Help me guys please. What is a mutation? What causes mutation?

A mutation is a change in a DNA sequence. Mutations are commonly caused by mistakes during DNA replication. However, they can also be caused by ultraviolet radiation, exposure to chemicals called mutagens, and infection by viruses. But, a mutation usually occurs when a nucleotide in the DNA gets changed, causing the nucleotide sequence to be copied incorrectly which leads to the wrong code for a protein. This can sometimes be fatal.

Hope that helps.