Deletion Mutation Definition
A deletion mutation is a mistake in the DNA replication process which removes nucleotides from the genome. A deletion mutation can remove a single nucleotide, or entire sequences of nucleotides. Deletions are thought to occur when the enzyme that synthesizes new DNA slips on the template DNA strand, effectively missing a nucleotide.
This enzyme, polymerase, must attach the template DNA nucleotides in its active site for DNA replication to occur. Larger strands of DNA can undergo a deletion mutation during crossing-over, which takes place in meiosis. If the segments of DNA that are exchanged are not the same size, large sections may experience a deletion mutation, as seen below.
A deletion mutation can be a serious mutation, or a harmless mutation. The effect of the deletion mutation will be determined by where in the gene it takes place, and how many nucleotides are deleted. The genetic code is read in triplets by protein producing enzymes. If three or more nucleotides are lost in a gene, entire amino acids can be missing from protein created which can have serious functional effect.
Losing a single nucleotide is often not better, as a frameshift mutation can occur. A frameshift mutation shifts the entire gene, and changes all of the original triplet codons. A mutation of this type can cause a gene to produce a completely non-functional gene, as it seriously alters the chain of amino acids the gene produces.
A deletion mutation may take place more often than we can measure, but mutations which are inherited in offspring are typically rare. In asexually reproducing animals the rate of mutations is kept relatively low. In part this is due to the specificity and accuracy of polymerase. However, cells also have another enzyme, exonuclease, which follows polymerase and cuts out sections of DNA which do not match their nucleotide counterpart on the template DNA. Due to this and other regulatory mechanisms, deletion mutations that cause phenotypic change are rare.
Examples of Deletion Mutation
A Simple Mutation
The following is an example of a single nucleotide deletion mutation. The short sequences of DNA are not representative of actual DNA, which contains many hundreds or thousands of base pairs. The top string represents the original strand of DNA, while the bottom strand lacks the nucleotide pair removed by the deletion mutation. The triplet codons are separated, to see the effects of the deletion mutation.
5’ TAC CCA GGG 3’
3’ ATG GGT CCC 5’
5’ TAC CCA GG 3’
3’ ATG GGT CC 5’
As you can see, if this were the end of the DNA molecule, the last amino acid would not be produced. Instead, a deletion mutation will usually occur in the middle of a chromosome or gene. This will cause the deleted nucleotide to be filled by shifting the DNA and causing a frameshift mutation, or inserting a new nucleotide in a mutation known as an insertion. This mutation can only be passed on if the organism’s mechanisms for repairing DNA do not catch the mistake, and the DNA exists in a cell that will produce offspring. In asexual organisms, this is every cell and mutations are more likely to persist. In sexually reproducing organisms, mutations can only be passed on if they arise in the gamete producing tissues of the sex organs.
Discovering the Genetic Code
Before the 1950s, the nature of the genetic code was not well understood. That all changed when Francis Crick and Sydney Brenner began experimenting on mutant strain of bacterial virus. Crick and Brenner analyzed the DNA of viruses that were exposed to a toxin known to cause mutations. During their trials, the noticed that the function of certain genes could be restored by a combination of mutations, which we know now to insertion and deletion mutations.
While the DNA between the two mutations would become nonsense, the insertion would offset the deletion. This would reset the reading frame of the gene, and cause a frameshift mutation to be avoided. This interaction between mutations was termed intragenic suppression. By comparing how individual mutations affected the proteins and amino acids produced, Crick and Brenner were able to formally theorize about the existence of the triplet genetic code, and its universal use in organisms.