Q.How does a mutagen induce mutation? Explain with example.

A mutagen is any physical or chemical agent that permanently alters the DNA sequence, causing a mutation. It works by directly damaging the DNA bases or by interfering with the replication or repair machinery, leading to errors that become fixed as permanent changes.

• Physical mutagens like UV radiation cause adjacent thymine bases to fuse together, forming thymine dimers. This distorts the DNA helix and, if not repaired correctly, leads to a mutation during replication. …

To understand how a mutagen works, you first need to see mutation as a change in the nucleotide sequence of DNA. This change can happen spontaneously — for example, due to a mistake during DNA replication — but many mutations are induced by external agents called mutagens. These mutagens are broadly of three types: physical mutagens (like radiation), chemical mutagens (like certain compounds), and biological mutagens (like viruses). The NCERT textbook focuses on the first two.

A physical mutagen, such as ultraviolet (UV) radiation, does not directly break the DNA backbone. Instead, UV light causes adjacent thymine bases on the same DNA strand to form covalent bonds with each other, creating a thymine dimer. This dimer distorts the DNA helix, and when the replication machinery encounters it, it may insert incorrect bases opposite the dimer, leading to a mutation. For example, a thymine-thymine dimer can cause a transition mutation where a cytosine is inserted opposite one of the damaged thymines, changing the base pair.

Chemical mutagens work by directly reacting with DNA bases. One classic example from the NCERT syllabus is nitrous acid (HNO₂). This compound deaminates adenine, converting it into hypoxanthine. Hypoxanthine pairs with cytosine instead of thymine. So, if the original DNA had an A-T base pair, after treatment with nitrous acid, the adenine becomes hypoxanthine, and during replication, cytosine pairs with it. In the next round, that cytosine directs the insertion of guanine, permanently changing the A-T pair to a G-C pair. This is a specific type of point mutation called a transition.

Classify mutagens by their molecular action first — base-modifying (nitrous acid), base-mimicking (5-bromouracil), or structure-distorting (UV) — then trace how each class produces a mispairing event …