Mutation: Definition
A mutation occurs when a gene or chromosome differs from the wild type. Changes in the gene or the chromosome itself may cause the mutation.
1. A gene mutation occurs when the allele of a gene is altered.
2. Chromosome mutation, in which chromosome fragments, complete chromosomes, or entire sets of chromosomes alter.
Different Types of Mutations
There are several classification schemes for different types of mutations. It is dependent on:
A. The Type of Cell Involved
1. Somatic mutations: Mutations are found in the body’s somatic tissues and these aren’t passed down from generation to generation.
2. Germinal mutations: Mutations can be found in the body’s germ tissues and these are passable from generation to generation.
B. Mode of Origin
(1) Spontaneous mutations
The origin of spontaneous mutations is unknown. They arise spontaneously in nature. Background or Spontaneous mutations have been observed in a variety of taxa, including Oenothera, maize, bread molds, microbes (bacteria and viruses), Drosophila, mice, and humans.
(2) Induced mutations
Aside from naturally occurring spontaneous mutations, living organisms can be subjected to abnormal environments such as radiation, specific physical variables (such as temperature), and chemicals to produce mutations.
C. Direction of Mutation
The following categories of mutations have been identified based on their mechanism of direction:
1. Forward mutations
Forward mutations occur when mutations cause a change in an organism’s phenotype from wild type to abnormal. The majority of mutations are of the forward type.
2. Reverse or back mutations
Forward mutations are frequently rectified by an error-correcting process, resulting in the transformation of a defective phenotype into a wild-type phenotype.
D. Size and Quality
Two categories of mutations have been identified based on their size:
1. Point mutation
Point mutations occur when heritable changes occur in a very tiny piece of DNA, such as a single nucleotide or nucleotide pair. The following forms of subnucleotide changes in DNA and RNA can cause point mutations.
A.Deletion mutations
Deletion mutation is a type of point mutation that occurs when a piece (single nucleotide pair) of a triplet codon in a cistron or gene is lost or deleted.
B.Insertion or addition mutation
Insertion mutations are point mutations that arise when one or more additional nucleotides are added to a gene or cistron. Frameshift mutations are caused by the insertion or deletion of particular nucleotides.
C.Substitution mutation.
Substitution mutation is a point mutation in which one nucleotide of a triplet is replaced by another nucleotide.
2. Multiple mutations or gross mutations.
Gross mutations are changes to more than one nucleotide pair or an entire gene. Rearrangements of genes throughout the genome cause massive mutations. It could be:
When a gene locus moves, new forms of symptoms can emerge, especially when the gene is moved near heterochromatin. The following procedure may result in the relocation of gene loci:
(i) Translocation
Translocation occurs when a gene moves from a homologous chromosome to a non-homologous chromosome.
(ii) Inversion.
Inversion is the migration of a gene within the same chromosome.
E. Effects of Phenotypic Variation
1. Morphological mutations are mutations that influence an organism’s external appearance (i.e. curly ears in cats)
2. Lethal mutations are mutations that damage the organism’s survivability
3. Conditional mutations occur when a mutant allele creates the mutant phenotype only in specific conditions (called the restrictive condition).
4. Biochemical mutations: are mutations that are not observable or impact a specific morphological attribute but influence the ability to develop or multiply in a general way.
The bacterium Escherichia coli, for example, can generate the amino acid tryptophan and so does not require it for growth. However, there are mutant E. coli strains with trp gene mutations. These mutants are tryptophan auxotrophs, which means that tryptophan must be supplied to the nutritional medium for them to develop.
F. The Degree of the Phenotypic Effect
The following types of mutations may occur based on their phenotypic effects:
1. Dominant mutations are the most common type of mutation.
A dominant mutant gene, for example, causes the genetic sickness aniridia (absence of the iris of the eyes) in humans.
2. Recessive mutations are the second type of mutation. The phenotypic effects of recessive gene mutations are visible only after one or more generations when the mutant gene can recombine with another recessive gene that is comparable.
3. Isoalleles are mutations in genes that result in slightly altered phenotypes. In homozygous or heterozygous pairings, they generate identical characteristics.
H. Chromosome Type Involved
The mutations can be one of two types, depending on the types of chromosomes:
1. Autosomal mutations. Autosomal chromosomes are vulnerable to this form of mutation.
2. Sex chromosomal mutations. This type of mutation can be seen in both male and female chromosomes.
I. Types of chromosomal mutations
Chromosome aberrations or chromosome mutations are alterations in the genome involving chromosome sections, complete chromosomes, or whole chromosome sets.
In practical biology, such as agriculture (including horticulture), animal husbandry, and medicine, chromosome mutations have proven to be extremely important.
Once a chromosome mutation occurs, it is passed on to the next generation.
A. Structure alterations in the chromosomes:
1. Changes in the number of genes
(a) Deletion: Deletion occurs when a damaged section of a chromosome is lost.
(b) Addition: Duplication including the addition of a chromosomal segment.
Chromosome number variations:
1. Euploidy: Euploidy (Greek: EU = even or true; ploid = unit) refers to genomes with multiples of a fundamental number of chromosomes (x). Euploid organisms are those that have a balanced set or sets of chromosomes in any number. The entire chromosomal set is lost or gained
A.The monoploid number, x, refers to the number of chromosomes in a basic set.
B.Polyploid euploid kinds have more than two sets of chromosomes; for example, 1x is monoploid, 2x is diploid, and 3x (triploid), 4x (tetraploid), 5x (pentaploid), 6x (hexaploid), and so on.
The mutation caused by The condition of having a chromosomal number that is an exact multiple of a basic chromosome set is referred to as euploidy. In euploidy, the number of chromosomal sets is increased.
Polyploidy
A set of chromosomes or more is added.
There could be more:
(a) Autopolyploidy: Polyploidy, in which the same basic set of chromosomes is multiplied, is involved in autopolyploidy.
(b) Allopolyploidy: Polyploidy occurs when the number of chromosomes in an F1 hybrid that is generated from two separate species doubles. An allopolyploid is a name given to the resulting species.
2. Aneuploidy: It is characterized by the loss or gain of a chromosomal set. It’s a situation in which one or a few chromosomes are added or removed from the usual amount of chromosomes. As a result, the number of chromosomes in aneuploidy can differ from the number of chromosomes in the wild type.
Nullisomy, monosomy, and trisomy are three different kinds of aneuploidy.
1. Nullisomy (2n-2) is the lack of both homologous pair chromosomes. Most creatures may die as a result of these conditions.
2. Monosomy (2n-1) is the loss of one of the homologous pair’s chromosomes.
3. Trisomy refers to the addition of a second chromosome (2n+1). Trisomy includes conditions such as Klinefelter syndrome (44+XXY/XYY) and Down syndrome.
Missense Mutation
A missense mutation is a type of mutation that occurs when A shift in the base transforms one codon into another. Many missense mutations are undetectable because the encoded amino acid remains unchanged or the amino acid substitution is minor enough to not affect the enzyme’s function. Missense mutations that have a significant effect are frequently found in the active site or cause severe protein folding disruption.
Nonsense mutation
A mutation in the nucleotide sequence changes a codon into a stop codon. It’s worth noting that a single base mutation can only convert a small number of sense codons to stop codons. Early in the gene sequence, nonsense mutations inactivate the gene. Occasionally, late-in-the-gene nonsense mutations will not alter gene function.
Frameshift mutation
The coding sequence is displaced out of the register due to the addition or deletion of a base or bases. A frameshift is not caused by the addition or deletion of a multiple of three bases. Missense codons will be read up to the first stop codon after the frameshift mutation is found. Frameshift mutations, like nonsense mutations, usually result in the gene’s total inactivation.