Principles of Biochemistry (2+1)

7.5.3 Mutations

A mutation is an error in base sequence in a gene that is reproduced during DNA replication. Such errors alter the genetic information that is passes on during transcription. The altered information can cause changes in amino acid sequence during protein synthesis. Sometimes, such changes have a profound effect on an organism.

A mutagen is a substance or agent that causes a change in the structure of a gene. Radiation and chemical agents are two important types of mutagens.

Radiation: Radiation, in the form of ultraviolet light, X rays, radioactivity, and cosmic rays, has the potential to be mutagenic. Ultraviolet light from the sun is the radiation that causes sunburn and can induce changes in the DNA of the skin cells. Sustained exposure to ultraviolet light can lead to serious problems such as skin cancer.

Chemical agents: Chemical agents can also have mutagenic effects. Nitrous acid(HNO₂) is a mutagen that causes deamination of heterocyclic nitrogen bases. e.g. HNO_2­ can convert cytosine to uracil. Deamination of a cytosine that was part of an mRNA codon would change the codon; for example CGG would become UGG.

A variety of chemicals-including nitrites, nitrates, and nitrosamines—can form nitrous acid in the body. The use of nitrates and nitrites as preservatives in foods such as bologna and hot dogs is cause of concern because of their conversion to nitrous acid in the body and possible damage to DNA.

Fortunately, the body has repair enzymes that recognize and replace altered bases. Normally, the vast majority of altered DNA bases are repaired, and mutations are avoided. Occasionally, however, the damage is not repaired, and the mutation persists.