Transcription is the process by which mRNA is created from DNA. Translation is the following process by which that mRNA is read through a ribosome and a polypeptide chain is created from it. However, the mRNA strand that reaches the ribosome may not be the same mRNA that was transcribed. This is due to the random occurrence known as a point mutation. There are two kinds of point mutations, the change of one nucleotide to another, called a base substitution, and the complete addition or removal of a nucleotide, which is called either an insertion or deletion, respectively.
A base substitution is the change from one base pair to another. The first subtype of substitution is called a transition. This occurs when a purine, a double-ringed base, changes to another purine (A to G or vice versa), or when a pyrimidine, a single-ringed base, changes to another pyrimidine (C to T or vice versa). Conversely, the other subtype of mutation is called a transversion is the opposite occurrence, where a purine changes to a pyrimidine or vice versa.
There are three kinds of mutations that result from transition and transverse mutations. The first kind is called a synonymous, or silent, mutation. This is when the resulting mutation does not change which amino acid the overall codon codes for. For example, if the codon UGC, which codes for cysteine, had a point mutation and was changed to UGU, there would be no repercussions because UGU also codes for cysteine.
The second type of mutation is called a missense mutation. This is when a mutation at one nucleotide of a codon changes the codon entirely, such as when cysteine (UGU) is changed to UUU, which codes for phenylalanine. However, there are two subtypes of missense mutations. If the new amino acid that the mutated codon codes for is not too chemically different from the original amino acid, it is called a conservative substitution. These types of mutations may not hinder the new protein enough to cause any damage. However, if the mutation changes to an amino acid that is drastically chemically different from the original amino acid, the consequences to the protein could be severe. This mutation subtype is called a nonconservative mutation.
The third type of mutation resulting from a base substitution is called a nonsense mutation. This is the most simple of the three, yet can have the most devastating effects. This mutation results from any substitution that changes the codon from its original amino acid to the codons UAA, UGA, and UAG, which are stop codons. This will immediately end the synthesis for that protein, which can either result in impairment or total loss of function or a harmful protein.
Lastly, the most lethal kinds of point mutations are insertion and deletion mutations, or indel mutations for short (in-del is taken from insertion and deletion). This is when a nucleotide is added to a codon or completely removed from a codon. Since codons must be read as three nucleotides at a time, the addition or deletion of a nucleotide will not only change that particular codon in the mRNA, but it will cause a frameshift mutation, which forces every other codon to change further along the mRNA strand, either due to being forced back by the insertion of a nucleotide or by moving up one space in order to make up for the lost nucleotide. This will completely change the rest of the protein that is synthesized, either resulting in harmful proteins or dysfunctional proteins that must be destroyed, which then results in no protein at all.