Point mutation is the change of a single base pair of DNA or a small set of close base pairs. Depending on what kind of point mutation it is, the consequences can have minor or major changes to not only the DNA sequence but also the protein structures.
The two main types of point mutations are base substitution and base deletion or insertion. Substitution is the replacement of one base pair with another. Base substitution can be divided into two groups, transitions and transversion. In a transition mutation, a base is replaced by another base of the same chemical category. For example, a purine would be replaced by a purine and a pyrimidine is replaced by a pyrimidine. A transversion mutation is the exact opposite of a transition mutation. In a transversion mutation, a base is replaced by another base of a different chemical category. For instance, a purine can be replaced by a pyrimidine and vise versa. On a molecular level, transition and transversion can lead to four different types of mutations that occur to codons and proteins. When a sequence is changed yet it still codes for the same codon, it is called a synonymous mutation. Sometimes termed silent mutation, the altered codon of the sequence specifies for the same amino acid and the amino acid chain is never altered. For example, AGA codes for arginine and if a synonymous mutation were to happen, changing that code to AGC, it would still code for arginine. Thus, the outcome is not altered in any way by the change in base. Then we have conservative missense and nonconservative missense mutations. In a conservative missense mutation, the changed codon codes for an amino acid with similar chemical properties. For instance, a polar amino acid is replaced by another polar amino acid. This mutation has a smaller chance of causing severe damage to the protein. In a nonconservative missense mutation the altered codon codes for a chemically different amino acid like a polar amino acid is replaced by a nonpolar one. This can cause severe changes to protein structure and function since it is no long the correct amino acid in any way. Last but not least in the nonsense mutation in which the altered codon signals for early termination. The premature termination caused by the nonsense mutation causes inactivity of protein products.
Insertion and deletion mutations are basically the addition or removal of nucleotide pairs. Together, they can be termed indel (insertion and deletion) since they are the simplest point mutations. The simplest form of this mutation is just the insertion or deletion of a single base pair. Insertions and deletions cause changes in the reading frame since a nucleotide pair is either added or removal. This can extend the reading frame or shorten it. This causes the entire amino acid sequence from the insertion or deletion to be completely different from the original sequence. A frameshift mutation results in the complete damage of the normal protein function and structure.
Point mutations can cause no change or major changes to not only the nucleotide sequence but can also affect protein structures and functions.