Epigenetics and DNA Methylation — MCQs
Sumoylation of histone proteins is associated with
Phenotypic expression of a gene depending on the parent of origin is referred to as:
Deamination of methylated cytosine forms which of the following?
Which of the following statements about RNA capping is INCORRECT?
Differential expression of the same gene depending on parent of origin is referred to as
Which of the following statements about gene therapy is false?
Which one of the following statements about chromatin is not true?
Assertion: DNA methylation leads to gene silencing. Reason: Methylation prevents binding of transcription factors to promoter regions.
Effect of environment on genes is called?
DNA Methylation is not related to?
Epigenetics and DNA Methylation Indian Medical PG Practice Questions and MCQs
Question 1: Sumoylation of histone proteins is associated with
- A. Activation of gene transcription
- B. Condensation of chromosome
- C. Transcription repression (Correct Answer)
- D. DNA replication
Explanation: ***Transcription repression*** - **Sumoylation** is a post-translational modification involving the covalent attachment of **Small Ubiquitin-like Modifier (SUMO) proteins** to target proteins, which leads to transcriptional repression. - When histones are sumoylated, it alters chromatin structure and recruits **transcriptional corepressors**, making the DNA less accessible for transcription factors. - This is the **primary and well-established function** of histone sumoylation in gene regulation. *Activation of gene transcription* - **Histone acetylation** and specific methylation patterns (e.g., H3K4me3, H3K36me3) are associated with **transcriptional activation**, not sumoylation. - Sumoylation typically creates a repressive chromatin environment, hindering gene expression. *Condensation of chromosome* - While sumoylation can influence chromatin structure, **chromosome condensation** during cell division is primarily regulated by **condensins** and **cohesins**. - Sumoylation's role in condensation is indirect and not its primary function. *DNA replication* - DNA replication is a separate process from transcriptional regulation and involves DNA polymerases and replication machinery. - Histone sumoylation specifically affects **gene transcription**, not DNA replication.
Question 2: Phenotypic expression of a gene depending on the parent of origin is referred to as:
- A. Genomic imprinting (parent-of-origin gene expression) (Correct Answer)
- B. Mosaic genetic variation
- C. Nonpenetrance of genotype
- D. Genetic anticipation
Explanation: ***Genomic imprinting (parent-of-origin gene expression)*** - **Genomic imprinting** is an epigenetic phenomenon where gene expression is dependent on whether the gene was inherited from the mother or the father. - This results in monoallelic expression of specific genes, with only one copy (maternal or paternal) being active. *Mosaic genetic variation* - **Mosaicism** refers to the presence of two or more populations of genetically different cells in one individual, all derived from a single zygote. - This typically arises from a somatic mutation during development, not from differential expression based on parental origin. *Nonpenetrance of genotype* - **Nonpenetrance** occurs when individuals carrying a disease-causing genotype do not express the associated phenotype. - This concept relates to the presence or absence of a phenotype, not the differential expression based on parental origin. *Genetic anticipation* - **Genetic anticipation** is the phenomenon where the symptoms of a genetic disorder become more severe and/or appear at an earlier age in successive generations. - This is commonly observed in disorders caused by expansions of trinucleotide repeats, such as Huntington's disease, and is distinct from parent-of-origin gene expression.
Question 3: Deamination of methylated cytosine forms which of the following?
- A. Uracil
- B. Thymine (Correct Answer)
- C. Cytosine
- D. Guanine
Explanation: ***Thymine*** - Deamination of **5-methylcytosine** removes the amine group at the 4-position and replaces it with a keto group, forming **thymine**. - This reaction can lead to a common type of point mutation, as the DNA repair machinery might fail to distinguish this naturally occurring base from normal thymine. *Uracil* - **Uracil** is formed by the deamination of **unmethylated cytosine**, not methylated cytosine. - Uracil is a base found in RNA but not typically in DNA, so its presence in DNA signals a repair event. *Cytosine* - **Cytosine** is the original base before deamination occurs; deamination is a chemical modification that changes cytosine into another base. - If a base remains cytosine, it means deamination has not taken place. *Guanine* - **Guanine** is a purine base and is structurally unrelated to cytosine or its deamination products. - Deamination primarily affects pyrimidine bases like cytosine and uracil, not purines like guanine.
Question 4: Which of the following statements about RNA capping is INCORRECT?
- A. RNA capping occurs in the cytoplasm after transcription is complete. (Correct Answer)
- B. RNA capping occurs in the nucleus and is co-transcriptional.
- C. RNA capping involves the addition of a cap structure at the 5' end of the RNA molecule.
- D. S-adenosyl methionine (SAM) acts as a methyl donor
Explanation: ***RNA capping occurs in the cytoplasm after transcription is complete.*** - **RNA capping** occurs in the **nucleus**, not the cytoplasm, making this statement incorrect. - The process is **co-transcriptional**, occurring during transcription rather than after it is complete. *RNA capping occurs in the nucleus and is co-transcriptional.* - This is a **correct statement** - RNA capping takes place in the **nucleus** of eukaryotic cells. - It occurs **co-transcriptionally**, beginning when the nascent RNA chain is about **20-40 nucleotides** long. *RNA capping involves the addition of a cap structure at the 5' end of the RNA molecule.* - This is a **correct statement** - the **7-methylguanosine cap** is indeed added to the **5' end** of mRNA. - The cap structure is linked via a **5' to 5' triphosphate bridge** to protect against degradation. *S-adenosyl methionine (SAM) acts as a methyl donor* - This is a **correct statement** - **SAM** serves as the **methyl donor** for cap methylation reactions. - **SAM** provides methyl groups for the **N7-methylguanosine** and **2'-O-methylation** of the first transcribed nucleotide.
Question 5: Differential expression of the same gene depending on parent of origin is referred to as
- A. Mosaicism
- B. Nonpenetrance
- C. Anticipation
- D. Genomic imprinting (Correct Answer)
Explanation: ***Genomic imprinting*** - This phenomenon describes the differential expression of a gene based on its **parent of origin**, meaning that the gene is expressed only from the allele inherited from a specific parent (either maternal or paternal). - This differential expression occurs without altering the underlying DNA sequence and is often mediated by **epigenetic mechanisms** such as DNA methylation. *Mosaicism* - This refers to the presence of **two or more populations of cells** with different genotypes within a single individual who has developed from a single fertilized egg. - It does not involve differential expression of the same gene based on parental origin, but rather **genetic differences arising after fertilization**. *Nonpenetrance* - **Nonpenetrance** describes a situation where an individual carries a disease-causing gene mutation but **does not express the associated phenotype** or clinical symptoms. - This concept explains variability in disease manifestation, not differential gene expression based on parental origin. *Anticipation* - **Anticipation** is a phenomenon in genetic disorders where the symptoms become **more severe** and/or appear at an **earlier age** in successive generations. - This is often seen in disorders caused by expansion of trinucleotide repeats, such as Huntington's disease, and is not related to parent-of-origin gene expression.
Question 6: Which of the following statements about gene therapy is false?
- A. Gene also considered as drug
- B. Gene therapy can be used to treat some cancers.
- C. Has been tried in cystic fibrosis
- D. Gene therapy is only used for genetic disorders. (Correct Answer)
Explanation: ***Gene therapy is only used for genetic disorders.*** - This statement is **false** because gene therapy has applications beyond just genetic disorders. It is also being explored and used in the treatment of acquired diseases such as **cancer** and **infectious diseases**. - While it's a prominent approach for correcting genetic defects, its scope is much broader, involving the introduction or modification of genes to achieve a therapeutic effect in various conditions. *Gene also considered as drug* - This statement is **true**. Gene therapy products are often regulated as **drugs** or **biological products** by regulatory bodies like the FDA. - This is because they involve the delivery of genetic material that acts to modify gene expression or cell function to produce a therapeutic effect, similar to how traditional drugs work. *Has been tried in cystic fibrosis* - This statement is **true**. Gene therapy has been extensively investigated as a potential treatment for **cystic fibrosis (CF)**. - CF is caused by mutations in the **CFTR gene**, and researchers have attempted to deliver functional copies of this gene to the affected cells, particularly in the lungs, to correct the underlying defect. *Gene therapy can be used to treat some cancers.* - This statement is **true**. Gene therapy is an active area of research and treatment for various **cancers** [1]. - Approaches include introducing genes that make cancer cells more susceptible to chemotherapy, enhancing the immune system's ability to fight cancer, or directly killing cancer cells through gene delivery [1].
Question 7: Which one of the following statements about chromatin is not true?
- A. DNA winds approximately 1.75 times around the nucleosomes
- B. Covalent modification of histones influence chromatin compaction
- C. Non-histone proteins are part of mitotic chromosomes
- D. H2A-H2B bind to both the entry and exit ends of DNA in nucleosomes (Correct Answer)
Explanation: ***H2A-H2B bind to both the entry and exit ends of DNA in nucleosomes*** - This statement is **not entirely true** as presented because while **H2A-H2B dimers** do make contacts with DNA near entry/exit regions, they do not bind **exclusively** at these ends. - In the nucleosome structure, two H2A-H2B dimers flank the central **(H3-H4)₂ tetramer** and interact with DNA throughout approximately **30 base pairs on each side**. - The **entry and exit points** of nucleosomal DNA are primarily stabilized by **linker histones (H1)**, which bind to the dyad axis and linker DNA regions. - The statement oversimplifies the complex three-dimensional interactions within the nucleosome core particle. *DNA winds approximately 1.75 times around the nucleosomes* - This statement is **true**; approximately **1.65 to 1.75 turns** of DNA (about 146-147 base pairs) wrap around the **histone octamer** to form the core nucleosome particle. - This precise winding is crucial for the compaction of DNA into eukaryotic chromatin and represents the fundamental repeating unit of chromatin structure. *Covalent modification of histones influence chromatin compaction* - This statement is **true**; **post-translational modifications** (PTMs) such as acetylation, methylation, phosphorylation, and ubiquitination on histone tails significantly impact **chromatin structure and accessibility**. - For example, **histone acetylation** generally leads to a more open chromatin conformation (euchromatin) by neutralizing positive charges, facilitating gene expression. - **Histone methylation** can lead to either open or compact chromatin depending on the specific residue modified (e.g., H3K4me3 for activation, H3K9me3 for repression). *Non-histone proteins are part of mitotic chromosomes* - This statement is **true**; mitotic chromosomes contain numerous **non-histone proteins** essential for chromosome structure and function. - Examples include **structural maintenance of chromosomes (SMC) proteins** like condensin and cohesin, topoisomerases (DNA topoisomerase II), and kinetochore proteins. - These non-histone proteins are crucial for chromosome condensation, sister chromatid cohesion, segregation, and proper mitotic progression.
Question 8: Assertion: DNA methylation leads to gene silencing. Reason: Methylation prevents binding of transcription factors to promoter regions.
- A. Assertion is true but reason is false.
- B. Both assertion and reason are true and reason is the correct explanation. (Correct Answer)
- C. Both assertion and reason are true but reason is not the correct explanation.
- D. Both assertion and reason are false.
Explanation: ***Both assertion and reason are true and reason is the correct explanation.*** - **DNA methylation** at **CpG islands** in promoter regions is a well-established **epigenetic mechanism for gene silencing** - The reason directly explains HOW methylation causes silencing: **methylation prevents transcription factor binding** to promoter regions, blocking transcriptional machinery - Both statements are factually correct AND the reason provides the mechanistic explanation for the assertion *Assertion is true but reason is false.* - While the assertion is correct (DNA methylation does lead to gene silencing), the reason is also TRUE, not false - Methylation preventing transcription factor binding is indeed a **primary mechanism** of gene silencing - This option would only be correct if the reason statement were factually incorrect *Both assertion and reason are true but reason is not the correct explanation.* - Both statements are individually true, but this option is incorrect because the reason IS the correct explanation - The prevention of transcription factor binding **directly explains** how methylation silences genes - If this were correct, the reason would describe an unrelated consequence of methylation, not the causal mechanism *Both assertion and reason are false.* - Both statements are well-established biological facts - DNA methylation-mediated gene silencing is a fundamental epigenetic mechanism - Prevention of transcription factor binding is a validated mechanism of this silencing
Question 9: Effect of environment on genes is called?
- A. Euthenics
- B. Positive Eugenics
- C. Negative Eugenics
- D. Epigenetics (Correct Answer)
Explanation: ***Epigenetics*** - **Epigenetics** refers to heritable changes in **gene expression** that do not involve alterations to the underlying **DNA sequence**. - These changes are often influenced by **environmental factors**, such as diet, stress, and exposure to toxins, which can affect how genes are turned on or off. *Positive Eugenics* - **Positive eugenics** aims to improve the human population by encouraging the reproduction of individuals deemed to have "desirable" traits. - This concept is ethically controversial and focuses on directed breeding rather than environmental gene influence. *Negative Eugenics* - **Negative eugenics** aims to reduce the prevalence of "undesirable" traits in the human population by discouraging or preventing the reproduction of individuals deemed to possess them. - Like positive eugenics, this is a highly controversial concept focused on limiting reproduction based on perceived genetic quality. *Euthenics* - **Euthenics** is a movement focused on improving human well-being and development by improving living conditions and the environment. - While it acknowledges the impact of the environment, it focuses on societal and lifestyle improvements rather than the direct molecular changes in gene expression.
Question 10: DNA Methylation is not related to?
- A. DNA Replication
- B. Gene silencing
- C. Capping (Correct Answer)
- D. Mismatch repair
Explanation: ***Capping*** - **Capping** is a modification of messenger RNA (mRNA) that occurs during **mRNA processing** in eukaryotes, involving the addition of a 7-methylguanosine cap to the 5' end of the mRNA molecule. - This process is crucial for mRNA stability, translation initiation, and nuclear export, and is entirely **independent of DNA modifications** like DNA methylation. *DNA Replication* - DNA methylation plays a role in **DNA replication** to distinguish newly synthesized strands from parental strands during **DNA repair**. - In bacteria, methylation at specific sites (**Dam methylase**) helps in **mismatch repair** by identifying the parental strand. *Gene silencing* - **DNA methylation** of CpG islands in promoter regions is a well-established mechanism for **gene silencing** by altering chromatin structure and preventing transcription factor binding. - This epigenetic modification leads to stable transcriptional repression and is critical for processes like X-chromosome inactivation and genomic imprinting. *Mismatch repair* - In prokaryotes, **DNA methylation** marks the parental strand, which is used by the **mismatch repair system** to correct errors on the newly synthesized, unmethylated strand. - In eukaryotes, while not directly marking strands, DNA methylation can influence the efficiency of mismatch repair pathways by altering chromatin accessibility.
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