Question

Give two differences between spontaneous and induced mutation.

Answer 1

Hint: Mutations are an essential tool in the study of genetics. They represent heritable changes in the DNA structure and can be either induced or spontaneous.

Complete answer:
Two major differences between spontaneous and induced mutation are:

Factor	Spontaneous mutation	Induced mutation
Definition	Spontaneous mutations are the type of heritable changes in the structure of DNA, occurring due to natural factors.	As the name suggests, it is a mutation that is triggered with a physical or chemical agent also called as Mutagen and changes the sequence of genetic material of a living organism.
Causes	The three sources of spontaneous mutations are the errors in DNA replication, spontaneous lesions like depurination and deamination, and transposable genetic elements.	Induced mutations occur due to the incorporation of base analogs, base mispairing, and base damage produced due to mutagens.
Example	Sickle cell anemia- It is caused due to the substitution of the amino acid glutamic acid with valine at the 6th position of the β-chain of hemoglobin.	5-bromouracil- It is a base analog (structurally similar) of thymine. So, when it gets incorporated into the DNA structure, the DNA polymerase cannot differentiate it leading to the formation of an incorrect sequence of DNA.

Additional Information:
1. Base analogs are the chemical compounds with similar structural properties to the nitrogenous bases in the DNA. They can incorporate into DNA, resulting in mutations. For example, 5-bromouracil (5-BU)- a base analog of thymine. 
2. Some mutagens change the sequence of nitrogenous bases in DNA, resulting in mispairing. Example- Alkylating agents like ethyl methanesulfonate, etc.
3. Base damage is caused by radiation, oxidizing, or reducing agents. They alter the base sequence leading to mutation.
4. Depurination- It refers to the loss of purine bases from the DNA fragment. In deamination, cytosine converts into uracil, which then pairs with adenine during replication. Therefore, this also causes GC to AT transition. 
5. Transposable genetic elements can move from one to another position of the same or a different chromosome thus producing unwanted chromosomal rearrangements.  

Notes:
1. Some examples of chemical mutagens include hydroxylamine, base analogs, alkylating agents, DNA adducts, intercalating agents, DNA crosslinkers, oxidative damage, nitrous acid, etc. In contrast, the two types of physical mutagens are UV radiation and ionizing radiation. 
2. Both induced and spontaneous mutations are heritable.