What types of external stimuli cause mutations?

Mutations can be caused by a variety of external stimuli, including:

1. Radiation: Ionizing radiation, such as X-rays and gamma rays, can damage DNA by breaking the chemical bonds between nucleotides. This can lead to mutations, as the cell may attempt to repair the damage incorrectly.

2. Chemicals: Certain chemicals, such as benzene and formaldehyde, can also damage DNA, leading to mutations. These chemicals can cause mutations by altering the structure of nucleotides or by interfering with the DNA replication process.

3. UV light: Ultraviolet radiation from the sun can also damage DNA, causing mutations. UV light can cause the formation of thymidine dimers, which are covalent bonds between adjacent thymine molecules. These dimers can interfere with DNA replication and can lead to mutations.

4. Oxidants: Reactive oxygen species (ROS), such as hydrogen peroxide and superoxide, are produced as by-products of cellular metabolism. These ROS can damage DNA by oxidizing nucleotides and breaking the sugar-phosphate backbone. Oxidative damage to DNA can lead to mutations.

5. Viruses: Some viruses, such as human papillomavirus (HPV) and hepatitis B virus (HBV), can also cause mutations. These viruses can insert their DNA into the host cell's genome, disrupting gene expression and leading to mutations.

6. Transposable elements: Transposable elements are segments of DNA that can move around the genome. When they insert themselves into a gene, they can disrupt its function and lead to mutations.