorrect base pairing during DNA replication, causing mutations.

Chemical mutagens

Chemical mutagens include alkylating agents and base analogues.

Alkylating agents insert methyl or ethyl groups to nucleotides, and this causes errors in base-pairing, which changes the nucleotide structure. Base analogues resemble nucleotides, and these are incorporated into DNA which again induces base-pairing errors.

Alkylating agents include diethyl sulphate and nitrogen mustards. Base analogues include 5-bromodeoxyuridine (BU) and 2-aminopurine.

Biological mutagens

Viruses can be biological mutagens, as these can cause changes in our DNA by inserting their own genomes into their host's DNA. The human immunodeficiency virus (HIV) is one example.

Gene mutation examples

Now that we have a broader understanding of what gene mutations are, their effects and the causes, let’s take a real-life look at individuals who harbour mutations. We will explore the following examples but don’t worry; you won’t need to know all these details for your exam. It’s just a great way to apply your knowledge.

Tumour suppressor gene mutations

Tumour suppressor genes encode transcription factors that regulate the cell cycle and DNA replication by preventing uncontrolled divisions and blocking mutated cells from dividing. These are negative regulators as they stop the cell cycle from being over-activated.

Transcription factors are polypeptide products that regulate the expression of genes during transcription by binding to DNA. You can learn more about transcription factors in our article Protein Synthesis.

For example, the tumour suppressor gene, p53, monitors DNA and detects mutations under normal conditions. If a DNA mutation is detected in the cell, p53 will induce cell cycle arrest whereby the DNA is repaired. If the DNA damage is too extensive, p53 will induce apoptosis. Therefore, p53 is a master regulator that prevents mutated DNA from replicating.

Apoptosis: programmed cell death.

The p53 gene undergoes frequent missense mutations, resulting in the loss of the transcription factor’s ability to bind to DNA. Consequently, this promotes tumour development, and cells have a greater chance of becoming cancerous.

MTHFR gene mutation

The MTHFR gene encodes the methylenetetrahydrofolate reductase enzyme, which is involved in folate metabolism. Folate metabolism is important as it synthesises nucleotides and amino acids. This enzyme is needed in protein synthesis and DNA methylation, important cellular processes.

DNA methylation is a process involved in gene expression regulation. It is an epigenetic mechanism whereby the genes are transcriptionally silent, meaning the protein the gene codes for is not synthesised.

One of the most common gene mutations occurs when a C is substituted with T, called the C677T mutation. This mutation increases an individual’s susceptibility to cardiovascular and hormonal conditions, such as blood clots and hypothyroidism. This is an example of how mutations can induce health risks to an individual.

Hypothyroidism describes a condition in which your thyroids are under-active. This means not enough hormones are synthesised. These hormones include thyroxine and triiodothyronine (T3).

Gene Mutations - Key takeaways

  • Gene mutations describe changes in the DNA base sequence. These can be substitution, deletion or insertion events.

  • Missense mutations describe the incorporation of incorrect amino acids.

  • Nonsense mutations occur when a premature stop codon is coded for.

  • Silent mutations describe neutral changes to the base sequence.

  • Frameshift mutations describe the dramatic changes in successive triplets following the mutation.

  • The rate of mutation events can increase due to physical, chemical or biological mutagenic exposure.

Gene Mutations

Gene mutations are caused by spontaneous errors during DNA replication. This can be induced by physical, chemical or biological mutagens. Physical mutagens include ionising radiation, chemical mutagens include base analogues and biological mutagens include viruses

These are three main types of mutations: substitutions, deletions and insertions. Substitutions occur when the original amino acid is replaced by an incorrect one, deletions occur when a nucleotide is lost from the base sequence and insertions occur when an extra nucleotide is added to the base sequence.

The MTHFR gene is important in folate metabolism. Mutations in this gene are closely associated with cardiovascular and hormonal problems, such as blood clots and hypothyroidism.

Gene mutations describe changes to a DNA base sequence. These occur randomly during DNA replication. Genes are read as triplets and triplets code for amino acids. This means changes to the DNA base sequence can alter the amino acid sequence of the corresponding polypeptide. This can induce beneficial, neutral or detrimental effects on the organism. 

Tumour suppressor genes regulate the cell cycle and DNA replication. These prevent uncontrollable divisions from happening. For example, the p53 gene detects DNA damage and induces cell cycle arrest or apoptosis to prevent mutated cells from replicating. If the p53 gene is mutated, this results in the loss of DNA damage detection, potentially enabling mutated cells to replicate and divide uncontrollably. This could lead to tumour development and cells growing cancerous. 

Final Gene Mutations Quiz

Question

What are gene mutations?

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Answer

Gene mutations describe changes to the DNA base sequence. 

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Question

What are triplets?

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Answer

Triplets correspond to three nucleotides. These code for one amino acid. 

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Question

What are the three types of gene mutations?


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Answer

Substitution, insertion and deletion. 

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Question

What is the term given to a change in one single nucleotide?


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Answer

Point mutations.

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Question

Describe silent mutations.


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Answer

Silent mutations do not change the protein being coded due to the degeneracy of the genetic code. This means, when a substitution event occurs, the new triplet codes for the same amino acid. 

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Question

What are nonsense mutations?


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Answer

Nonsense mutations occur when a codon becomes a premature stop codon. This results in a shortened polypeptide product that is non-functional. This causes detrimental effects to the organism. 

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Question

Describe missense mutations. 


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Answer

Missense mutations occur when an incorrect amino acid is incorporated. This can cause beneficial, silent or detrimental effects to the organism.

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Question

Identify an example of a beneficial mutation. 


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Answer

Antibiotic resistance in bacteria. This is beneficial for the bacterial strain. 

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Question

What are frameshift mutations?


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Answer

Frameshift mutations occur when every successive triplet code after a deletion or insertion event changes. This is largely detrimental as all amino acids being coded for following the mutation are dramatically different. 

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Question

Identify the enzyme which drives DNA replication. 


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Answer

DNA polymerase. 

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Question

Identify a physical mutagen and its effect on DNA. 


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Answer

Ionising radiation (e.g X-rays) can cause DNA strand breaks. 


Non-ionising radiation (e.g UV light) can alter the structure of nucleotides. 

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Question

Identify a chemical mutagen and its effect on DNA. 


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Answer

Identify a chemical mutagen and its effect on DNA. 

Alkylating agents insert alkyl groups to nucleotides. This induces errors in base-pairing. 


Base analogues resemble nucleotides and are incorporated into DNA. This also induces base-pairing errors. 

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Question

Why are viruses described as biological mutagens?


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Answer

Viruses have the ability to insert their viral DNA into the host genome. This effectively changes the DNA base sequence of the host. 

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Question

What is the function of tumour suppressor genes?


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Answer

Tumour suppressor genes regulate the cell cycle and DNA replication to prevent uncontrolled divisions and tumour growth. 

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Question

Identify some health risks associated with mutations in the MTHFR gene.


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Answer

Blood clots and hypothyroidism. 

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