Mutations in
DNA sequences generally occur
through one of two processes:
- DNA damage from environmental agents such as ultraviolet light (sunshine), nuclear radiation or certain chemicals
- Mistakes that occur when a cell copies its DNA in preparation for cell division.
1. DNA damage from environmental agents
Ultraviolet light, nuclear radiation, and certain chemicals can damage DNA by altering nucleotide bases so that they look like other nucleotide bases.
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.
Environmental agents such as nuclear radiation can damage DNA by breaking the bonds between oxygens (O) and phosphate groups (P).
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.
Next, a protein called DNA polymerase copies each strand of DNA
to create two double-stranded DNA molecules.
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.
