Subtopic 3.6: Mutation
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Mutations are changes in the genetic sequence (DNA or RNA sequence) , and they are a main cause of diversity among organisms. Although some of mutations are beneficial, offering resistance to disease or improved structure and/or function, some other specific mutations can lead to disease and/or death of the cell or organism. Mutations can occur due to assaults from the environment or spontaneous mutation may occur during the DNA replication. Mutations are estimated to occur at an approximate rate of 1000–1,000,000 per cell per day in the human genome, and every new cell is believed to contain approximately 120 new mutations. |
Figure 8. Human diversity
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Types of mutation
Point mutations when only a single base pair is changed into another base pair. They can be classified as:
Insertion and deletion mutations, which are together known as indels. Indels can have a wide variety of lengths. At the short end of the spectrum, indels of one or two base pairs within coding sequences have the greatest effect, because they will inevitably cause a frameshif, i.e. change the entire reading of the mRNA sequence. At the intermediate level, indels can affect parts of a gene or whole groups of genes. At the largest level, whole chromosomes or even whole copies of the genome can be affected by insertions or deletions. At this high level, it is also possible to invert or translocate entire sections of a chromosome, and chromosomes can even fuse or break apart.
If a large number of genes are lost as a result of one of these processes, then the consequences are usually very harmful.
Point mutations when only a single base pair is changed into another base pair. They can be classified as:
- Transition when a purine nucleotide is changed to a different purine (A ↔ G) or a pyrimidine nucleotide is changed to a different pyrimidine nucleotide [C ↔ T(U)].
- Transversion when the orientation of a single purine and pyrimidine nucleotide is reversed [A/G ↔ C/T(U)].
- Silent when the same AA is coded.
- Missense when a different AA is coded.
- Neutral when an AA change occurs but does not affect the protein's structure or function.
- Nonsense when a stop codon results, terminating translation and shortening the resulting protein.
Insertion and deletion mutations, which are together known as indels. Indels can have a wide variety of lengths. At the short end of the spectrum, indels of one or two base pairs within coding sequences have the greatest effect, because they will inevitably cause a frameshif, i.e. change the entire reading of the mRNA sequence. At the intermediate level, indels can affect parts of a gene or whole groups of genes. At the largest level, whole chromosomes or even whole copies of the genome can be affected by insertions or deletions. At this high level, it is also possible to invert or translocate entire sections of a chromosome, and chromosomes can even fuse or break apart.
If a large number of genes are lost as a result of one of these processes, then the consequences are usually very harmful.
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