Which of the following statements are incorrect with respect to mutations?
i. During the S phase of cell cycle, chances of mutation are higher.
ii. In spontaneous mutations, presence of mutagens is required.
iii. Cigarette compounds belong to the class of physical mutagens.
iv. UV rays can cause dimerisation in the DNA.

1. iii, iv
2. ii, iii
3. i, ii, iii
4. i, iii

The correct option is B ii, iii
Spontaneous mutations occur all of a sudden causing unpredictable changes in the DNA. It occurs spontaneously and thus don’t need any mutagens to bring the changes.

Cigarettes contain a polycyclic aromatic compound benzopyrene, which is produced when the tobacco leaves are burned down. In the cells of the respiratory tract, benzopyrene is converted to an epoxide which reacts readily with guanine (G) positions of the DNA helix. Hence, it belongs to the category of chemical mutagens.

There is a high chance of mutations occuring in the S phase of the cell cycle. DNA replication process during S-phase involves the addition of new nucleotides forming the new strand. If an error occurs during this addition, it would result in a mutation.

Ultraviolet rays, UV-A & UV-B cause damage to the skin. If exposed for a long time, UV radiation can cause DNA damage; it interacts with the adjacent thymine bases to form thymine dimers, thereby causing changes in the DNA structure and arrangement.