Can you detect if a mutation is spontaneous or induced?

Can you detect if a mutation is spontaneous or induced?

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Is it possible to determine if a certain specific mutation had a spontaneous origin (for example from a mistake of the DNA polymerase) as opposed to an induced origin (for example, from some genotoxic agent)?

Here's a quick answer; hopefully someone will give a more complete one. But meanwhile you've got something.

Some genotoxic agents have predictable results. For example, they cause Gs to substitute for Cs, or they cause mutations at specific spots on the genome. So if you had a bunch of mutations in a single cell (or collection of related cells) that matched the modus operandus of a known genotoxic agent, then it would be reasonable to conclude that the mutations were induced. But not all genotoxic agents are like that. And if you only have 1 or 2 mutations, that's not enough to prove they were induced. Also, of course, it depends on your sample size and your controls.

The above answer is mostly right. In the lab one can show that induced mutations are caused by exposure, compared to unexposed controls (e.g. in bacteria). The induced mutations will have a frequency, position, and type which can be typified. As a group, theyc ompris a "spectrum" of mutations. In a real case problem, such as humans exposed to diesel exhaust, the number of mutations and their trype are usually going to be too few to have a"spectrum". Naturally occurring (a serious misnomer in its own right) could also occur in those same positions and types. The usual language is that the observed mutations are consistent with an exposure, but does not prove the exposure caused them.

2.9: Mutations

Errors occurring during DNA replication are not the only way by which mutations can arise in DNA. Mutations, variations in the nucleotide sequence of a genome, can also occur because of physical damage to DNA. Such mutations may be of two types: induced or spontaneous. Induced mutations are those that result from an exposure to chemicals, UV rays, x-rays, or some other environmental agent. Spontaneous mutations occur without any exposure to any environmental agent they are a result of spontaneous biochemical reactions taking place within the cell (including errors of replication).

Mutations may have a wide range of effects. Some mutations have no effect on gene function these are known as silent mutations. Point mutations are those mutations that affect a single base pair. The most common nucleotide mutations are substitutions, in which one base is replaced by another. Mutations can also be the result of the addition of a nucleotide, known as an insertion, or the removal of a base, also known as deletion. Sometimes a piece of DNA from one chromosome may be joined to another chromosome or to another region of the same chromosome this is known as translocation.

When a mutation occurs in a protein coding region it may have several effects. Nucleotide substitutions may lead to no change in the protein sequence (known as silent mutations), change the amino acid sequence (known as missense mutations), or create a stop codon (known as a nonsense mutation). Insertions and deletions in protein coding sequences lead to frameshift mutations. Missense mutations that lead to conservative changes result in the substitution of similar but not identical amino acids. For example, the acidic amino acid glutamate substituted for the acidic amino acid aspartate would be considered conservative- they have the same charge. In general we do not expect these types of missense mutations to be as severe as a non-conservative amino acid change such as a glutamate substituted for a valine (changing from charged to hydrophobic). Drawing from our understanding of functional group chemistry we can correctly infer that this type of substitution may lead to severe functional consequences, depending upon location of the mutation.

Note that the preceding paragraph had a lot of potentially new vocabulary - it would be a good idea to learn these terms.

Mutations can lead to changes in the protein sequence encoded by the DNA.

Based on your understanding of protein structure, which regions of a protein would you think are more sensitive to substitutions, even conserved amino acid substitutions? Why?

A insertion mutation that results in the insertion of three nucleotides is often less deleterious than a mutation that results in the insertion of one nucleotide. Why?

Mutations: Some nomenclature and considerations


The term mutation simply means a change or alteration. In genetics, a mutation is a change in the genetic material - DNA sequence - of an organism. By extension, a mutant is the organism in which a mutation has occurred. But what is the change compared to? The answer to this question is that it "depends". The comparison can be made with the direct progenitor (cell or organism) or to patterns seen in a population of the organism in question. It mostly depends on the specific context of the discussion. Since genetic studies often look at a population (or key subpopulations) of individuals we begin by describing the term "wild-type". Different forms of a gene, including those associated with "wild type" and respective mutants, in a population are termed alleles.

Wild Type vs Mutant

What do we mean by "wild type"? Since the definition can depend on context, this concept is not entirely straightforward. Here are a few examples of definitions you may run into:

Possible meanings of "wild-type"

  1. An organism having an appearance that is characteristic of the species in a natural breeding population (i.e. a cheetah's spots and tear-like dark streaks that extend from the eyes to the mouth).
  2. The form or forms of a gene most commonly occurring in nature in a given species.
  3. A phenotype, genotype, or gene that predominates in a natural population of organisms or strain of organisms in contrast to that of natural or laboratory mutant forms.
  4. It is also generally safe to say that a completely defective allele (say, an allele with an early stop codon), that cannot encode a functioning version of the gene, is a mutant derivative of the wild-type allele.

That said, however, it might be the "norm" for an entire species to lack a functioning version of a gene that serves a purpose in a closely related species. In which case a reasonable person might describe this nonfunctioning gene as the wild-type allele- for that species! However, genes that cannot be, and never are, expressed in a species are referred to as psuedogenes.

The common thread to all of the definitions listed above is based on the "norm" for a set of characteristics with respect to a specific trait compared to the overall population. In the "Pre-DNA sequencing Age" species were classified based on common phenotypes (what they looked like, where they lived, how they behaved, etc.). A "norm" was established for the species in question. For example, Crows display a common set of characteristics, they are large, black birds that live in specific regions, eat certain types of food and behave in a certain characteristic way. If we see one, we know its a crow based on these characteristics. If we saw one with a white head, we would think that either it is a different bird (not a crow) or a mutant, a crow that has some alteration from the norm or wild type.

In this class we take what is common about those varying definitions and adopt the idea that "wild type" is simply a reference standard against which we can compare members of a population.

If you were assigning wild type traits to describe a dog, what would they be? What is the difference between a mutant trait and variation of a trait in a population of dogs? Is there a wild type for a dog that we could use as a standard? How would we begin to think about this concept with respect to dogs?

Mutations can lead to changes in the protein sequence encoded by the DNA that then impact the outward appearance of the organism. Source: y-in-the-life/

Mutations are simply changes from the "wild type", reference or parental sequence for an organism. While the term "mutation" has colloquially negative connotations we must remember that change is not inherently "bad". Indeed, mutations (changes in sequences) should not primarily be thought of as "bad" or "good", but rather simply as changes and a source of genetic and phenotypic diversity on which evolution by natural selection can occur. Natural selection ultimately determines the long-term fate of mutations. If the mutation confers a selective advantage to the organism, the mutation may eventually become very common in the population. Conversely, if the mutation is deleterious, natural selection will ensure that the mutation will be lost from the population. If the mutation is neutral, that is it neither provides a selective advantage nor disadvantage, then it may persist in the population.

Consequences of Mutations

For an individual, the consequence of mutations may mean little or it may mean life or death. Some deleterious mutations are null or knock-out mutations which result in a loss of function of the gene product. These mutations can arise by a deletion of either the entire gene, a portion of the gene, by a point mutation in a critical region of the gene that renders the gene product non-functional, through a nonsense mutation early in the coding sequence, or through a frame-shift mutation. These types of mutations are also referred to as loss-of-function mutations. Alternatively, mutations may lead to a modification of an existing function (i.e. the mutation may change the catalytic efficiency of an enzyme, a change in substrate specificity, or a change in structure). In rare cases a mutation may create a new or enhanced function for a gene product this is often referred to as a gain-of-function mutation. Lastly, mutations may occur in non-protein-coding regions of DNA. If these mutations occur in regions of the gene that are non-coding, but still important for gene expression (such as a promoter), they can also strongly affect gene function.

In the discussion above what types of scenarios would allow such a gain-of-function mutant the ability to out compete a wild type individual within the population? How do you think mutations relate to evolution?

Mutations and cancer

Mutations can affect either somatic cells or germ cells. Sometimes mutations occur in DNA repair genes, in effect compromising the cell's ability to fix damage, and possibly increasing mutation rate. If, as a result of mutations in DNA repair genes, many mutations accumulate in a somatic cell, they may lead to problems such as the uncontrolled cell division observed in cancer. Cancers, including forms of pancreatic cancer, colon cancer, and colorectal cancer have been associated with mutations like these in DNA repair genes. Similarly, defects in repair can be passed through the generations (due to their recessive nature). Although the frequency of such mutations in the general population is low, occasionally individuals unknowingly heterozygous for the repair defect do have children who inherit both defective alleles. In the case of XP homozygous individuals with compromised DNA repair processes become very sensitive to UV radiation. In severe cases these individuals may get severe sunburns with just minutes of exposure to the sun. Nearly half of all children with this condition develop their first skin cancers by age 10.

Consequences of errors in replication, transcription and translation

Something key to think about:

Cells have evolved a variety of ways to make sure errors of replication are both detected and corrected, from proofreading by the various DNA-dependent DNA polymerases, to more complex mismatch repair systems. Why did so many different mechanisms evolve to repair errors in DNA? By contrast, similar proof-reading mechanisms did NOT evolve for errors in transcription or translation. Why might this be? What would be the consequences of an error in transcription? Would such an error effect the offspring? Would it be lethal to the cell? What about translation? Ask the same questions about the process of translation. What would happen if the wrong amino acid was accidentally put into the growing polypeptide during the translation of a protein? Contrast this with DNA replication.

Mutations as instruments of change

Mutations are how populations can adapt to changing environmental pressures.

Mutations are randomly created in the genome of every organism, and this in turn creates genetic diversity and a plethora of different alleles per gene per organism in every population on the planet. If mutations did not occur, and chromosomes were replicated and transmitted with 100% fidelity, how would cells and organisms evolve? Whether mutations are retained in a population depends largely on whether the mutation provides selective advantage (meaning, increases the frequency of reproduction of that allele), poses some selective cost or is at the very least, not harmful. Indeed, mutations that appear neutral may persist in the population for many generations and only be meaningful when a population is challenged with a new environmental challenge. At this point the apparently previously neutral mutations may provide a selective advantage.

Example: Antibiotic resistance

The bacterium E. coli is sensitive to an antibiotic called streptomycin, which inhibits protein synthesis by binding to the ribosome. The ribosomal protein L12 can be mutated such that streptomycin no longer binds to the ribosome and inhibits protein synthesis. Wild type and L12 mutants grow equally well and the mutation appears to be neutral in the absence of the antibiotic. In the presence of the antibiotic wild type cells die and L12 mutants survive. This example shows how genetic diversity is important for the population to survive. If mutations did not randomly occur, when the population is challenged by an environmental event, such as the exposure to streptomycin, the entire population would die.

Uncorrected errors in DNA replication lead to mutation. In this example, an uncorrected error was passed onto a bacterial daughter cell. This error is in a gene that encodes for an important ribosomal protein. The mutation results in a different final 3D structure of the ribosome protein. While the wild-type ribosome can bind to streptomycin (an antibiotic that will kill the bacterial cell by inhibiting the ribosome function) the mutant ribosome cannot bind to streptomycin. This bacterium is now resistant to streptomycin and will of course outgrow bacteria carrying the wild-type sequence, in the presence of the drug.
Source: Intelligent Design Center (yes, really)

Mutation: Definition, Detection and Types | Genetics

Heredity results accurate reproduction of genes. During cell-division the chromo­somes divide to give rise to daughter chromosomes. The chromosomes are beset with genes and consequently chromosome duplication means in a way the duplication of genes.

Genes always arise from existing genes although the material for the synthesis comes from nutritional sources. The newly formed gene is an exact replica of the parent gene.

The process of gene reproduction though exact, sometimes presents an error in copying. The copy of the gene differs from the original and the modified gene re­produces its changed structure. This error in copying is Mutation.

Thus mutation is a change in gene, potentially capable of being transmitted in the changed form. Mutation occurs in genes (gene mutation) as well as in chromosomes (chromosomal mutation). Chromosomes usually repro­duce faithfully and accurately. Sometimes a chromosome may alter by loss of some parts of genes, by reduplication, by trans­location or by inversion.

Instances of sudden appearance of new hereditary types in plants and ani­mals were in record by breeders. These new types were referred to as sports. The appea­rance of Ancon lamb (lamb with short and bowed legs), hornless cattle, double toed cats, albino rats are all examples of spon­taneous mutations.

In Darwin’s time these sports were re­garded as insignificant. They were consi­dered as monstrosities rather than the origin of new viable types. Appearance of a number of sports in Oenothera lamarckiana led de Vries to put forward his muta­tion theory.

The striking mutants in Oeno­thera are forms like giants which had large size, nanella which were dwarfs and many other forms which differ in colour, size or shape of various parts. De Vries introduced the term ‘Mutation’ or ‘Saltation’ to ascribe for these sports.

3. Aniridia in Man as a Case of Mutation:

Genes are responsible for characters. In man the existence of gene is known which regulates the formation of normal iris of the eye. Sometimes this gene becomes so altered that the iris fails to develop. Failure of the iris to develop is known as aniridia. Muta­tion of this gene occurs in the germ cell of a man and is transferred to the zygote which finally develops into a new man.

The mutated gene is a dominant gene and as a result the new individual is born with no iris in the eye. The mutated gene per­sists in the germinal cells of this aniridic man who will eventually produce half of the germ cells with this mutant gene (Fig. 2.28).

4. Detection of Mutation:

Mutations produce both visible and lethal effects and it is possible to detect both these effects.

i. Detection of visible mutation:

Mutations may be produced artificially by applying different methods. If a visible nonlethal mutation is produced it will be detected readily provided the gene that mutates is a dominant one.

In one peculiar strain of Drosophila melanogaster known as ‘attached-X’ strain a very favourable condition is present and because of this condition a visible but recessive mutation can be detected very easily. In the ‘attached-X’ strain the females have two X chromosomes attached to one another at the ends nearest the centro­mere and a Y chromosome.

Because the two X chromosomes are attached they always tend to go to the same pole during meiosis. When such a female is crossed to a normal male then comes a theoretical possibility of the formation of four types of individuals.

(a) Attached X+Y—when the two attached X of the female unite with the Y chromosome of the male.

(b) X+Y male—When the Y chromo­some of the female unites with the X chro­mosome of the male. This is a normal male.

(c) Attached X+X—When the two attached X of the female unite with the X of the male. Such individuals are with a total of three X chromosome? and are called super females which dies at a very early stage of development.

(d) Y+Y—When the Y chromosome of the female unites with the Y chromosome of the male. These are incomplete indivi­duals and die at an early stage.

Thus a cross between attached X fe­males and normal males produces ‘attached X’ females, normal males and some non­viable super females and incomplete indi­viduals. The most noteworthy feature of such a cross is that the normal male off­spring receives its X chromosome from the male and not from the female.

In the attached X method, normal males are treated with X’-rays and are then mated with attached-X females. As the sons in such a cross receive their X chromo­some from the father, it becomes very easy to detect any mutation that may occur to the treated X chromosome-of the male.

ii. Detection of induced lethals:

In order to estimate the frequency of the induced lethal mutations that are produced in X chromosomes, male Drosophila flies are exposed to radiation of known kind and intensity. They are then-mated with non-radiated or normal females.

If a lethal is produced in the X chromosome of the male it will not be detected in the first generation of the females because they will be ‘masked’ by the ‘non-lethal’ of the X chromosome of the female parent. But if the resultant female offsprings are again mated to normal males the mutation will reappear in the next generation and will be manifested as a deficiency in the male population.

It is obvious that the treated X chromosome of the male passes into a female and subsequently again into the male. These grandsons have no X chro­mosome other than the one received from the grandfather and the presence of a lethal in that X chromosome results in the early death of all males.

A very simple and standard technique has been devised by Muller for the detec­tion of new lethal in the X chromosome of Drosophila melanogaster. The technique is known as CIB method.

The method employs a special stock of female Drosophila fly. One X chromosome of such a female fly bears a recessive lethal gene (1) a dominant gene (B) for bar eye and a dominant gene (G) which is a cross­over suppressor besides other genes.

Such females with one X chromosome as marker are mated to irradiated males. One fourth of the offsprings produced out of such a cross will receive a CIB X chromosome and an irradiated X chromosome and will be females.

One fourth of the offsprings, will be male with a GIB chromosome and will eventually die because of the presence of the lethal character (1). The remaining half of the progeny will not have the GIB chro­mosome and as such they will be normal in their eye structure. Such flies will be discarded. The CIB females with treated X chromosome will then be mated with normal males.

It is to be remembered that in a normal female crossing-over takes place between the two X chromosomes. But in CIB chromosome such a cross-over is not possible because of .the presence of the gene, C, which is a cross-over suppressor.

Thus the chance of transference of the induced lethal from the irradiated chro­mosome to the non-irradiated chromosome- becomes nil. The use of B gene is to iden­tify the flies that have the cross-over suppressor (Fig. 2.29).

Thus when females with one CIB X- chromosome and one treated X chromo­some are mated with normal males half of the male offsprings will fail to survive be­cause of the presence of the lethal gene, 1. In case the irradiation induces any new lethal the other half of the males will die.

5. Types of Mutation:

Mutations occurring in the genes which are somatic or vegeta­tive in this location have been termed somatic mutation. Most of the mutations observed by de Vries in Oenothera lamarkiana were somatic mutations. Subsequently the presence of somatic mutation in the endospermic tissue of maize has been demons­trated by Emerson.

Mutation occurring in the germ cells (sperm or ovum) has been termed germinal mutation. Such mutations are again divided into gametic when occur in the gametes and zygotic when occur in the zygote.

iii. Spontaneous mutation:

The naturally occurring mutations are the spontaneous mutations. Natural agencies are respon­sible for the production of such mutations. The appearance of-a white-eyed male in a culture of wild type Drosophila of Morgan is a classical case of spontaneous mutation. Presence of such mutation speaks in favour of evolution.

It is possible to in­duce mutation by the application of arti­ficial means. Any agent that can produce mutation is called a mutagenic agent. Many of such agents are known. The important ones are X-rays, ultra-violet rays, radium, heat and temperature, mustard gas, coal tar, formaldehyde, caffine, etc.

v. Anomozygous mutation:

Mutations due to structural changes of a chromosome or a set of chromosomes have been termed anomozygous mutation. Structural changes in the chromosomes are caused by trans­location, inversion, duplication and deficiency.

vi. Biochemical mutation:

Mutations that affect specific biochemical processes have been called biochemical mutation. Such mutation occurring in an individual inhi­bits its ability to synthesise essential mate­rials like vitamin or amino-acid. The in­hibition may be total or partial.

In partial inhibition there occurs a partial blockage in the steps of a synthetic process. Bioche­mical mutations of Neurospora is well studied. The alkaptonuria and phenyl­ketonuria of man are examples of bioche­mical mutations.

Most mutations are lethal. Such mutations cause loss or altera­tion of a function during embryogenesis. Biochemical mutations are mostly lethal. But in some cases, organisms with bioche­mical mutations can thrive well if the metabolite which they cannot synthesise is supplied exogenously and in such cases biochemical mutations are not regarded as lethal.

Quick Notes on Gene Mutation

Here is a compilation of notes on Gene Mutation. After reading these notes you will learn about: 1. Introduction to Gene Mutation 2. Origin of Gene Mutation 3. Effects 4. Direction 5. Types 6. Induction 7. Molecular Basis 8. Detection 9. Importance.

  1. Notes on Introduction to Gene Mutation
  2. Notes on the Origin of Gene Mutation
  3. Notes on the Effects on Gene Mutation
  4. Notes on the Direction of Gene Mutation
  5. Notes on the Types of Gene Mutation
  6. Notes on the Induction of Gene Mutation
  7. Notes on the Molecular Basis of Gene Mutation
  8. Notes on the Detection of Gene Mutation
  9. Notes on the Importance of Gene Mutation

Note # 1. Introduction to Gene Mutation:

Inheritance is based on genes that are faithfully transmitted from parents to offspring’s during reproduction. Different mechanisms have evolved to facilitate the faithful transmission of genetic materials (information) from generation to generation. Nevertheless, ‘mistakes’ or changes in the genetic material do occur. Such sudden, heritable changes in the genetic material are called mutations.

Hugo de Vries used the term ‘mutation’ to describe phenotypic changes which were inheritable. The term ‘mutation’ refers both to the, change in the genetic material and to the process by which the change occurs.

An organism exhibiting a novel phenotype as a result of the presence of mutation is referred to as mutant. However, the term mutation is often used in a rather strict sense to cover only those changes which alter the chemical structure of the gene at the molecular level.

These are commonly called gene mutations or point mutations. A gene which represents a particular segment of DNA with characteristic base sequence transcribes m-RNA with particular code sequence, codon is triplet to be translated into protein of definite amino acid sequence. Mutation involves the change in the base sequence of DNA which is reflected in amino acid sequence of protein through RNA.

Note # 2. Origin of Gene Mutation:

A. Spontaneous mutation — mutation occurs during normal celjular activities, primarily DNA replication and repair.

B. Induced mutation — mutation occurs as a result of treatment with a mutagenic agent or environment mutation rate is usually higher than background levels.

i. Ionizing radiation — α-, β-, y- or X-rays usually results in deletions or insertions of DNA.

ii. Non-ionizing radiation — UV light causes adjacent thymines on one DNA strand to bond together (thymine dimer) resulting in a structure that must be repaired in order for DNA replication to proceed inefficient repair can lead to point mutations.

iii. Chemicals — chemical substances that interact with DNA to create base changes.

(a) Base analogues — chemicals that are structurally similar to bases in DNA, but may have different base pairing properties bromouracil (BU) is structurally similar to thymine so will be incorporated in a growing DNA strand in place of T, but due to its properties it base pairs more frequently with G than with A. The mutagenic effect is mostly due to incorrect base pairing with G, leading to GC-AT transitions.

(b) Base modifiers — chemicals that make changes to a specific base changing its ability to base pair properly e.g., deamination of cytosine creates a uracil base that will pair with an A instead of the G previously designated by the original C, or alky­lating agents that add a methyl group causing guanine to mispair with thymine.

(c) Intercalating agents — chemicals that insert themselves into the DNA helix cau­sing DNA replication and transcription problems usually results in deletions or insertions.

C. Mutator mutations — mutations that influence the mutability of other genes.

i. Specific mutators — limited to one locus.

ii. Nonspecific mutators — effect is not specific to one locus these mutations are gene­rally in genes that control DNA repair.

Note # 3. Effects on Gene Mutation:

i. Effect on Protein (codons):

A. Silent mutation — change in a codon (usually in the third position) that does not change the amino acid coded for.

B. Nonsense mutation — change in a codon from amino acid specificity to a stop codon results in premature amino acid chain termination during translation.

C. Missense mutation — change in a codon that changes the specificity to a different amino acid changes the primary sequence of the polypeptide chain and alters the function of the protein.

D. Neutral mutation — change in the codon such that a different amino acid is specified how­ever, the new amino acid behaves similarly to the original one (e.g., has a similar functio­nal group) and does not alter the function of the protein.

E. Frame shift mutation — a shift of the reading frame caused by a deletion or insertion of one or a few nucleotides creates numerous missense and nonsense codons downstream of the mutational event.

ii. Effect on Gene Function:

A. Loss-of-function mutation — a mutation that results in a lack of gene function, this can result from a number of different types of mutations and is recessive in nature.

B. Gain-of-function mutation — a mutation that results in a new or different gene function this can result from a number of different types of mutations and is dominant in nature.

iii. Effect on DNA:

A. Structural mutations — changes in the nucleotide content of the gene.

1. Base substitution mutations — substitution of one nucleotide for another.

(a) Transition mutations substitute one purine for another purine or one pyrimidine for another pyrimidine.

(b) Transversion mutations substitute one purine for a pyrimidine or vice versa.

2. Deletion mutations — loss of some portion of DNA.

3. Insertion mutations — addition of one or more extra nucleotides.

B. Chromosomal rearrangements — changing the location of a piece of DNA within the genome can result in large structural changes (translocations or inversions) in genes or may change the expression of a gene by placing it under the control of a different promoter (called a “position effect”).

1. Translocations — movement of DNA to a nonhomologous chromosome usually an exchange occurs between two nonhomologous chromosomes.

2. Inversions — movement of DNA within the same chromosome a 180° rotation or “flip”.

iv. Magnitude of phenotypic effect:

A. Change in mutation rate — alleles mutate at different rates some can be distinguished based on their rate of mutation.

B. Isoalleles — produce identical phenotypes in homozygous or heterozygous combinations with each other, but prove to be distinguishable when in combination with other alleles.

C. Mutants affecting viability

1. Subvitals — relative viability is greater than 10% but less than 100% compared with wild type.

2. Semilethals — cause more than 90% but less than 100% mortality.

3. Lethals — kill all individuals before adult stage.

Note # 4. Direction of Gene Mutation:

A. Forward mutation — creates a change from wild type to abnormal phenotype.

B. Reverse or back mutation — changes an altered nucleotide sequence back to its original sequence.

C. Suppressor mutations — produces a change from abnormal (i.e., mutated) phenotypes back to wild type. There are two types of suppressor mutations.

1. Intragenic suppressor — a mutation in the same gene as was originally mutated, but at a different site, that results in restoration of wild-type function (e.g., if an arginine codon CGU was originally mutated to serine codon-, AGU, the suppression causes a change back to an arginine codon, AGA also, restoration of a reading frame by additions or deletions)

2. Intergenic suppressor — a mutation in another gene that results in restoration of wild- type function (e.g., a nonsense mutation may be suppressed by a mutation in the tRNA for that codon so that it now inserts an amino acid). These are sometimes referred to as suppressor genes or extragenic suppressors.

Note # 5. Types of Gene Mutation :

i. Morphological Mutation:

This involves changes in morphology including colour, shape, size, etc., e.g., albino ascospores in Neurospora, kernel colour in corn, curly wings in Drosophila, and dwarfism in pea.

This involves genotypic changes leading to death of an individual. Example includes albino mutation resulting from chlorophyll deficiency in plants.

iii. Biochemical Mutation:

Biochemical mutations are identified by a deficiency, so that the defect can be overcome by supply­ing the nutrient or any other chemical com­pound, for which the mutant is deficient. Such mutation has been studied in bacteria and fungi, as well as blood disorders in human.

iv. Resistant Mutation:

Resistant mutations are identified by their ability to grow in pres­ence of an antibiotic (e.g., streptomycin, ampicillin, cycloheximide) or a pathogen, to which wild type is susceptible.

v. Conditional Mutation:

Conditional muta­tions are those which permit the mutant phenotype to be expressed only under cer­tain restrictive conditions (e.g., high temp.). Under normal condition termed as permis­sive condition the mutants express normal phenotype.

vi. Somatic and Germinal Mutation:

During development of organisms, mutation may occur in any cell at any stage of the cell cycle. If a mutation occurs in the somatic cell of the organism, it immediately reproduces other cells like itself resulting chimera, but not the whole organism being mutated. When the new individual develops from such cells through vegetative means, it is said to be somatic mutation.

When, however, mutation occurs in the germ cells, it can pro­duce entirely a new organism and the type of mutation is known as germinal mutation.

vii. Missense Mutation:

A missense mutation is one which results in replacement of one amino acid in a polypeptide chain by ano­ther. As a result of mutation, one base of a codon may be substituted by another base. The changed codon may then code for another amino acid.

viii. Nonsense Mutation:

Of the 64 codons 61 code for amino acids, while three are termi­nation codons which do not specify any amino acid. The three termination codons are UAA, UGA and UAG. Any mutation resulting in the alteration of a codon speci­fying an amino acid to a termination codon is called nonsense mutation. Thus if the codon UAC (for tyrosine) undergoes a one base substitution (C G) it becomes UAG, a termination codon.

A nonsense mutation leads to termination of polypeptide synthe­sis. As a result the polypeptide is incom­plete. Such chains are likely to be biologi­cally inactive. A nonsense mutation causes a relatively drastic change in the enzyme synthesized and as such likely to have a deleterious effect on the phenotype.

A mutation which does not result in phenotypic change is called a silent mutation. Silent mutations are of different types.

(a) The genetic code is degenerate, i.e., more than one codon may specify an amino acid. Therefore, when a mutated codon codes for the same amino acid as the original, there is no change in amino acid.

(b) The codon change may result in one amino acid substitution but this is not sufficient to modify the function of pro­tein appreciably.

(c) The mutation may occur in a non­functional gene.

x. Suppressor Mutation:

The effect of a muta­tion on the phenotype can be reversed so that original wild type phenotype is brought back. A second mutation at a different site neutralizes the effects of the first mutation.

xi. Spontaneous and Induced Mutation:

Mutations may arise spontaneously in nature. They may be artificially induced, or may be caused by environment agents. Induced mutations are thus resulting from exposure of organisms to mutagenic agents such as ionizing irradiation, ultraviolet light or various chemicals which react with genes. Mutations can be classified on the basis of several criteria (Table 13.1).

Note # 6. Induction of Gene Mutation:

Mutations can be artificially induced with the help of mutagenic agents or mutagens which can be broadly grouped into physical mutagens and chemical mutagens (Table 13.2).

These include various kinds of radiations including X-rays whose mutagenic effect was first demonstrated by Muller and Stadler. Radiations may be ionizing or non-ionizing. Ionizing radiations will cause ionization and will force ejection of an electron from the atom it attacks. X-rays, gamma rays, beta rays and neutrons are common ionizing radiations used for inducing mutations.

Non­-ionizing radiations like UV do not cause ioniza­tion, but cause excitation through energy trans­fer.

Auerbach in Drosophila was the first to demonstrate that mutation can be induced by certain chemicals. Later Oehelkers demonstrated the same effect in plants. At pre­sent, there are a variety of chemical substances known which cause mutations in plants and ani­mals.

Majority of chemicals, even water from certain sources may cause dis-balance in metabolism and mutation. As such, any com­mercial or medical products, before being released, are tested for the mutagenic effects. Mustard gas, EMS have been most extensively used for induction of mutation.

In addition, low pH as well as high temperature may cause mutation. Mutation rate is also influenced by aging of the organism.

Clastogens, Carcinogens and Teratogens:

Clastogens are agents causing effects include chromosomal alterations — breaks, gaps, fragments, lagging, sticky bridge, pulve­rization, stickiness, waviness, wooliness, poly­ploidy, inversion, translocation, sister chromatid exchanges and associated aberrations. Clasto­gens include both chemical (gammexene) and physical (X-ray) agents.

The clastogenic effects are often used as parameters of genotoxicity. The end points for the test of genotoxicity at the microscopic level are chromosomal changes, fragments and micronuclei, mostly observed at the metaphase and later stages.

The standard test system used in higher plants are Allium cepa, Tradescantia virginiana, Vicia faba, Hordeum vulgare and Zea mays. All clastogens, in general, are mutagens as well, but all mutagens are not necessarily clastogens. Specially those which cause point mutation like X-rays in low dosage is a mutagen whereas in high dosage it may be clastogen.

Carcinogens are a group of chemicals which causeancer in animals and humans. The carcino­gens affect DNA, preventing it from giving the necessary directions for the synthesis of sub­stances which control cell growth. Most carcino­gens act as mutagens and both kinds of effects are related to DNA damage.

Radiation and many chemical carcinogens act by damaging DNA and inducing mutations. Most common carcinogens are aflatoxin, dimethyl nitrosamine, nickel- carbonyl, benzo (α-) pyrene, α-naphthylamine, vinyl chloride, etc.

Teratoma are considered as abnormal body developments in early embryogenesis. The agents either physical (X-ray) or chemical (cocaine) which cause such abnormalities are termed as teratogens. The susceptibility to terato­gens is also a factor controlled by individual genetic system.

Ames test, developed by Bruce Ames, is a rapid inexpensive and easy test for mutagens. This is a screening method for measuring the ability of potential carcinogens to induce mutations (most carcinogens act as mutagens). He worked with a strain of Salmonella typhimurium that requires histidine to grow.

However, it will grow in presence of a carcinogenic muta­gen that causes the defective gene in histidine pathway to revert to the wild type.

Histidine auxotrophic bacteria are plated onto agar containing very little histidine and treated with substance under test. Only those bacteria that revert to the wild type, be able to synthesize histidine, will form colonies (Fig. 13.1A). Colony counting indicates the mutagenicity of the substance to be tested.

The test can be modified to detect pro-carcinogens by adding rat liver homogenates to the medium which causes biochemical modifications of the chemi­cal to become carcinogenic.

Note # 7. Molecular Basis of Gene Mutation:

The mutation may arise out of:

A. Base-pair Substitution:

Base-pair sub­stitution results in the incorporation of wrong bases during replication or repair of DNA. In base-pair changes, one base of triplet codon is substituted by another, resulting in changed codon. If the original message or reading frame is CAC GAC CAC GAC CAC, after A being substi­tuted by G in the third codon, it will be CAC GAC CGC GAC CAC.

Sickle cell anaemia in human is due to base- pair substitution, a kind of point mutation. The RBC of such individual contains an abnormal haemoglobin and are elongated filamentous sickle shaped. It is inheritable and recessive in nature.

B. Frame Shift Mutation:

A mutation in which there is deletion or insertion of one or a few nucleotides is called frame shift mutation. The name is derived from the fact that there is shift in the reading frame backward or forward by one or two nucleotides. Addition or deletion of one or two bases results in new sequence of codons which may code for entirely different amino acids and the proteins often become non­functional.

If shift involves three nucleotides, the resulting protein is with minor changes in amino acid sequence (only against the region from first base change to third base change of the reading frame). If the original message or reading frame is CAC GAC CAC GAC CAC GAC, then dele­tion of base C at seventh position will change the sequence to CAC GAC ACG ACC ACG AC.

Similarly, the insertion of base G at same position will make the message out of frame — CAC GAC GCA CCA CCA CGA C.

Note # 8. Detection of Gene Mutation:

Different methods have been devised for detection of mutations in different organisms.

A. Sex-linked Lethals Detection (Drosophila):

Lethal mutations induced on sex chromo­some of Drosophila have been detected by the following methods:

This method involves use of a GIB stock of Drosophila which carries

(i) An inversion in heterozygous state to work as crossover suppressor (C)

(ii) A recessive lethal (I) on X-chromosome in heterozygous state, and

(iii) A dominant marker Bar (B) for barred eye. One of the two X-chromosomes in a female fly carried all these 3 features and the other X chromosome was normal.

Male flies irradiated for induction of mutations were crossed to GIB females (Fig. 13.20). Male progeny receiving GIB X-chromosome will die. The GIB female flies obtained in progeny can be detected by barred phenotype.

These are crossed to normal males. In the next generation 50% of males receiving GIB X-chromosome will die. The other 50% males will receive X-chromosome which may or may not carry induced mutation. In case lethal mutation was induced, no males will be observed.

On the other hand, if no lethal mutation was induced, 50% males will survive. Thus, GIB method of Muller was the most effi­cient method for detecting sex linked lethal mutations.

Muller-5 Drosophila stock carries two marker genes-barred and apricot eye on X chromosome and a complex inversion with better crossover suppressor. Muller-5 stock when crossed with irradiated normal male, the F1 generation shows that 50% males are barred, apricot and remaining 50% are normal. But if lethal mutation is induced in X-chromosome of irradiated male, no wild male would appear.

Therefore, absence of wild type males in F2 is an indication of an induced lethal mutation (Fig. 13.21).

B. Fluctuation Test (Bacteria):

Variations occurring in bacteria, e.g., resistance to phage or antibiotics is due to genetic changes through mutation or due to adaptation to environmental condition, was confirmed by fluctuation test carried out by Luria and Delbruck. They allowed the growth of E. coli cells (10 3 cells per ml) in two sets: independent culture – 40 tubes each with aliquots of 0.5 ml bulk culture – one tube with 20 ml.

After an incubation of 36 hours at 37°C small aliquots (0.1 ml) from each tube of the independent cultures, as well as bulk culture were spread over a large number of replica plates coated with T, phage.

Number of phage-resistant colonies growing on each plate was counted which revealed a much greater fluctuation (i.e., a wider variation) exists among the plates prepared from independent cultures than the plates prepared from bulk culture.

The greater fluctuation in independent cultures is mainly due to origin of spontaneous mutation arising independently in different tubes at diffe­rent times during growth.

The number of each resistant mutant arising at different times in inde­pendent culture multiplied during incubation and final number of resistant bacteria in different tubes was widely variable at the time of plating (before coming in contact with phage).

In contrast, the bulk culture contained a uniform population of both sensitive and resistant bacte­ria at the time of plating, i.e., before the bacteria come in contact with phage. Development of resistance due to adaptation will occur only after the bacteria come in contact with phage. This experiment thus proved that resistance appeared due to random mutation, not due to physiological adaptation (Fig. 13.22).

Note # 9. Importance of Gene Mutation:

Mutation is the major source of genetic variation it provides raw mate­rial for evolution. Without mutation all genes will exist in only one form, alleles would not exist. Different organisms would not be able to evolve and adapt to environmental changes.

(b) Application in Plant Breeding:

Mutations are normally deleterious. Gustaffsson estimated that less than one in thousand mutants produced, may be useful in plant breeding. Several important mutants have, however, been obtained in different crops.

(i) In wheat, several useful mutations, viz, branched ears, lodging resistance, amber seed colour and awned spikelet were obtained and utilized in plant breeding. The most remarkable mutation obtained by Swaminathan is Sharbati Sonora. Other important varieties released in India are Pusa Lerma, NP 836.

(ii) In rice, several high yielding elite vari­eties – Reimei, Japonica, Indica have been obtained through mutations. Mutants were also obtained in rice for increased protein and lysine content. Jagannath, I/T48, l/TGO are the products of induced mutation in India.

(iii) In barley, mutant known as erectoides is of high yield. RBD-1, DL-253 are induced mutants in India.

(iv) In Legumes, Hans-pea, Ranjan-lentil, MUM 2-mung bean are mutants developed in India.

(v) Other important mutant varieties relea­sed in India are S 12-tomato, Rasmi-cotton, RLM 514-mustard, Co997-sugarcane, JRC 7447-Jute.

Regular survey by joint FAO and IAEA reported that there has been a highly significant increase in the number of mutant varieties deve­loped in different crops.

(c) Another valuable application of induced mutation is the increased production of antibio­tics, such as penicillin from species of Penicillium.

(d) Somatic mutations have also been found useful in many ornamentals. In tissue cul­ture as well, several somaclonal mutants leading to somatic mutants have been obtained in horti­cultural species.

Clinical Significance

Antibiotic Resistance

Antibiotics work through a variety of mechanisms:[17]

When an antibiotic loses theꃊpacity to kill or control bacterial growth, antibiotic resistance occurs. This can occur in two ways:

These circumstances exacerbate under selective pressure (i.e., the use of antibiotics). Antibiotic resistance can spread both vertically and horizontally through a population. Horizontal transfer is considered the primary mediator of antibiotic resistance. The following are non-comprehensive examples of how two of the classes of antibiotics mentioned above encounter resistance mutations.

DNA Synthesis Inhibitor

In gram-negative bacteria, such as Helicobacter pylori, mutation resistance occurs relatively quickly to fluoroquinolones and thereby poses clinical issues for these therapies. Levofloxacin, moxifloxacin, and ciprofloxacin, examples of fluoroquinolones, inhibit DNA synthesis by targeting two homologous enzymes (DNA topoisomerase II and IV).[18] These enzymes are necessary for the supercoiling of bacterial DNA.  

Gram-negative bacterial resistance to fluoroquinolones includes the accumulation of substitution mutations in the coding regions for particular subunits of DNA topoisomerase II. Resistance can be enhanced further by efflux pump modification.[19]਌iprofloxacin targets only the parC subunit while other quinolones target one or more of these subunits.[20]ਏor example, garenoxacin targets both DNA topoisomerases II and IV thus is less prone to resistance. Resistance to Garenoxacin requires both proteins to have resistance mutations.[21]

Combination therapy for Helicobacter pylori typically includes clarithromycin (protein synthesis inhibitor), metronidazole (DNA synthesis inhibitor), amoxicillin (Cell wall synthesis inhibitor), or tetracycline (protein synthesis inhibitor), and a proton pump inhibitor.[22][23][24]

Protein Synthesis Inhibitor

Linezolid prevents protein synthesis and is active against resistant Gram-positives.[25] Linezolid inhibits the formation of the 70S ribosomal initiation complex through binding to the 23S portion of the 50S subunit.[26] Infrequent resistance found in strains of S.ਊureus,ਊnd coagulase-negative staphylococci has mutations in the central loop of the domain V region of the 23S rRNA gene. More specifically,਌linical isolates had a substitution of Thymine for Guanine at the 2576 position.[27][28]

Intrinsically, resistant bacteria have a characteristic resistance within all members of a species or genus. Such resistance may arise because:

Antibiotic resistance mechanisms can also occur by incorporating resistance genes into plasmids, transposons, and integrons. These genes spread through horizontal transfer by conjugation, transformation, or transduction mechanisms. However, the mutation is essential for the evolution or assortment of these genes.

Single-strand viruses show higher mutation rates than double-strand viruses

Single-strand DNA viruses tend to mutate faster than double-strand DNA viruses, although this difference is based on work with bacteriophages, as no mutation rate estimates have been obtained for eukaryotic single-strand DNA viruses [1]. Within RNA viruses, there are no obvious differences in mutation rate among Baltimore classes (Fig.  2 a). The mechanisms underlying these differences are not well understood. One possible explanation for the differences between single and double-strand viruses is that single-strand nucleic acids are more prone to oxidative deamination and other types of chemical damage. Elevated levels of reactive oxygen species (ROS) and other cellular metabolites during viral infections can induce mutations in the host cell and in the virus. For instance, ethanol is likely to synergize with virus-induced oxidative stress to increase the mutation rate of HCV [21]. Differences among single- and double-strand DNA viruses may also be explained in terms of their access to post-replicative repair. Work with bacteriophage ϕX174 has provided interesting clues on this issue. In enterobacteria, methyl-directed mismatch repair (MMR) is performed by MutHLS proteins and Dam methylase. Dam methylation of GATC sequence motifs is used to differentiate the template and daughter DNA strands and is thus required to perform mismatch correction [22]. Mismatches are recognized by MutS, which interacts with MutL and leads to the activation of the MutH endonuclease, which excises the daughter strand. However, the genome of bacteriophage ϕX174 has no GATC sequence motifs, even if approximately 20 such sites are expected by chance. As a result, the ϕX174 DNA cannot undergo MMR. This contributes to explaining the relatively high mutation rate of this virus, which falls on the order of 10 𢄦  s/n/c, a value three orders of magnitude above that of Escherichia coli and highest among DNA viruses [23]. Avoidance of GATC motifs may be a consequence of selection acting on mutation rate, but also of other selective factors. For instance, inefficient methylation of the phage DNA may render it susceptible to cleavage by MutH, therefore imposing a selection pressure against GATC sequence motifs [24].

As opposed to bacteriophage ϕX174, the link between post-replicative repair and mutation rate is still unclear in eukaryotic viruses. Numerous studies have shown that viruses interact with DNA damage response (DDR) pathways by altering the localization or promoting the degradation of DDR components [25, 26]. For instance, the adenoviral E4orf6 protein promotes proteasomal degradation of TOPBP1, a DDR component [27]. DDR activation can occur as an indirect consequence of cellular stress due to the infection per se or as a part of an antiviral response, which would be in turn counteracted by viruses. Although DNA viruses tend to promote genomic instability in the host cell, it remains to be shown whether DDR dysregulation can determine DNA virus mutation rates.

Can you detect if a mutation is spontaneous or induced? - Biology

Most mistakes during replication are corrected by DNA polymerase during replication or by post-replication repair mechanisms.

Learning Objectives

Explain how errors during replication are repaired

Key Takeaways

Key Points

  • Mismatch repair enzymes recognize mis-incorporated bases, remove them from DNA, and replace them with the correct bases.
  • In nucleotide excision repair, enzymes remove incorrect bases with a few surrounding bases, which are replaced with the correct bases with the help of a DNA polymerase and the template DNA.
  • When replication mistakes are not corrected, they may result in mutations, which sometimes can have serious consequences.
  • Point mutations, one base substituted for another, can be silent (no effect) or may have effects ranging from mild to severe.
  • Mutations may also involve insertions (addition of a base), deletion (loss of a base), or translocation (movement of a DNA section to a new location on the same or another chromosome ).

Key Terms

  • mismatch repair: a system for recognizing and repairing some forms of DNA damage and erroneous insertion, deletion, or mis-incorporation of bases that can arise during DNA replication and recombination
  • nucleotide excision repair: a DNA repair mechanism that corrects damage done by UV radiation, including thymine dimers and 6,4 photoproducts that cause bulky distortions in the DNA

Errors during Replication

DNA replication is a highly accurate process, but mistakes can occasionally occur as when a DNA polymerase inserts a wrong base. Uncorrected mistakes may sometimes lead to serious consequences, such as cancer. Repair mechanisms can correct the mistakes, but in rare cases mistakes are not corrected, leading to mutations in other cases, repair enzymes are themselves mutated or defective.

Mutations: In this interactive, you can “edit” a DNA strand and cause a mutation. Take a look at the effects!

Most of the mistakes during DNA replication are promptly corrected by DNA polymerase which proofreads the base that has just been added. In proofreading, the DNA pol reads the newly-added base before adding the next one so a correction can be made. The polymerase checks whether the newly-added base has paired correctly with the base in the template strand. If it is the correct base, the next nucleotide is added. If an incorrect base has been added, the enzyme makes a cut at the phosphodiester bond and releases the incorrect nucleotide. This is performed by the exonuclease action of DNA pol III. Once the incorrect nucleotide has been removed, a new one will be added again.

DNA polymerase proofreading: Proofreading by DNA polymerase corrects errors during replication.

Some errors are not corrected during replication, but are instead corrected after replication is completed this type of repair is known as mismatch repair. The enzymes recognize the incorrectly-added nucleotide and excise it this is then replaced by the correct base. If this remains uncorrected, it may lead to more permanent damage. How do mismatch repair enzymes recognize which of the two bases is the incorrect one? In E. coli, after replication, the nitrogenous base adenine acquires a methyl group the parental DNA strand will have methyl groups, whereas the newly-synthesized strand lacks them. Thus, DNA polymerase is able to remove the incorrectly-incorporated bases from the newly-synthesized, non-methylated strand. In eukaryotes, the mechanism is not very well understood, but it is believed to involve recognition of unsealed nicks in the new strand, as well as a short-term continuing association of some of the replication proteins with the new daughter strand after replication has been completed.

Mismatch Repair: In mismatch repair, the incorrectly-added base is detected after replication. The mismatch-repair proteins detect this base and remove it from the newly-synthesized strand by nuclease action. The gap is now filled with the correctly-paired base.

In another type of repair mechanism, nucleotide excision repair, enzymes replace incorrect bases by making a cut on both the 3′ and 5′ ends of the incorrect base. The segment of DNA is removed and replaced with the correctly-paired nucleotides by the action of DNA pol. Once the bases are filled in, the remaining gap is sealed with a phosphodiester linkage catalyzed by DNA ligase. This repair mechanism is often employed when UV exposure causes the formation of pyrimidine dimers.

DNA Ligase I Repairing Chromosomal Damage: DNA damage, due to environmental factors and normal metabolic processes inside the cell, occurs at a rate of 1,000 to 1,000,000 molecular lesions per cell per day. A special enzyme, DNA ligase (shown here in color), encircles the double helix to repair a broken strand of DNA. DNA ligase is responsible for repairing the millions of DNA breaks generated during the normal course of a cell’s life. Without molecules that can mend such breaks, cells can malfunction, die, or become cancerous. DNA ligases catalyse the crucial step of joining breaks in duplex DNA during DNA repair, replication and recombination, and require either Adenosine triphosphate (ATP) or Nicotinamide adenine dinucleotide (NAD+) as a cofactor.

Nucleotide Excision Repairs: Nucleotide excision repairs thymine dimers. When exposed to UV, thymines lying adjacent to each other can form thymine dimers. In normal cells, they are excised and replaced.

DNA Damage and Mutations

Errors during DNA replication are not the only reason why mutations arise in DNA. Mutations, variations in the nucleotide sequence of a genome, can also occur because of damage to DNA. Such mutations may be of two types: induced or spontaneous. Induced mutations are those that result from an exposure to chemicals, UV rays, X-rays, or some other environmental agent. Spontaneous mutations occur without any exposure to any environmental agent they are a result of natural reactions taking place within the body.

Mutations may have a wide range of effects. Some mutations are not expressed these are known as silent mutations. Point mutations are those mutations that affect a single base pair. The most common nucleotide mutations are substitutions, in which one base is replaced by another. These can be of two types: transitions or transversions. Transition substitution refers to a purine or pyrimidine being replaced by a base of the same kind for example, a purine such as adenine may be replaced by the purine guanine. Transversion substitution refers to a purine being replaced by a pyrimidine or vice versa for example, cytosine, a pyrimidine, is replaced by adenine, a purine. Mutations can also be the result of the addition of a base, known as an insertion, or the removal of a base, known as a deletion. Sometimes a piece of DNA from one chromosome may get translocated to another chromosome or to another region of the same chromosome.

Spontaneous and ultraviolet-induced mutations on a single-stranded shuttle vector transfected into monkey cells

The shuttle vector plasmid PCF3A, carrying the supF target gene, can be transfected into monkey COS7 cells as single-stranded or double-stranded DNA. Single strand-derived plasmid progeny exhibited a 10-fold higher spontaneous mutation frequency that double strand-derived progeny. The location of spontaneous mutations obtained after transfection of the single-stranded vector shared similarities with that for double-stranded vectors. However, the nature of base changes was very different. Single-stranded PCF3A DNA was used to study ultraviolet-induced mutagenesis. An earlier report (Madzak and Sarasin, J. Mol. Biol., 218 (1991) 667–673) showed that single-stranded DNA exhibited a lower survival and a higher mutation frequency than double-stranded DNA after ultraviolet irradiation. In the present report, sequence analysis of mutant plasmids is presented. The use of a single-stranded vector allowed us to show the targeting of mutations at putative lesion sites and to determine the exact nature of the base implicated in each mutation. Frameshift mutations were more frequent after transfection of control on irradiated plasmid as single-stranded DNA than as double-stranded DNA. Multiple mutations, observed at a high frequency in the spontaneous and ultraviolet-induced mutation spectra following single-stranded DNA transfection, could be due to an error-prone polymerisation step acting on a single-stranded template.

D. Size and Quality

According to size following two types of mutations have been recognized:

1. Point mutation

When heritable alterations occur in a very small segment of DNA molecule, i.e., a single nucleotide or nucleotide pair, then this type of mutations are called “point mutations”. The point mutations may occur due to following types of subnucleotide change in the DNA and RNA.

– Deletion mutations. The point mutation which is caused due to loss or deletion of some portion (single nucleotide pair) in a triplet codon of a cistron or gene is called deletion mutation.

– Insertion or addition mutation. The point mutations which occur due to addition of one or more extra nucleotides to a gene or cistron are called insertion mutations.

The mutations which 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 a nucleotide of a triplet is replaced by another nucleotide, is called substitution mutation.

2. Multiple mutations or gross mutations.

When changes involving more than one nucleotide pair, or entire gene, then such mutations are called gross mutations. The gross mutations occur due to rearrangements of genes within the genome. It may be:

  1. The rearrangement of genes may occur within a gene. Two mutations within the same functional gene can produce different effects depending on gene whether they occur in the cis or trans position.
  2. The rearrangement of gene may occur in number of genes per chromosome. If the numbers of gene replicas are non-equivalent on the homologous chromosomes, they may cause different types of phenotypic effects over the organisms.
  1. Due to movement of a gene locus new type of phenotypes may be created, especially when the gene is relocated near heterochromatin. The movement of gene loci may take place due to following method:

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

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

Can you detect if a mutation is spontaneous or induced? - Biology

10 9 his - Salmonella bacteria, which cannot grow in the absence of the amino acid histidine . In this control experiment, the small number (

10 2 ) of white colonies are derived from single bacteria that have undergone spontaneous reversion mutations to his + . The reversion test is thus extremely sensitive, because it can detect mutation rates as small as 1/10 9 - 2 = 10 -7 / cell.

In the experiment on the right, the disc at the center of the dish contains a mutagenic chemical. As it diffuses outward, the chemical at high concentration is toxic and at first kills all the bacteria (clear circle around the disc), but at lower concentrations gives rise to induced reversion mutations , seen as closely-packed revertant ( his + ) colonies. As the concentration continues to decrease towards the outer periphery of the plate, the frequency of revertant colonies falls to about the same as in the control experiment at left. In cell culture, it is therefore possible to measure the precise degree of mutagenicity at a range of concentrations.

The Ames Test uses the bacterial reversion assay to measure mutagenicity as the difference between the induced and spontaneous rates of reversion mutation at various concentrations of the mutagenic substance.