Mutation MCQ Quiz in தமிழ் - Objective Question with Answer for Mutation - இலவச PDF ஐப் பதிவிறக்கவும்

The correct answer is Frameshift

Explanation:

• A frameshift mutation occurs when there is an insertion or deletion (indel) of nucleotides that are not in multiples of three, altering the reading frame of the genetic code.
• This leads to a complete shift in how the codons are read during translation, potentially altering all downstream amino acids or introducing premature stop codons.
• Comparing the sequencing gels of the wild type (Figure A) and the mutant (Figure B), there is a clear shift in the alignment of bands after a specific point. This is indicative of an insertion or deletion, which causes a change in the reading frame.
• This disruption continues throughout the sequence, consistent with the nature of a frameshift mutation.

Other Options:

• Nonsense Mutation: A nonsense mutation introduces a premature stop codon, truncating the protein. This would result in a shorter sequence on the gel. The gel shows a continued sequence with a shift in the pattern, rather than a premature truncation.
• Missense Mutation: A missense mutation results in a single base substitution, leading to one amino acid being replaced by another. The gel shows multiple shifted bands, indicating a frameshift caused by insertion or deletion rather than a single substitution.
• Transversion: A transversion mutation is a point mutation where a purine (A or G) is replaced by a pyrimidine (C or T), or vice versa. A transversion affects only a single base pair, which would appear as a single difference between the wild type and mutant sequences. The gel demonstrates a shift in multiple bands, 

The correct answer is A, B and C

Concept:

• Translocation refers to exchange of chromosomal segments between two non-homologous chromosomes. It can be of two types: reciprocal and non-reciprocal.
  • Non-reciprocal translocation involves the transfer of segment in one direction from one chromosome to another. 
  • Reciprocal translocation involves exchange of the segments of chromosomes between non-homologous chromosomes, this results in the generation of two translocated chromosomes simultaneously. 

Explanation:

Types of Segregation: The three modes of segregation (adjacent 1, adjacent 2, and alternate) occur in reciprocal translocation heterozygotes.

• Alternate Segregation: Opposite chromosomes (diagonally) in the cross segregate into the same gamete. This leads to viable gametes, as these gametes either contain both normal chromosomes or both translocated chromosomes.
• Adjacent 1 Segregation: Adjacent chromosomes segregate vertically, resulting in one normal and one translocated chromosome per gamete. This often leads to non-viable gametes due to duplications and deletions of genetic material.
• Adjacent 2 Segregation: Adjacent chromosomes segregate horizontally, resulting in one normal and one translocated chromosome per gamete. This also leads to non-viable gametes because of genetic imbalances.

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Dicentric (two centromeres) and acentric (no centromere) chromosomes are not a consequence of reciprocal translocations during alternate segregation. Instead, they occur during structural rearrangements like inversion. Thus, statement D is incorrect.

The correct answer is C and D.

Concept:

• Synonymous Mutations: These mutations occur in a DNA sequence but do not change the amino acid sequence of the protein that is produced. This occurs because of the redundancy in the genetic code, where multiple codons can encode the same amino acid. Since the resulting protein remains unchanged, synonymous mutations are often considered neutral, meaning they neither benefit nor harm the organism.
• Non-Synonymous Mutations: These mutations result in a change in the amino acid sequence of the protein, which can alter the protein's structure and function. Non-synonymous mutations are further divided into two types:
  • Missense Mutations: These mutations change a single amino acid in the protein sequence.
  • Nonsense Mutations: These mutations introduce a premature stop codon, leading to a truncated and often non-functional protein.

Explanation:

Purifying selection, also known as negative selection, is a type of natural selection that acts to eliminate deleterious mutations from a population. Under purifying selection, mutations that are harmful to the organism are selected against, while neutral or beneficial mutations persist.
A. Most non-synonymous mutations are selected against.

• Non-synonymous mutations result in a change in the amino acid sequence of proteins. Many such changes can impair protein function, thus, under purifying selection, most non-synonymous mutations are indeed selected against to maintain protein functionality.
• Under purifying selection, these harmful mutations are typically removed from the population
• True under purifying selection: Yes, this statement is correct for purifying selection.

B. Synonymous mutations can accumulate.

• Synonymous mutations do not change the amino acid sequence of proteins due to the redundancy of the genetic code. Since they do not affect the protein function, they are generally neutral and can accumulate over time because they are not subject to strong selective pressure.
• True under purifying selection: Yes, this statement is correct for purifying selection.

C. The ratio of non-synonymous to synonymous substitutions is high.

• Under purifying selection, non-synonymous mutations (which could alter protein function and often reduce organism fitness) are selected against much more strongly than synonymous mutations. Thus, the ratio of non-synonymous to synonymous substitutions is expected to be low.
• True under purifying selection: No, this statement does not fit the expectations of purifying selection.

D. Non-synonymous sites accumulate mutations at higher rates.

• Given that non-synonymous mutations are often deleterious, purifying selection acts to remove them, leading to a lower rate of accumulation compared to synonymous mutations.
• True under purifying selection: No, this statement is incorrect for purifying selection.

Conclusion
Under purifying selection, harmful mutations, particularly those that change the amino acid sequence of proteins, are removed from the population. Therefore, The correct option that is NOT true about sequence evolution under purifying selection is:

• C. The ratio of non-synonymous to synonymous substitutions is high.
• D. Non-synonymous sites accumulate mutations at higher rates.

The Correct answer is Population size

Explanation:

The mutation rate equation takes into consideration the number of observed mutations and the total number of opportunities for mutations to occur. The factors influencing the opportunities for mutations include:

• Generation time: Shorter generation times can result in more frequent cell divisions, thereby increasing the opportunities for mutations.
• DNA repair efficiency and replication fidelity: High efficiency in DNA repair and high fidelity in DNA replication can reduce the number of mutations that occur, thereby influencing mutation opportunities.
• Exposure to mutagens: Increased exposure to mutagens (e.g., UV light, chemicals) can increase the probability of mutations occurring.

However, population size does not influence the intrinsic opportunities for mutations to occur within an organism’s genome. While larger population sizes may lead to more total mutations observed at the population level, the rate at which mutations occur per genome per generation is not directly affected by population size. Therefore, population size does not influence the mutation rate in the context of opportunities for individual mutations.

Key Points

• Generation Time: Influences opportunities for mutations by affecting the frequency of cell divisions.
• DNA Repair Efficiency and Replication Fidelity: Reduce or increase the error rate during DNA replication.
• Exposure to Mutagens: Directly influences the likelihood of mutations occurring by causing DNA damage.
• Population Size: Does not affect the opportunities for mutations within an individual organism’s genome. Although a larger population may show more mutations overall, it does not increase the mutation rate per individual.
• Nutrient Availability: Can influence cellular stress and metabolic activity, potentially affecting mutation rates.
• Environmental Factors: Conditions such as temperature and radiation levels can impact the likelihood of mutations.
• Genetic Background: Certain genetic variations can make some genomes more prone to mutations.
• Replication Machinery Variability: Differences in the fidelity of the enzymes involved in DNA replication can influence mutation rates.
• Cell Cycle Regulation: Proper control of cell cycle checkpoints can reduce the occurrence of mutations.
• Oxidative Stress: High levels of reactive oxygen species can damage DNA, increasing the mutation rate.

The correct answer is recessive, dominant

Explanation:

Loss-of-function mutations in enzymes:

• Enzymes are catalytic proteins, and in most cases, a single functional allele is sufficient to produce enough enzyme activity to meet the cellular or physiological requirements.
• When one allele is non-functional due to a loss-of-function mutation, the remaining functional allele compensates for the loss. This is why such mutations are typically autosomal recessive.
• Examples: Phenylketonuria (PKU) (Loss-of-function mutation in the phenylalanine hydroxylase (PAH) enzyme) and Tay-Sachs disease (Loss of hexosaminidase A enzyme function)

Loss-of-function mutations in structural proteins:

• Structural proteins, such as those in the cytoskeleton (e.g., collagen or keratin), often function as part of larger complexes or assemblies.
• When one allele is mutated, the defective structural protein may be incorporated into the complex, disrupting its overall function. This phenomenon is known as a dominant-negative effect.
• Therefore, mutations affecting structural proteins are typically autosomal dominant.
• Examples: Osteogenesis imperfecta (collagen mutations) and Marfan syndrome (fibrillin mutations).

The correct answer is ethylmethane sulfonate

Explanation:

• EMS is an alkylating agent that induces mutations by adding ethyl groups to DNA bases, particularly guanine.
• This modification can lead to improper base pairing during DNA replication.
• Specifically, the alkylated guanine can mispair with thymine instead of cytosine, leading to a G-C to A-T transition mutation upon DNA replication.
• This type of mutation alters the base pairing in the DNA, potentially causing significant effects on gene function or protein expression.

Additional Information

• Proflavin and Acridine Dye: These are intercalating agents that insert themselves into the DNA structure, distorting the helix and causing frameshift mutations (additions or deletions of nucleotide bases), not base pair substitutions.
• Ethidium Bromide: Also an intercalating agent like proflavin and acridine dye, ethidium bromide inserts between DNA base pairs which can lead to frameshift mutations. Though known for its use in laboratory as a nucleic acid stain, it does not specifically cause a G-C to A-T transition.

The correct answer is Option 3

Explanation:

Ser → Cys (Serine to Cysteine)

• Serine (Ser) consists of a chain with a hydroxymethyl group attached to it, making it C₃H₇NO₃.
• Cysteine (Cys) has a thiol side chain, making it C₃H₇NO₂S.
• Difference: They have a relatively small difference in atomic composition, mainly differing by the presence of sulfur in cysteine instead of an oxygen in serine.

Tyr → Phe (Tyrosine to Phenylalanine)

• Tyrosine (Tyr) is C₉H₁₁NO₃ due to its phenol side chain.
• Phenylalanine (Phe) is C₉H₁₁NO₂, lacking the hydroxyl group present in tyrosine.
• Difference: The main difference is the absence of an oxygen atom in phenylalanine.

Lys → Ala (Lysine to Alanine)

• Lysine (Lys) is a longer amino acid, C₆H₁₄N₂O₂, with an amine group in its side chain.
• Alanine (Ala) is much smaller, C₃H₇NO₂.
• Difference: The transition from lysine to alanine removes a significant portion of the molecule, resulting in a loss of 3 carbons, 7 hydrogens, and 1 nitrogen, which is the largest change among the options provided.

Arg → Lys (Arginine to Lysine)

• Arginine (Arg) is C₆H₁₄N₄O₂, notable for its guanidinium group.
• Lysine (Lys), as mentioned, is C₆H₁₄N₂O₂.
• Difference: Changing arginine to lysine removes 2 nitrogen atoms, which is significant but not the largest change in terms of total number of atoms.

Conclusion:

Therefore, the mutation from Lys to Ala (Lys → Ala) results in the largest difference in terms of the number of atoms between the wild type and the mutant amino acid. 

The correct answer is None of these

Explanation:

• Statement 1: Correct. Intercalating agents, such as ethidium bromide, insert themselves between the bases of DNA strands. This can lead to insertions or deletions during DNA replication, commonly resulting in frameshift mutations.
• Statement 2: Correct. The Luria-Delbrück fluctuation test was a landmark experiment that demonstrated mutations arise in bacteria spontaneously and not as a result of exposure to a selective agent. It provided strong evidence for the random occurrence of mutations.
• Statement 3: Correct. A tautomer is indeed an alternative form of a molecule that differs by the position of a single hydrogen atom and the rearrangement of single and double bonds. In the context of nucleotide bases, tautomeric shifts can lead to incorrect base pairing during DNA replication, which can result in mutation.
• Statement 4: Correct. A missense mutation occurs when a single nucleotide change results in the substitution of one amino acid for another in the resulting protein. This can affect the protein’s structure and function but does not always do so deleteriously.

The correct answer is Option 1

Explanation:

The suppression of the mutant phenotype, known as a suppressor mutation, is a genetic event that restores the wild-type phenotype in the presence of a primary mutation. The correct mechanism by which a suppressor mutation typically operates is by misreading of the mutant codon and incorporation of a correct amino acid.

• This describes a mechanism that can occur with certain types of suppressor mutations, specifically tRNA suppressor mutations. These mutations occur in tRNA genes and can lead to the incorporation of the correct amino acid at positions where the mutant mRNA specifies a wrong amino acid, due to the altered tRNA's ability to recognize the mutated codon as if it were the wild-type codon. This can partially or completely restore the function of the protein and hence the wild-type phenotype.
• Option 2, "by insertion of another normal copy of the gene," refers to gene duplication or addition techniques, which don't necessarily represent the classical definition of a suppressor mutation.
• Option 3, "by reversion of the mutation to wild type," is describing a true revertant rather than a suppressor mutation. In a revertant, the original mutation is undone by a second mutation at the same site or a closely linked site, restoring the wild-type sequence and phenotype.
• Option 4, "by deletion of the mutant gene," might in rare cases restore a wild-type phenotype if the deletion removes a dominant negative mutation or a toxic gene product, but it is not a typical suppressor mutation mechanism, which generally operates by compensating at the level of gene expression or protein function rather than removing the mutant gene.

The correct answer is Option 4

Explanation:

• Ethidium bromide acts as a mutagen primarily through intercalating between DNA bases, interfering with proper base stacking.
• Ethidium bromide is a flat, planar molecule that can slide between the stacked base pairs at the core of the DNA double helix. This intercalation process distorts the DNA structure by unwinding the double helix and extending the distance between adjacent base pairs.

The insertion of ethidium bromide into DNA:-

• Affects DNA Replication and Transcription: The altered DNA structure can lead to errors during DNA replication and transcription because the DNA polymerase and RNA polymerase enzymes may incorporate incorrect nucleotides opposite the distorted sites.
• Fluorescence Upon Binding: Ethidium bromide is fluorescent when intercalated into DNA, a property that researchers exploit to visualize DNA in gel electrophoresis assays.

Additional Information(a) Substituting adenine by its structural analogue: This describes a mechanism characteristic of specific chemical mutagens like purine analogs (e.g., 2-aminopurine) that can replace adenine or guanine in DNA, causing incorrect base pairing. Ethidium bromide does not substitute for DNA bases.

(b) Chemical modification of base: Some chemical mutagens directly modify the chemical structure of DNA bases, leading to mispairing. For example, alkylating agents add alkyl groups to bases, altering their pairing properties. Ethidium bromide does not chemically modify DNA bases.

(c) Production of interstrand cross-links in DNA: Certain mutagens and chemotherapeutic agents (e.g., cisplatin) can cause covalent links between the two strands of DNA, hindering separation of strands and thus DNA replication and transcription. Ethidium bromide does not create such cross-links.