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What is a Mutation?
Before we can discuss spontaneous self-correction of gene mutations, we must first understand what a mutation is. Not all of them are bad. In scientifically precise terms, a "mutation" is any change from one hereditary state to another, and a "gene mutation" is any change to the genetic code. (There are also "chromosome mutations.") Each slightly different version of the same gene is called an allele, and there can be many alleles that are equally good to have.
Genes are encoded as strings of nucleotides in a molecule of DNA. There are four nucleotides - adenine, thymine, cytosine, and guanine. A DNA molecule is two strands of nucleotides paired to each other (adenine to thymine, cytosine to guanine), arranged in a double helix. When it's being read, one strand contains the code for the gene and the other is uninvolved.
The nucleotides code for strings of amino acids in a polypeptide chain that, when properly folded into a 3D structure, becomes a protein (or part of a protein - some are multiple polypeptide chains folded together). There are 20 amino acids. Three nucleotides code for one amino acid, and most amino acids are coded for by more than one nucleotide triplet.
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A gene mutation is when any change happens to the nucleotides in the DNA. For example, in a point mutation, one nucleotide gets switched out for another (such as an adenine for a cytosine). In some cases this has no real effect on the final protein, because the triplet still ends up coding for the same or a similar amino acid. In other cases, the protein is rendered non-functional. Very occasionally, the protein turns out to work better than it did before (which could be either more or less effectively, depending on the needs of the overall organism in the environment it's in at the time).
There are also frameshift mutations, which is where nucleotides get inserted or deleted. If the number of these is a multiple of three, the resulting polypeptide chain will have fewer or extra amino acids - and then the overall effect depends on whether they alter the folding of the chain to final protein, or have crucial positions for the protein's function. If it's not a multiple of three, all the nucleotide triplets downstream of the change will shift frame, changing all the amino acids in the chain. This usually renders the final protein non-functional.
Mutations are called "spontaneous" when they occur due to natural causes. Sometimes a nucleotide will wander off on its own, and DNA repair mechanisms fail to replace it (by looking at its pair on the other strand of the helix). Sometimes there are errors in DNA replication. Sometimes transposons pop in.
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Is spontaneous self-correction of gene mutations possible? The short answer is yes, but if the correction happens in a somatic cell, there won't be much effect. To counter the effects of a mutation, the self-correction would have to occur in every cell with the mutation - which in a heritable genetic disease is every cell in the body. A self-correction can only be passed on to future generations if it occurs in a germinal cell that forms a sex cell that then becomes part of a zygote.
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Self-Correction of Gene Mutations
The mechanisms that cause gene mutations to occur can also cause mutations back to the "original" (in scientific terms, "wild-type") version of the nucleotide chain, or allele. An adenine that has switched out for a cytosine might later switch back for an adenine. A triplet that started coding for the wrong amino acid changes back to coding for the original one. These are called reversion mutations.
There can also be new mutations that cancel out the effects of the first mutation. A frameshift caused by an inserted nucleotide at one spot is "fixed" by deleting a nucleotide in another. A misfold in one part of a polypeptide chain is balanced by a second misfold elsewhere in the chain or protein. A defect in one chemical pathway is circumvented by a mutation that opens another. These are called suppressor mutations.
So is spontaneous self-correction of gene mutations possible? To answer the literal genetics question, yes. However, many people asking the question are probably more interested in spontaneous self-correction of genetic diseases.
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Somatic vs. Germinal Mutations
For the purposes of genetics, there are two types of cells in a multicellular organism - somatic and germinal. Germinal cells are the ones that form sex cells and can pass on genetic information to the next generation. Somatic cells are everything else.
Mutations can happen in any cell at any time. Much of the time, mutations in a somatic cell of a mature individual won't cause any major problems overall. Cancers can start in this way, however. This can occur when the genes that regulate cell division get mutated, leading to uncontrolled cell division and the growth of tumors.
Somatic mutations in a developing individual can also cause birth defects. All cells descended from the original mutant cell will carry the mutation - and the earlier in development the mutation happened, the more final cells will also be mutants.
Somatic mutations only affect the individual with the mutation. Generally, they can't be passed on to offspring, though exceptions exist for certain species of plants which can generate germinal cells from somatic tissues.
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Self-Correction of Genetic Diseases
Self-correction of mutations can also happen in any cell at any time. However, once cell division has started after a somatic mutation, self-correction would have to occur in each and every mutant cell to cancel the effects on the overall individual. Likewise, in heritable genetic diseases, the affected individual has the mutation in every cell of their body.
If the spontaneous self-correction occurs in a germinal cell, then there's a chance it can pass on to offspring, where it might stick as a permanent change for the individual's future descendants. But the probability of it happening is fairly small.
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Griffiths, Anthony J.F., Jeffrey H. Miller, David T. Suzuki, Richard C. Lewontin, and William M. Gelbart. 1993. An Introduction to Genetic Analysis 5th ed. W.H. Freeman and Company.
Horton, Robert H., Laurence A. Moran, Raymond S. Ochs, J. David Rawn, and K. Gray Scrimgeour. 1996. Principles of Biochemistry 2nd ed. Prentice Hall, Inc.