A mutation is a change that occurs in a DNA sequence, either due to mistakes when the DNA is copied or as the result of environmental factors; it can occur during DNA replication if errors are made and not corrected in time.

Mutations contribute to genetic variation within species, and can also be inherited, particularly if they have a positive effect. However, a mutation can also disrupt regular gene activity and cause diseases, like cancer. Cancer is the most common human genetic disease; it is caused by mutations occurring in several growth-controlling genes. Sometimes faulty, cancer-causing genes can exist from birth, increasing a person’s chance of getting cancer.

Mutation. An illustration of a DNA mutation.

Errors during DNA replication are not the only reason why mutations arise in DNA. Mutations, variations in the nucleotide sequence of a genome, can also occur because of damage to DNA. Such mutations may be of nine types:

• Random mutation occurs when there are accidental changes in the DNA sequence that are due to radiation, chemicals, replication error, etc.
• Translation and transcription errors cause the expression of a mutant phenotype.
• Base substitution is a mutation involving a DNA base (ATGC) changing to a different base.
• Inversion is a type of mutation where a stretch of DNA (a segment of a chromosome) breaks off, then reattaches in the opposite orientation.
• Addition, happens when an extra DNA base is added/inserted into the DNA sequence.
• Deletion is a type of mutation when a DNA base is taken out of the DNA sequence.
• Translocation mutation occurs when a stretch of DNA (a segment of a chromosome) breaks off, then reattaches somewhere else.
• Mispairing is a type of mutation that mispairs the DNA such that A does not pair with T or G with C.

Mutations can also be classified as harmful or beneficial. A harmful or deleterious mutation decreases the fitness of the organism. One example is a mutation that causes an organism to be sterile. Common mutations lead to changes in the triplet base pair code which means the codons of the mRNA do not match up to the anticodons of the tRNA this results in the incorrect translation and incorrect amino acid structure. Often this will result in misfolded proteins and inactive enzymes and hormones.

A favourable or advantageous mutation increases the fitness of the organism. For instance, the mutation that causes flies to become wingless is beneficial in a very windy environment.

Mutations are often caused by chemicals, radiation or dangerous substances. These are known as mutagens, which can be defined as an agent which causes mutations in the DNA of the cell. Carcinogens are those agents that lead to cancer, i.e. converts a normal cell to cancerous cell. Mutagens are often also carcinogens, agents that cause cancer. However, whereas nearly all carcinogens are mutagenic, not all mutagens are necessarily carcinogens.

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Key Points

• Mutation can occur during DNA replication if errors are made and not corrected in time.

• Most mistakes are corrected, but if they are not, they may result in a mutation defined as a permanent change in the DNA sequence.

• Mutations can be of many types, such as random mutation, translation error, transcription error, base substitution, inversion, addition, deletion, addition/insertion, deletion, translocation, and mispairing.

• Mutations can be classified as deleterious or advantages which decreases or increases the fitness of the organism, respectively.

• Inborn errors of metabolism are rare genetic (inherited) disorders in which the body cannot properly turn food into energy.

• Mutations often lead to the misfolding of proteins as the amino acid sequence is different.

• Almost all carcinogens are mutagens, but not all mutagens are carcinogens.

Key Terms

DNA base: a unit of the DNA; AGTC adenine (A), guanine (G), thymine (T), and cytosine (C)

Mutagens:  anything that causes a mutation

Carcinogens: any substance or radiation that promotes the formation of cancer

Species: a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding.

Mutation: the changing of the structure of a gene, resulting in a variant form that may be transmitted to subsequent generations, caused by the alteration of DNA

Deleterious mutation: decreases the fitness of the organism

Advantageous mutation: increases the fitness of the organism.

Gene: A section of DNA that codes for a characteristic

DNA replication: is the process by which DNA makes a copy of itself during cell division

Cancer: the uncontrolled growth of abnormal cells