Mutations: Definition, Types, Causes, and Facts
Mutations Definition
o Any alteration in the genome that is not genetic recombination is called a mutation.
o A gene mutation is the alteration in the sequence of DNA nucleotides in a single gene.
o A chromosomal mutation occurs when the structure of a chromosome is changed.
o Gene mutation rates vary depending on size of gene and differences in susceptibility of particular genes to various mechanisms that cause mutations.
o A mutation in a somatic cell is unlikely to have drastic effects, but a mutation in a germ cell, from which all other cells arise, can be very serious for the offspring.
o Mutations can be spontaneous or they can be induced (occurring due to physical or chemical agents called mutagens).
o A mutagen is any physical or chemical agent that increases the frequency of mutation above the frequency of spontaneous mutations.
Types of Mutations
o If a mutation changes a single base-pair of nucleotides in a double strand of DNA, that mutation is called a point mutation.
o One type of point mutation is called a base-pair substitution results when one base-pair is replaced by another.
Schematic Representation of Point Mutations
o A base pair mutation involving a complete switch from A-T base-pair to the G-C base pair or the opposite is called a transition mutation.
o A base-pair substitution that involves a reversal of the same base-pairs is called a transversion mutation.
o Mnemonic: “In transversion mutations there is a conversion of purine ↔ pyramadine”
o A missense mutation is a base-pair mutation that occurs in the amino acid coding sequence of a gene.
o If there is no change in protein function, the mutation is called a neutral mutation, and if the amino acid is not changed, it is called a silent mutation.
o Even a silent mutation may be significant b/c it my change the rate of transcription.
o Ex. Deamination: Spontaneous loss of C and turns to U
Schematic Representation of Frameshift Mutations
o A second type of point mutation, an insertion or deletion of a base-pair, may result in a frameshift mutation.
o A frameshift mutation results when the deletions or insertions occur in multiples other than 3.
o A nonframeshift mutation occurs when a multiple of 3 is added or deleted.
o Frameshift mutations often result in a completely nonfunctional protein, whereas non-frameshift mutations may still result in a partially or even completely active protein.
o Base pair mutations are nonframeshift mutations as well.
o Ex. Depurinination – A&G are lost (resembles missing teeth)
o If a base-pair substitution or insertion or deletion mutation creates a stop codon, a nonsense mutation results.
o Nonsense mutations are very serious for the cell because they prevent the translation of a functional protein entirely.
o Mnemonic: “Stop the Nonsense”
Chromosomal Deletion
Chromosomal deletions occur when a portion of the chromosome breaks off, or when a portion of the chromosome is lost during homologous recombination and/or crossing over events.
Chromosomal Duplication
Chromosomal duplication is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination, a retrotransposition event, or duplication of an entire chromosome.
o The second copy of the gene is often free from selective pressure — that is, mutations of it have no deleterious effects to its host organism.
o Thus it mutates faster than a functional single-copy gene, over generations of organisms.
o Deletion/Duplication can occur with entire chromosomes (aneuploidy) or even sets of entire chromosomes (polyploidy).
DNA Recombination
o Homologous recombination is a type of genetic recombination in which genetic material is exchanged between two similar or identical strands of DNA.
o Although most widely used in cells to accurately repair double-strand breaks in DNA, homologous recombination also produces new combinations of DNA sequences during chromosomal crossover in meiosis.
o Homologous chromosomes are chromosomes in a biological cell that pair (synapse) during meiosis.
o The pair are non-identical chromosomes that both contain information for the same biological features and contain the same genes at the same loci but possibly each have different alleles (that is, different genetic information) at those genes.
o When a segment of DNA from one chromosome is inserted into another nonhomologous chromosome, the resulting mutation is called a translocation.
o In inversion the orientation of a section of DNA is rotated 180 degrees in the chromosome.
o Translocation and inversion can be caused by transposition, which takes place in BOTH prokaryotic and eukaryotic cells.
o The DNA segments of transposable elements or transposons can excise themselves from a chromosome and reinsert themselves at another location.
o They are usually flanked by identical nucleotide sequences (the circles in this case).
o When moving, the transposon may excise itself from the chromosome and move; it may copy itself and move, or copy itself and stay, moving the copy.
o Transposition is one mechanism that a somatic cell, of a multicellular organism, can alter its genetic makeup without meiosis.
o A mutation can be a forward mutation or a backward mutation.
o These terms refer to an already mutated organism that is mutated again.
o The mutations can be forward, tending to change the organism even more from its original state, or backward, tending to revert the organism back to its original state.
o The original state is called the wild type.
o Forward mutations usually occur more than reverse mutations.
"Mutations are usually bad"
Mutation Citations
- Germline mutation: de novo mutation in reproductive lineage cells. Genes Genet Syst . 2019 Apr 9;94(1):3-12.
- Evolution of recombination rates between sex chromosomes. Philos Trans R Soc Lond B Biol Sci . 2017 Dec 19;372(1736):20160456.
- Meiotic recombination mechanisms. C R Biol . Jul-Aug 2016;339(7-8):247-51.
- En passant mutagenesis: a two step markerless red recombination system. Methods Mol Biol . 2010;634:421-30.
- λ Recombination and Recombineering. EcoSal Plus . 2016 May;7(1).
- Meiotic Recombination: The Essence of Heredity. Cold Spring Harb Perspect Biol . 2015 Oct 28;7(12):a016618.
- Recombination, Pairing, and Synapsis of Homologs during Meiosis. Cold Spring Harb Perspect Biol . 2015 May 18;7(6):a016626.
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