Point Mutation

A point mutation occurs when one single nucleotide base is added, removed, or modified in DNA or RNA, the genetic material of the organism. Nucleotides are the building blocks of DNA and RNA. Cytosine, guanine, adenine, thymine (in DNA), and uracil (in RNA), abbreviated C, G, A, T, and U, are five distinct compounds that can make up nitrogenous bases on nucleotides. All of the information required to carry out all cell operations is encoded in a precise sequence of nucleotides. A mutation occurs when a gene is altered due to a change in DNA structure; this can include entire chromosome portions. Although numerous point mutations can occur in one strand of DNA or RNA, a point mutation occurs when only one nucleotide base is altered in some way.

From DNA to Protein

DNA and RNA have a double helix structure. The backbone is made up of phosphate groups and 5-carbon sugars, whereas the midsection of the double helix is made up of pairs of nitrogenous bases. Each nitrogenous base is paired with a different base. In DNA and RNA, cytosine pairs with guanine, while adenine pairs with thymine and uracil, respectively. DNA must be transcribed by messenger RNA to produce proteins (mRNA). 

The mRNA “reads” the DNA three bases at a time, matching them to its complementary bases. Codons are three-base groupings that each code for a distinct amino acid. Proteins are made up of chains of amino acids. As a result, DNA must have the precise base pair sequence to build proteins correctly. A single point mutation may have little effect or may change the protein produced, rendering it worthless.

Mutations that occur spontaneously during DNA replication can sometimes produce point mutations. When a cell is exposed to mutagens, which are environmental stimuli that can alter an organism’s DNA, the rate of mutations may also increase. X-rays, UV radiation, intense heat, and some compounds like benzene are all mutagens. 

Point Mutations and Their Types

1. Substitution

A replacement mutation occurs when one base pair is replaced by another. If a cytosine-containing nucleotide is accidentally exchanged with a guanine-containing nucleotide, this can happen. Substitution mutations are classified into three types:

• Nonsense

• Missense

• Silent

A nonsense mutation arises when one nucleotide is replaced, resulting in the development of a stop codon rather than an amino acid-coding codon. A stop codon is a sequence of nucleotides (TAG, TAA, or TGA in DNA, and UAG, UAA, or UGA in RNA) that causes the amino acid chain to stop being produced. When a protein is made, it is always found near the end of the mRNA sequence, but if a substitution causes it to appear anywhere else, it will prematurely end the amino acid sequence and prevent the right protein from being made.

A missense mutation occurs when one nucleotide is substituted and a new codon is created, similar to a nonsense mutation; however, the new codon is not a stop codon. Instead, the codon generates a different amino acid in the amino acid sequence. If a missense substitution changes a codon from AAG to AGG, instead of lysine, the amino acid arginine is generated. If the amino acid generated as a result of a missense mutation has similar qualities to the one that was meant to be formed instead, it is considered conservative. Non-conservative amino acids have features that differ from the structure and function of a protein.

A nucleotide is substituted in a silent mutation, but an identical amino acid is generated. As numerous codons might code for the same amino acid, this can happen. For example, AAG and AAA both code for lysine, therefore changing the G to an A will produce the identical amino acid and have no effect on the protein. It depicts the DNA codon, the RNA codon that results, and the amino acid that is created.

2. Deletion and insertion

When an extra-base pair is introduced to a base sequence, it is called an insertion mutation. When a base pair is deleted from a sequence, it is called a deletion mutation. These two forms of point mutations are grouped because they both have a significant impact on the amino acid sequence produced. All three-base codons change when one or two nucleotides are added or removed. A frameshift mutation is a name for this type of mutation. If an extra A is placed between the two cytosine bases in a sequence of codons in DNA that is typically CCT ATG TTT, the sequence becomes CAC TAT GTT T. This fundamentally alters the amino acids synthesised, altering the structure and function of the final protein and perhaps rendering it unusable. Similarly, deleting one base would cause the sequence to change.

Conclusion

We conclude that when it comes to protein, a point mutation can have one of three effects: (1) a change to a different amino acid, known as a missense mutation; (2) a change to a termination codon, known as a nonsense mutation; or (3) the formation of a new sequence that is silent in terms of protein sequence but changes some feature of it, known as a nonsense mutation.