What Causes DNA Mutations?

Mutations in DNA sequences generally occur through one of two processes:

  1. DNA damage from environmental agents such as ultraviolet light (sunshine), nuclear radiation or certain chemicals
  2. Mistakes that occur when a cell copies its DNA in preparation for cell division.
1. DNA damage from environmental agents

normal DNA structure

Modifying nucleotide bases

Ultraviolet light, nuclear radiation, and certain chemicals can damage DNA by altering nucleotide bases so that they look like other nucleotide bases.

environmental DNA damage

When the DNA strands are separated and copied, the altered base will pair with an incorrect base and cause a mutation. In the example below a "modified" G now pairs with T, instead of forming a normal pair with C.

incorporating DNA mistakes

Breaking the phosphate backbone

Environmental agents such as nuclear radiation can damage DNA by breaking the bonds between oxygens (O) and phosphate groups (P).

breaking the phosphate backbone

Breaking the phosphate backbone of DNA within a gene creates a mutated form of the gene. It is possible that the mutated gene will produce a protein that functions differently.

Cells with broken DNA will attempt to fix the broken ends by joining these free ends to other pieces of DNA within the cell. This creates a type of mutation called "translocation." If a translocation breakpoint occurs within or near a gene, that gene's function may be affected.

2. Mistakes created during DNA duplication

Prior to cell division, each cell must duplicate its entire DNA sequence. This process is called DNA replication.

DNA replication begins when a protein called DNA helicase separates the DNA molecule into two strands.

helicase separates DNA strands

Next, a protein called DNA polymerase copies each strand of DNA to create two double-stranded DNA molecules.

polymerase copies DNA strands

Mutations result when the DNA polymerase makes a mistake, which happens about once every 100,000,000 bases.

Actually, the number of mistakes that remain incorporated into the DNA is even lower than this because cells contain special DNA repair proteins that fix many of the mistakes in the DNA that are caused by mutagens. The repair proteins see which nucleotides are paired incorrectly, and then change the wrong base to the right one.