Point Mutation in Genetics and DNA Sequence Change

The alteration that occurs to the nucleotide sequence present in the genome of a virus or an organism or extrachromosomal DNA  is called a mutation. There are chances that mutation can either produce detectable changes that are observable in an organism or it cannot produce it. They can either prevent the genes from functioning properly or can have no effect or it can alter the product of the gene. It involves the duplication of DNA in large sections. 

There are different types of mutations that occur in an organism; they are chromosomal mutation and point mutation. If the mutation occurs as a result of crossing over in the meiosis is called a chromosomal mutation. When there is an alteration in the single base pair is known as a point mutation. 

Point mutation, also known as substitution, is a type of genetic mutation where the nucleotide base is inserted, deleted, or changed in the DNA or RNA of the genome of an organism. These have a variety of effects on the products, where the consequences are predictable with the specific mutation. 

In regard to the synthesis of protein, its function, and its composition the range of these consequences can be determined from no effect to deleterious effects. Point mutation examples include sickle-cell anaemia and cystic fibrosis. 

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Types of Point Mutations

In the case of point mutations there are two different types of mutations these are further divided depending on the form they mutate

1. Substitution Mutations: If the mutation occurs by the substitution of a nucleotide in the genome of an organism then it is known as a substitution mutation. It is further subdivided into three types:

• Silent mutation

• Mis-sense mutation

• Nonsense mutation

In the case of silent mutation, a nucleotide can be substituted that results in the formation of the same amino acid, and this situation can make the multiple codons code for the same amino acid. The effect on the protein will be less, for example, codons AAA and AAG codes for lysine, and in the case instead of ‘G’ if ‘A’ is produced then the same amino acid is formed thus the effect on the protein is not found.

In the case of non-sense mutation, the nucleotide is substituted resulting in the formation of a stop codon instead of the formation of the codon that codes for the amino acid. These stop codons are certain sequences of the base chain that have the capability to stop amino acid chain production. At the end of the mRNA sequence in the production of the protein, it is always found and when the substitution occurs it will terminate the sequence of amino acids and prevent the formation of the correct protein.

The missense mutation occurs when the nucleotide is substituted which results in the formation of the different codon. It is the same as that of the non-sense mutation but in this case, the difference is the newly produced codon is not a stop codon but it is a different amino acid in the sequence. For example, if AAG is substituted as AGG then this codon relates to arginine instead of lysine. This type of mutation is said to be conservative if the amino acid that has to be formed instead of that of the amino acid that is formed from the missense mutation shares similar properties. The mutation is said to be non-conservative if different properties are found in the amino acid that has to be formed instead of that of the amino acid that is formed from the missense mutation. 

2. Insertion or Deletion Mutations: When an extra-base pair is added to the sequence of the amino acid then the insertion mutation occurs. If an extra-base pair is removed from the sequence of the amino acid then it is said to be a deletion mutation. These types of mutation are grouped together since they can affect the sequence of the amino acid drastically. 

When one or two bases are deleted or added the change in all three base codons occurs that results in the mutation, it is also known as frameshift mutation. Suppose the sequence in the DNA is CCT ATG TTT if ‘A’ is added in between the cytosine and the change in the sequence will be as CAC TAT GTT T this changes the structure and functioning of the protein formed and sometimes can make this protein useless. The same effect can be found if a base is deleted.

Consequences of Point Mutation

In the non-coding sequences, most of the time the point mutation occurs without any consequences. If the mutated base pair is present in the promoter sequence then the gene expression will vary. If the splicing site of an intron the point mutation is involved then it interferes with the splicing site of the transcribed mRNA in the correct form.

By altering one amino acid the entire peptide chain will change this, in turn, changes the entire protein. Thus the newly formed protein is called a protein variant. If this original protein is involved in the functioning of the cellular reproduction then the single point mutation involves the change in the entire process of cellular reproduction. 

The point germline mutations can be beneficial as well as can cause diseases. Depending on the environment where the organism lives the adaptations can happen. The scientific theory of evolution is completely based on the point mutation that happens in the cells. This theory explains the history and diversity of the organisms present on the Earth. The beneficial mutations can help the organism to reproduce where the positively affected genes are passed to the next generation. The harmful mutations can make the organism reduce the process of reproduction or it can make the organism die this happens through a phenomenon called natural selection.

In mutations, the long-term and short-term effects can arise. Where the long-term effects are permanent by changing the chromosome that leads to mutation, short-term effects are involved in the halting of the cell cycle at different stages. For example, a codon that codes for glycine is changed to form a stop codon makes the protein stop the tasks that are to be performed. Mutations can affect the DNA and prohibit the process of mitosis due to the absence of the complete chromosome. An example of the long-term effects is cancer.

The other effects of the point mutation involve the location where the mutation happens in the gene. If the mutation occurs in the gene that is responsible for the coding then the amino acid sequence of a protein can be altered. This alteration leads to protein localization, changes in the function, or protein complex. Many of the methods have been proposed in the determination of the effects of missense mutations. While these methods provide only the binary classification of the effects of the mutations if they are benign or damaging, another level is required to provide the explanation of why and how the mutations are capable of damaging the proteins.

If the mutation occurs in the region where the proteins are bound with the transcriptional machinery then the mutation can affect the factors of binding. Thus the rate of efficiency of the gene transcription can be affected. This in turn alters the levels of mRNA and proteins. The transcription mechanism of binding to a protein is through the recognition of the short nucleotide sequence. Depending on the region of the amino acid sequence of the protein the point mutation can affect the behaviour and the reproduction of the protein in several ways. If the mutation occurs in the region where the gene is responsible for the coding of the protein then the alteration in the amino acid can be found. This change can affect the protein activation that is how the protein is bound to the enzyme or the change in the function. 

Diseases Caused by Point Mutations

1. Cystic Fibrosis: It is most commonly found in people of European descent, it is an inherited recessive disorder. There are many types of mutations that can cause CF, but the common one is the deletion of the three nucleotide bases in the CFTR gene that is abbreviated as cystic fibrosis transmembrane conductance regulator gene. This results in the loss of the phenylalanine amino acid and makes the protein folding incorrect. The symptoms are thick sticky mucus found in the lungs. Salty sweat, trouble while breathing, shortened life expectancy, and in some individuals it can cause infertility.

2. Sickle-Cell Anaemia: The single substitution in the gene of the haemoglobin that carries the oxygen in the blood causes sickle-cell anaemia. It is a recessive disorder. The valine is produced instead of the glutamic acid in the chain by the substitution. When there is the presence of two copies in the people this leads to the change of the blood cells from disc-shaped to sickle-shaped that lacks the supply of oxygen to the blood. Almost 80 percent of the people with this disease can protect against malaria. The symptoms are chest pain, obstruction of the blood vessels, and anaemia. 

3. Tay-Sachs: It is another recessive disorder caused due to point mutation where the effects are found on the HEXA gene of chromosome 15. It can cause the nerve cells to deteriorate which results in the decline of the mental and physical functioning of the body. 

Conclusion

The point mutations can be beneficial as well they can cause harmful effects. It is depending on the environment it is adapted to. The point mutations are sometimes caused by the replication of DNA. The rate of these mutations can increase when these are exposed to the mutagens such as extreme heat, X-rays, UV rays, or due to some of the chemicals such as benzene.