Physical Mechanisms include:
pyrimidine dimers occur when adjacent thymines or cytosines are linked together, usually by ultraviolet light. The fusion of adjacent nucleotides may impede replication or alter the sequence of bases, resulting in one of the various point mutations.
ionization by x-rays. The resulting damage to DNA can result in permanent change to the genetic code. Ionizing radiation often results in the formation of highly reactive chemicals called free radicals which if part of a DNA molecule may produce a number of unwanted changes.
Chemical Mechanisms are:
intercalating agents -- flat molecules that can squeeze between adjacent nucleotides. This addition of a nucleotide will distort the DNA molecule and result in a point mutation.
base analogs -- compounds which are very similar to the normal nucleotide bases. Base analogs are readily incorporated into the DNA as it replicates. Because many base analogs can change their hydrogen bonding preferences the wrong complementary base may be added during the next round of replication.
DNA modifying agents chemically combine with DNA. This may result in misreading by DNA or RNA polymerase
Once a nucleotide or nucleotides have been altered the genetic code will be subject to one of the following mutations.
Base substitution. A single base pair is replaced by a different pair. This can result in a
- silent mutation - the new base pair codes for the same amino acid. Usually the third nucleotide may change without changing the meaning of code --the degenerate or synonymous codes
- neutral mutation - the new base pair codes for a different amino acid but the shape of the resulting protein is unchanged so there is no noticeable effect.
- an actual MUTATION - while most mutations are disastrous, some can actually benefit the organism.
The disasters include:
- Chain termination mutations
Point mutations normally affect only one codon but when that codon is a stop codon the results are much more drastic. Premature chain termination usually results in totally nonfunctional polypeptides.
- Additions and Deletions
When the number of nucleotides added or removed is not equal to three, the resulting code from the insertion or deletion point forward is converted into nonsense. Such errors are called frameshift mutations. The end result is a grossly abnormal polypeptide.
Point Mutations may also occur in non-coding regions -- introns or regulatory regions. As you might expect the severity of the mutation depends upon the region it occurs in. Mutations in introns have little or no effect.