Introduction of Mutation

DNA is the inherited genetic material known for imparting genotypic and phenotypic characteristics. Nucleotides are DNA building blocks consisting of a five-carbon sugar, nitrogenous base, and one phosphate. DNA is a genetic macromolecular or polymer of small nucleotide monomeric units which are joined together by a phosphodiester bond. The DNA carries the genetic instructions that are responsible for imparting certain characteristics, and the production of proteins necessary for the growth, development, and functioning of a living organism. However, DNA mutations are sudden or random changes in the genetic sequence causing phenotypic variations, genetic changes, or even causes leading to genetic disease or syndrome. The topic is all about DNA mutations, and how it impacts or affects an organism. 

Understand the DNA building block

The nucleotides are DNA building blocks consisting of majorly three components:

• A ribose sugar
• Phosphate molecule 
• Nitrogenous bases – The nucleotide bases consist of Adenine (A), Guanine (G), Thymine (T), and Cytosine (C).

Adenine and guanine make the purine, whereas cytosine and thymine make the pyrimidine. The sugar and phosphate make the backbone for the double helix DNA. Adenine forms a double bond with thymine, whereas guanine forms a triple bond with cytosine in the DNA double helix. 

What are DNA Mutations?

The DNA mutation is a change in the genetic sequence. There are some mutations which are lethal and cause genetic disease, whereas some are silent and do not cause an impact. Most of the mutations occur during DNA replications if the errors are not corrected during the replication process. However, mutations in the DNA building block can also occur as a result of exposure to various environmental factors like sunlight, radiation, smoking, etc. 

The genetic mutations lead to permanent alteration in the genetic sequence. The mutations affect either a large chromosomal segment, or alteration of a single nucleotide base. The DNA mutations are classified into two major types:

• Hereditary mutations – These types of mutations are passed on from the parents to the offspring. Such types of mutations are also known as germline mutations, as they are present in the parent’s egg or sperm cell. The mutation exists in every cell of the offspring when passed on to the offspring. 
• Acquired mutations – These are only known as somatic mutations, but are present only in certain cells. These are not inherited, but caused due to exposure to environmental conditions like UV radiation, smoking, etc. It is further categorised into induced and spontaneous mutations. 
• Induced mutations in the DNA building blocks are due to exposure to UV rays, chemicals, X-rays, or any environmental agent. Spontaneous mutations occur in the body due to natural reactions without environmental agent exposure.

Basic Types of DNA Mutations 

• Frameshift Mutations – It is important to understand the concept of the codon. It is a sequence of three nitrogenous bases which represent an amino acid. A frameshift mutation is caused by either insertion, deletion, or duplication of DNA building block nucleotides in the sequence. It leads to a shifting of the genetic frame.
• Point mutations – When there is a change in the DNA building block, i.e., a single nucleotide change in DNA, it is known as a point mutation. It is further categorised into silent, missense, nonsense, and nonstop mutation. 

1. Silent mutations are when the replacement of a nucleotide base does not change the amino acid in the genetic sequence. 
2. Missense mutation leads to the formation of entirely different amino acids.
3. A nonsense mutation is when the substitution of a single DNA building block leads to the formation of a stop codon. 
4. The nonstop mutation is when the otherwise stop codon is changed into an amino acid codon. 

• Transition mutations are those that substitute purines for purines and pyrimidine for pyrimidine. Transversion mutations substitute a purine for pyrimidine, and vice versa. 
• Large scale mutations are those which can lead to chromosomal rearrangement or aberrations. 

Mutations that do not involve a single DNA building block

Structural mutations are changes in the structure of chromosomes, which can be categorised as inversion, translocation, duplication, and deletion. 

• Chromosomal inversions are those in which a chromosome segment is broken, and the broken segments rotate by 180° to rejoin. 
• Chromosomal translocation is the interchange of chromosome segments between non-homologous chromosomes. 
• Chromosome deletion is when a DNA segment is lost from one arm of chromosomes. Also, two segments on either side of deleted parts join together. 
• Chromosomal duplication is the duplication of the DNA segment within an arm of a chromosome, leading to the segment getting duplicated or copied. 

Numerical mutations are a common type of mutation in the genetic sequence. In such types of mutations, it leads to a change in the number of chromosomes, leading to aberrations or genetic change. 

Conclusion 

The DNA building block nucleotide is connected by a phosphodiester bond leading to the formation of a genetic sequence or DNA. The four types of nucleotides present in the DNA are adenine, guanosine, cytosine, and thymine. However, there is a spontaneous or sudden change in the DNA sequence, known as a mutation, that can change characteristics or even might cause lethal changes. The mutations vary from single nucleotide base changes, known as point mutations, to chromosomal rearrangement or aberrations. There are changes in the sequence of DNA or point mutation that do not cause any effect. However, there are mutations which can lead to several genetic diseases. Hence, there are some mutations which can lead to the activation of oncogenes and might lead to the formation of cancer.