A mutation is a change from the original or wild type genetic sequence. These changes can arise within an organism (or particular cell) by errors in the processes of DNA replication, repair and recombination, or from an outside source such as a mutagen. Mutations may be detrimental, beneficial, or even have no effect on the organism's survivability.
A point mutation involves the change of a nucleotide at a single location. The nucleotide can be exchanged for another, it could be deleted, or an additional one can be added at that location. This type of mutation can have numerous possible effects on the resulting protein when it occurs within a coding region. These are outlined below:
Missense mutations cause the genetic code to signal for a different amino acid to be incorporated into a growing polypeptide chain. These mutations can be further classified as either conservative or non-conservative. In conservative missense mutations the newly encoded amino acid is similar in structure and chemical properties to the original, so that the structure and function of the resulting protein is not affected. In a non-conservative missense mutation the resulting amino acid differs in either structure or chemistry, thereby altering the activity of the original wild-type protein.
Nonsense mutations create a stop codon within the sequence. This causes an abrupt halt in transcription, which results in a truncated (and often non-functional, depending on where the stop codon occurs) protein.
A silent mutation is a change in the DNA sequence that alters the codon, but does not specifiy a different amino acid . This can occur because the genetic code is redundant; that is, there are multiple different codons that specify the same amino acid. For instance, both GAA as well as GAG code for glutamic acid. These mutations are called silent because there is no resulting effect downstream of the DNA.
Point Mutations in Non-coding RegionsEdit
Most point mutations that occur in non-coding regions of DNA do not have any changes or conseuqneces in the organism's phenotype. However, there are a few possible exceptions that can have drastic effects. Should a mutation occur in the promoter region of a gene, it could significantly alter the expression level of the resulting protein. Furthermore, if a mutation occurs at a splice site it could change the splicing patterns of the protein, or prevent the removal of an intron.