Mutations are genetic aberrations. Either single nucleotides can be exchanged (point mutation, substitution), deleted or inserted, or few bases up to whole DNA regions can be altered such a kind. In addition, entire pieces of the code can be spelled backwards, excised and inserted elsewhere in the genome, or truncated.

Mutations can change the form and function of the resulting protein. Environmental factors causing such alterations are radiation, chemical agents or viral infections. The cell disposes of DNA-repair mechanisms for preventing mutations and maintaining the integrity of the genome, but even these repair systems cannot assure complete precision of DNA-replication.

Due to the degeneracy of the genetic code, some mutations in protein-coding genes are silent, i.e. do not result in changes of the amino acid sequence. The impact of a mutation depends also on the environment the organism lives in. The same mutation can be harmful in one environment, neutral in another, and beneficial in yet another one.
Other mutations may not become harmful to an individual unless induced by toxic chemicals, carcinogens, allergens, or specific environmental stimuli.

Mutations can on one hand cause genetic malfunction and hereditary diseases, but on the other hand drive evolution due to acquiring of new traits and adaption to changing environments. Noteworthly, genetic alterations create heterozygousity in populations, thus leaving room for more plasticity in populations´ genomes.

Learn about the cell cycle.