A mutation, or a change in an organism's deoxyribonucleic acid (DNA), can be the result one or several factors. Two processes that account for a majority of mutations are:
1. Inaccurate DNA replication
2. External influences
Inaccurate DNA replication can occur during cellular division, when the cell makes a copy of its DNA. Occasionally in this process, the 'copied' DNA is not an identical replicate of the DNA template. This replication mistake will lead to a mutation, which may be the result of an unusual chemical environment or external radiation, causing the DNA to break down. Consequently, the cell is forced to repair the damaged DNA. However, instances occur where the cell does not repair the DNA correctly, which again results in a mutation.
When a mutation occurs, effects are seen at the DNA, protein, cellular and whole organism level. At the DNA level (see example below), a 'mutant' codon (triplets of nucleotides) may be present. This initiates the translation and transcription of an unwanted protein. As a result of the formation of the unwanted protein, protein structures may change. For example, normally spaced out proteins may become clumped together. Mutations may also cause cells to lose specific functions, or alter essential cellular functions. Additionally, mutations often have either a negative or positive effect on the whole organism. For example, individuals that carry the sickle cell allele occasionally experience pain and fatigue at high altitudes or during intense exercise. However, sickle cell allele carriers are resistant to the infectious disease, malaria, because the parasites causing it cannot survive in sickle-shaped blood cells.
Types of mutations
Substitution occurs when one codon base is replaced by another. This mutation could result in one or more of the following:
A codon is changed so that it is no longer the code for the desired amino acid; thus changing the protein that is supposed to be produced.
A silent mutation: the change in the codon does not prevent it from coding for the desired amino acid. Therefore it causes no change in the protein that is supposed to be produced.
A codon is changed so that the codon that was supposed to be the code for an amino acid now codes for 'Stop'; thus resulting in an incomplete protein. This can have serious effects, as the incomplete protein probably won't be able to function properly.
Insertions are mutations where extra base pairs are inserted into a new place in the DNA. Insertions can be any size -- from one base pair to several.
Deletion - as the name implies - results in missing DNA. Any number of DNA nucleotides may be deleted; thus resulting in lost DNA information. This type of mutation may occur during the chromosome-crossover part of meiosis, which often results in a serious genetic disease.
A frameshift is the result of deleting or inserting one or more nucleotides in such a way that multiple codons are affected. Since the codons are groups of three consecutive nucleotides, the insertion or deletion of just one nucleotide will shift the groupings of the remaining nucleotides into codons, thereby changing the function of all of them. As a result of such a frameshift, the desired amino acids and proteins are no longer developed.
In the following example, you will examine four common types of mutations: substitution, insertion, deletion and frame shift. Compare the original DNA strand, RNA strand and proteins to the mutated components by selecting one of the options from the drop down menu.
Select a type of mutation from the drop down menu (below) to see the effect it has on the DNA strand, RNA strand and proteins formed.
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