In the last post, we mentioned the different types of mutation:
1. Based on the changes in sequence (insertion, deletion, etc),
2. Based on the ability to express (dominant or recessive) and
3. Based on the cause of mutation (spontaneous or induced).
Let’s see the more other types of the mutations in this post:
4. Based on the origin of the mutation:
Gene mutations can be classified in two major ways based on the origin of the gene in an individual, i.e. the way a person receives the mutated gene; they are either Hereditary or Acquired.
• Hereditary mutation:
These are inherited from either or both of the parents. These mutation are given off to the individual through the parent’s germ cells i.e. egg and/or sperms. Hence also known as germline mutation. As the mutation is present in the germ cells, the mutation is passed on to every cell arising from the union of such germ cells.
(Just for info: Read this article on ‘Hereditary mutations in cancer: the use of panels and genetic counselling‘)
• Acquired mutations
These mutations are acquired by the person during his lifetime and is not inherited from the parents. It usually arises in a single cell and is passed on to all the cells arising from it after division. Hence this mutation is not present in all the cells in an individual, but only a few set of cells.
These changes can be induced by the environmental factors such as ultraviolet radiation or can arise spontaneously due to error DNA replication or repair machineries. Acquired mutations occurring in the somatic cells are not passed on to the next generation. However, in case of germ line cells of the individual getting affected by the mutation, the mutation may be passed on to the next generation.
(Just for info: Read a paper on how an acquired mutation of the tyrosine kinase JAK2 causes myeloproliferative disorders in humans.)
5. Based on the effect on protein structure/ function:
The mutations are the changes in the DNA bases. As we all know the sequence of the DNA bases, i.e. the exons, determines the sequence of amino acids. Hence any change in the sequence of the exons, will have an effect on the sequence of the amino acids in the protein, which in turn, will effect the structure and ultimately the function of the given protein.
Depending on the effect of the mutation on the function of the protein, the mutation can be of different types:
a. Missense mutation:
Missense mutation is a type of point mutation, in which the change in one DNA base pair (and hence the codon) results in substitution of the original amino acid with another amino acid in the given protein. Hence there is a change in the structure and function of the protein.
For e.g. as in the fig 3, if there is a mutation at a region, causing the ‘T’ in the codon AAT (which codes for leu) to be substituted by A, giving AAA (coding for phe), there will be replacement of the amino acid ‘leu‘ by the amino acid ‘phe‘ in the protein structure. This will be a missense mutation, wherein an amino acid is being replaced by another one.
The missense mutation may cause loss of function or incorrect folding of the protein. The change in the structure may or may not effect the function of the protein. Sometimes the change in structure may also be beneficial to the functioning of the protein and thus help the evolutionary process. However, missense mutation may also cause a number of congenital disorders, e.g. sickle cell anemia.
(Just for info: Read the paper titled ‘Predicting the Impact of Missense Mutations on Protein–Protein Binding Affinity.’)
b. Nonsense mutation:
A nonsense mutation is also a type of point mutation. However, in this type of mutation the change in the base pair causes a codon coding for an amino acid, to be converted to a stop codon.
For e.g. if the middle ‘A’ in the codon AAT coding for leucine, is substituted by T, giving ATT, the resulting codon is a stop codon.
This causes the protein translation to be stopped prematurely and results in a truncated or partial protein which may function improperly.
Duchenne muscular dystrophy (Medlineplus page), cystic fibrosis (NIH-GHR page), spinal muscular atrophy (NIH-GHR page) and Cancer are the few examples of disorders caused due to nonsense mutation.
c. Silent mutation:
A silent mutation is a point mutation wherein there is a change in bases pairs in the DNA sequence, but no change in the sequence of the amino acids. That is due to redundancy of the codons. The new mutated codon codes for the either the same amino acid or an amino acid with similar properties as the original one.
If the same amino acid is incorporated into the protein, the mutation is not visible at the phenotypic level. If the amino acid with similar property is incorporated the function of the protein may or even may not change slightly. Hence the this type of mutation is silent at the phenotypic level.
For e.g. in the fig 3, if the ‘T’ within the codon AAT (coding for leu) is substituted by ‘C’ to give AAC (coding for leu, too), the amino acid leu will be incorporated despite the mutation. Hence the structure or the function of the protein will not be changed and the mutation is known as silent mutation.
(Just for info: Read this interesting article ‘The Sound of a Silent Mutation‘.)
d. Frameshift mutation:
Frameshift mutation is a type of mutation in which an insertion, duplication or deletion of one or more base pairs results in the shift in the reading frame of the codons.
Each codon is made up of three DNA (or RNA) bases, each coding for a particular amino acid (or stop codons). As can be understood from the fig 4, an insertion will result in a change in grouping or framing of the codons ahead of the mutation.
As a result different amino acids are inserted upstream of the mutation. These mutations can occur only in coding regions or the exons. The mutations occurring in the non-coding regions or introns can not cause frameshift mutations as they do not effect any amino acid sequence.
As a result of this mutation the protein has a completely different structure and function which may cause various disorders. Tay Sachs disease is an example of the disease caused due to frameshift mutation.
(Just for info: Read this paper to know about a frameshift mutation causing Tay Sachs disease.)
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Read other posts by The Biotech Notes:
Lodish et al. (2000) Mutations: Types and Causes. Molecular Cell Biology. 4th edition. New York: W. H. Freeman.
Fay and Spencer (2005) Dominant mutations. Copyright © 2005, WormBook Research Community.
Ziad & Stypczynska (2013) Clustering algorithms in radiobiology and DNA damage quantification. Data Security, Data Mining and Data Management: Technologies and Challenges, Nova Science Pub Inc.