Mutations definition, Gene Mutation, Types & Structural

The mutation is a process that produces a gene or chromosome that differs from the wild type (arbitrary standard of what is “normal” for an organism).

The mutation can result from changes in the gene or in the chromosome itself. Therefore, generally speaking, the mutation can:

  1. Genetic mutation where the allele of a gene changes.
  2. Chromosome mutation in which chromosome segments, complete chromosomes, or complete sets of chromosomes change.

A mutation occurs when a DNA gene is damaged or altered in such a way that the genetic message transmitted by that gene is altered.

Mutations definition, Gene Mutation, Types & Structural

A mutagen is an agent of substance that can cause permanent changes in the physical makeup of DNA genes in such a way that the genetic message is altered.

Once a gene has been damaged or replaced, the mRNA transferred from that gene will now carry an altered message.

The polypeptide formed by translating the transformed mRNA will now have a different amino acid sequence. The function of proteins formed by turning this polypeptide will likely change or be lost. In this example, the enzyme catalyzing the production of the flower color pigment has been altered in such a way that it no longer catalyzes the production of red pigment.

The transformed protein does not produce any product (red pigment).

Microscopically or very clearly, the phenotype of the organism carrying the mutation will change. In this case, the flower without pigment is no longer red.

Mutagens

Chemical Mutagens change the sequence of bases in a DNA gene in a number of ways;

  • They mimic the correct nucleotide bases in a DNA molecule but do not match correctly during DNA replication.
  • removing parts of the nucleotide (such as the amino group in adenine), again causing inadequate base pairing during DNA replication.
  • it adds hydrocarbon groups to various nucleotides, which also causes incorrect base pairing during DNA replication.

Types of Mutation

There are various schemes for the classification of different kinds of mutations. Depending on:

A. The Type of Cell Involved

Mutations definition, Gene Mutation, Types & Structural

Somatic mutations

  • Mutations found in the somatic tissues of the body.
  • Mutations are not transmitted to offspring.
  • The extent of the phenotypic effect depends on whether the mutation is dominant or recessive (dominant mutations generally have a greater effect).
  • The extent of the phenotypic effect depends on whether it occurs early or late in development (mutations that arise early have a greater effect).

Germinal mutations

  • Mutations found in the germ tissues of the body.
  • Mutations can be transmitted to offspring.
  • Dominant mutations are seen in the first generation after the mutation occurs.
  • If a female gamete containing an X-linked mutation is fertilized, males will display the mutant phenotype
  • Recessive mutations will only be seen in casual mating with an individual carrying the recessive allele as well; thus, the recessive mutation can remain hidden for many generations.

B. Mode of Origin

  (1) Spontaneous mutations

Spontaneous mutations occur suddenly in nature and their origin is unknown. Also called “background mutation”, they have been reported in many organisms such as Oenothera, corn, bread molds, microorganisms (bacteria and viruses), Drosophila, mice, man, etc.

(2) Induced mutations

In addition to naturally occurring spontaneous mutations, mutations can be artificially induced in living organisms by exposing them to an abnormal environment such as radiation, certain physical conditions (i.e. temperature), and chemicals.

C. Direction of Mutation

Depending on their mode of direction, the following types of mutations have been recognized:

1. Forward mutations

In an organism, when mutations create a change from wild type to abnormal phenotype, those types of mutations are known as direct mutations. Most mutations are of the direct type.

2. Reverse or back mutations

Direct mutations are often corrected by the error correction mechanism so that an abnormal phenotype becomes a wild type phenotype.

D. Size and Quality

Depending on size, two types of mutations have been recognized:

1. Point mutation

When hereditary changes occur in a very small segment of the DNA molecule, that is, a single nucleotide or nucleotide pair, then these types of mutations are called “point mutations.” Point mutations can occur due to the following types of sub nucleotide changes in DNA and RNA.

Mutations definition, Gene Mutation, Types & Structural

– Deletion of mutations. 

The point mutation that occurs due to the loss or deletion of some portion (single nucleotide pair) in a triplet codon of a cistron or gene is called a deletion mutation.

– Insertion or addition mutation.

Point mutations that occur due to the addition of one or more additional nucleotides to a gene or cistron are called insertion mutations.

Mutations that arise from the insertion or deletion of individual nucleotides and cause the rest of the message downstream of the mutation to be read out of phase are called frameshift mutations.

– Substitution mutation.

 A point mutation in which the nucleotide of a triplet is replaced by another nucleotide is called a substitution mutation.

2. Multiple mutations or gross mutations.

When the changes involve more than one nucleotide pair or an entire gene, those mutations are called macroscopic mutations. Macroscopic mutations occur due to gene rearrangements within the genome. Can be:

  1. Gene rearrangement can occur within a gene: Two mutations within the same functional gene can produce different effects depending on the genes, whether they occur in cis or in transposition.
  2. Gene rearrangement can occur in multiple genes per chromosome: If the number of gene replicas is not equivalent to homologous chromosomes, they can cause different types of phenotypic effects on organisms.
  3. Due to the movement of a genetic locus, new types of phenotypes can be created: especially when the gene is relocated near heterochromatin. The movement of genetic loci can take place due to the following method:

(i) Translocation. Movement of a gene can take place to a non-homologous chromosome and this is known as translocation.

(ii) Investment. The movement of a gene within the same chromosome is called inversion.

E. Phenotypic Effects

  1. Morphological mutations are mutations that affect the visible external properties of an organism (i.e., curly ears in cats).
  2. Lethal mutations are mutations that affect the viability of the body (i.e., the Manx cat).
  3. Conditional mutations are mutations in which the mutant allele causes the mutant phenotype only in certain environments (called the restrictive condition).

In the permissive condition, the phenotype is no longer mutant

Example. Siamese cat: The mutant allele causes the albino phenotype at the restrictive temperature of most of the cat’s body, but not at the permissive temperature in the extremities where the body temperature is lowest.

  • Biochemical mutations are mutations that may not be visible or affect a specific morphological characteristic but may have a general effect on the ability to grow or proliferate.

For example, the Escherichia coli bacteria do not require the amino acid tryptophan to grow because they can synthesize tryptophan. However, there are mutants of E. coli that have mutations in the trp genes. These mutants are auxotrophic for tryptophan, and tryptophan must be added to the nutrient medium for growth.

F. The magnitude of Phenotypic Effect

Dominant mutations

Mutations that have dominant phenotypic expression are called dominant mutations. For example, in man, aniridia mutation disease (absence of the iris of the eyes) occurs due to a dominant mutant gene.

Recessive mutations

Most types of mutations are recessive in nature and therefore are not phenotypically expressed immediately. The phenotypic effects of mutations of a recessive gene are seen only after one or more generations when the mutant gene can recombine with another similar recessive gene.

Isoalleles

Some mutations alter the phenotype of an organism so slightly that they can only be detected by special techniques. Mutant genes that give slightly modified phenotypes are called alleles. They produce identical phenotypes in homozygous or heterozygous combinations.

G. Loss of Function or Gain of Function

  • Loss of function mutation

The loss-of-function mutation is also called inactivating mutations, which results in the gene product having little or no function (being partially or totally inactivated).

  •  The gain of function mutations

The gain of function mutations also called activating mutations, changes the gene product so that its effect is strengthened (enhanced activation) or even replaced by a different and abnormal function.

Mutations definition, Gene Mutation, Types & Structural

H. Type of Chromosome Involved

Depending on the types of chromosomes, mutations can be of the following two types:

  1. Autosomal mutations: This type of mutation occurs on the autosomal chromosomes.
  2. Sex chromosomal mutations: This type of mutation occurs on the sex chromosomes.

H.Chromosomal Mutation and Types

  1. Changes in the genome that involve chromosomal parts, complete chromosomes, or sets of complete chromosomes are called chromosomal aberrations or chromosomal mutations.
  2. Chromosomal mutations have been shown to be of great importance in applied biology: agriculture (including horticulture), livestock, and medicine.

Chromosome mutations are inherited once they occur and are of the following types

Structural changes in chromosomes:

Changes in the number of genes

(a) Loss: A deletion that involves the loss of a broken part of a chromosome.

(b) Addition: Duplication involving the addition of a part of a chromosome.

Changes in gene arrangement:

(a) Rotation of a group of 1800 genes within a chromosome:

Inversion in which the broken segment is reattached to the original chromosome in reverse order.

(b) Exchange of parts between chromosomes of different pairs: Translocation in which the broken segment joins a non-homologous chromosome, resulting in new link relationships.

 Changes in the number of chromosomes:

Euploidy

  • It involves the loss, or gain, of a complete set of chromosomes.
  • The term euploidy (Gr., Eu = even or true; ploidy = unity) designates genomes that contain chromosomes that are multiples of some basic number (x).
  • Euploids are those organisms that contain a balanced set or sets of chromosomes in any number.
  • The number of chromosomes in a basic set is called the monoploid number, x.
  • Euploid types whose number of seats is greater than two are called polyploids.
  • Therefore, 1x is monoploid, 2x is diploid; and the polyploid types are 3x (triploid), 4x (tetraploid), 5x (pentaploid), 6x (hexaploid), and so on.
  • The euploidy mutation refers to the state of having a number of chromosomes that are an exact multiple of a basic chromosome set. This means that the number of sets of chromosomes increases in euploidy.

Polyploidy

Addition of one or more sets of chromosomes.

They may be further:

(a) Autopolyploidy. Autopolyploidy implies polyploidy, in which the same basic set of chromosomes multiplies.

(b) Alopolyploidy. The results of polyploidy are due to the doubling of the number of chromosomes in an F1 hybrid that is derived from two clearly different species. The resulting species is called an allopolyploid.

Aneuploidy

  • It involves the loss or gain of a part of the set of chromosomes.
  • This refers to a condition in which one or a few chromosomes are added or removed from the normal number of chromosomes. Therefore, the number of chromosomes in aneuploidy may be greater or less than the number of chromosomes in the wild type.
  • Various types of aneuploidy can be identified as nullisomy, monosomy, and trisomy.
  1. Nullisomy (2n-2) is the loss of both chromosomes of the homologous pair. These conditions can be lethal in most organisms.
  2. Monosomy (2n-1) is the loss of a single chromosome of the homologous pair.
  3. Trisomy is the gain of an extra chromosome (2n + 1). Klinefelter syndrome (44 + XXY / XYY) and Down syndrome are examples of trisomy.

Mutations definition, Gene Mutation, Types & Structural

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Mutations definition, Gene Mutation, Types & Structural

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