In genetics, a mutation is a change that occurs to the DNA found within the cell of a living organism. Mutations can be caused by many different factors, including the age of the organism and external reasons like the sun. They can even occur due to errors that take place when the DNA is being copied in preparation for the creation of a new cell. Where the mutation occurs and how much DNA it involves determines how much of an effect the mutation has on the cell. A point mutation occurs when one nucleotide within the strand of DNA is replaced with another.
There are four different nucleotides that make up DNA, which are adenine (A), thymine (T), guanine (G) and cystine (C). The order of these nucleotides is what determines the gene that is found within that strand of DNA. Most genes have several different variations, which are caused by slight changes within the nucleotides. The location of the change determines how significant it is; while changes in certain strands produce subtle differences, others can be the cause of defects and diseases.
Genes are the templates for all the proteins that are made by a cell. Changing the nucleotides can mean that a similar protein is made, a different protein is made, or that no protein is made. While a point mutation is the change of a single nucleotide, it doesn’t only cause subtle variations. Where the point mutation occurs within the gene sequence can have a significant effect on the protein that is eventually produced.
One way of categorizing point mutations is to group them based on which nucleotide is replacing which other. The nucleotides are either purines or pyrimidines based on their chemical makeup and shape. Adenine and guanine are purines, while thymine and cytosine are pyrimidines. A transition point mutation occurs when a purine replaces a purine or a pyrimidine replaces a pyrimidine. When a purine replaces a pyrimidine or vice versa, the point mutation is called a transversion mutation.
Another way of categorizing point mutations is based on the effect they have on the functioning of the cell. When a protein is being created, the nucleotides are grouped into codons made up of three nucleotides. Each codon codes for an amino acid within a protein chain. Changing a single nucleotide within a codon can significantly affect the resultant protein.
In some cases, the point mutation changes the codon to a stop code, which means the protein is a lot smaller than it should be as the rest of the sequence after the mutation is missing. This type of point mutation is called a nonsense mutation. Missense mutations occur when the substituted nucleotide results in the codon for a different amino acid. Amino acids can have more than one codon, so a silent mutation occurs when the change results in the same amino acid. The term silent point mutation is also used when the substitution results in a different amino acid being coded for within the protein, but this change has no effect on the protein’s function.
