Mutation

= **What are mutations?** = A mutation is a change in genotype that can manifest as "an alteration in the nucleotide sequence of a DNA segment, the arrangement of a segment within a chromosome, the number of copies of a segment, the physical structure of a chromosome, or even the number of copies of a structurally normal chromosome" (Weinberg, Robert A. The Biology of Cancer, Glossary, G:13).

As far as our Cancer Project is concerned, considering a mutation as an alteration of the nucleotide sequence in DNA suffices.

**Substitution**
Substitution mutations, also known as point mutations, occur when a single base pair is changed for another. For example, in the DNA sequence of AAT-GTG-CCG, a substitution mutation would yield the sequence of A** G **T-GTG-CCG. What appears to be a small, single error results in drastic change in amino acid sequence, from Leucine-Histidine-Glycine to **Serine** -Histidine-Glycine (Freeman, Scott. Biological Science 3rd ed. pg. 321). Substitutions are classified as missense and nonsense. Missense mutations are changes in nucleotides that result in amino acid alterations, while nonsense mutations are changes in nucleotides that result in stop codons.

**Addition**
Addition mutations occur when one or more nucleotide is inserted into a DNA sequence. Addition of extra nucleotides not only shifts the reading frame of codons but also results in changes in amino acid sequence.



**Deletion**
Deletion mutations occur when one or more nucleotide is deleted from a DNA sequence. Like addition mutations, deletion of nucleotides from a DNA segment shifts the codon reading frame as well as changes the amino acid sequence. In some cases, a deletion mutation can manifest as the complete deletion of a gene.

Back to Main Page