1. DNA methylation primarily occurs at which site in mammals?
A. Histones
B. Cytosines in CpG islands
C. Adenines in promoter regions
D. Guanines in exons
Answer: B
Explanation: DNA methylation in mammals primarily involves the addition of a methyl group to the cytosines within CpG dinucleotides, especially in CpG islands located near gene promoters.
2. Which enzyme is responsible for maintaining DNA methylation during replication?
A. DNA ligase
B. DNA methyltransferase 1 (DNMT1)
C. DNA helicase
D. Topoisomerase
Answer: B
Explanation: DNMT1 maintains DNA methylation patterns during DNA replication by methylating the daughter strand based on the parental strand.
3. How does DNA methylation lead to gene silencing?
A. Enhances RNA polymerase activity
B. Recruits repressor proteins
C. Increases histone acetylation
D. Removes promoter regions
Answer: B
Explanation: Methylated DNA recruits proteins such as methyl-CpG-binding proteins, which block transcription machinery and lead to gene silencing.
4. Which of the following is a characteristic of hypermethylation in cancer?
A. Gene activation
B. Tumor suppressor gene silencing
C. Increased protein synthesis
D. Overexpression of oncogenes
Answer: B
Explanation: Hypermethylation of CpG islands in promoter regions of tumor suppressor genes often silences them, contributing to cancer progression.
5. Hypomethylation in the genome is associated with:
A. Chromosomal instability
B. Increased DNA repair
C. Reduced gene expression
D. Enhanced transcription fidelity
Answer: A
Explanation: Global hypomethylation can lead to chromosomal instability and activation of transposable elements, promoting genomic instability.
6. What is the role of DNMT3A and DNMT3B?
A. DNA replication
B. De novo DNA methylation
C. Maintaining histone modifications
D. RNA splicing
Answer: B
Explanation: DNMT3A and DNMT3B are involved in de novo methylation, establishing new methylation patterns during development.
7. Which of the following is a methylation-sensitive transcription factor?
A. CREB
B. SP1
C. MECP2
D. STAT3
Answer: C
Explanation: MECP2 (methyl-CpG-binding protein 2) specifically binds to methylated DNA and mediates gene silencing.
8. Which histone modification is commonly associated with DNA methylation?
A. Histone acetylation
B. Histone phosphorylation
C. Histone ubiquitination
D. Histone deacetylation
Answer: D
Explanation: DNA methylation is often associated with histone deacetylation, which leads to a more compact chromatin structure and gene silencing.
9. Demethylation of DNA occurs through which process?
A. Hydroxylation by TET enzymes
B. Deacetylation by HDACs
C. Methylation by DNMTs
D. Phosphorylation by kinases
Answer: A
Explanation: TET (Ten-Eleven Translocation) enzymes hydroxylate methylated cytosines, initiating active or passive DNA demethylation.
10. Which of the following techniques is used to study DNA methylation patterns?
A. RNA sequencing
B. Chromatin immunoprecipitation (ChIP)
C. Bisulfite sequencing
D. Gel electrophoresis
Answer: C
Explanation: Bisulfite sequencing converts unmethylated cytosines to uracil, allowing for precise analysis of DNA methylation patterns.
11. DNA methylation is heritable through:
A. RNA molecules
B. Chromosome segregation
C. Epigenetic inheritance
D. Histone replacement
Answer: C
Explanation: DNA methylation is an epigenetic mechanism that can be inherited during cell division without altering the DNA sequence.
12. Loss of imprinting in DNA methylation can lead to:
A. Normal gene expression
B. Activation of both parental alleles
C. Reduced chromosomal recombination
D. Histone acetylation
Answer: B
Explanation: Loss of imprinting disrupts monoallelic expression, often activating both parental alleles, which can contribute to diseases like cancer.
13. DNA methylation patterns are established during:
A. Zygote formation
B. Gametogenesis
C. Embryogenesis
D. Meiosis
Answer: C
Explanation: During embryogenesis, de novo methylation establishes specific DNA methylation patterns critical for development.
14. CpG islands are primarily found in:
A. Exons
B. Introns
C. Promoter regions
D. Telomeres
Answer: C
Explanation: CpG islands are regions rich in CpG sites located near promoter regions and are often subject to regulation by DNA methylation.
15. Global hypomethylation often leads to:
A. Silencing of all genes
B. Loss of genome integrity
C. Increased transcription fidelity
D. Histone deacetylation
Answer: B
Explanation: Global hypomethylation activates transposable elements and disrupts genome stability.
16. Aberrant DNA methylation in aging typically results in:
A. Increased gene expression
B. Loss of methylation in repetitive sequences
C. Enhanced DNA replication
D. Activation of tumor suppressor genes
Answer: B
Explanation: Aging is associated with loss of methylation in repetitive sequences and promoter hypermethylation of certain genes.
17. DNA methylation is reversible because of:
A. DNMT activity
B. Histone acetylation
C. TET enzyme-mediated hydroxylation
D. RNA interference
Answer: C
Explanation: TET enzymes can convert methylated cytosines to hydroxymethylcytosines, enabling DNA demethylation.
18. Which molecule acts as a methyl donor in DNA methylation?
A. ATP
B. S-adenosylmethionine (SAM)
C. NADPH
D. Acetyl-CoA
Answer: B
Explanation: SAM is the methyl donor used by DNA methyltransferases during the process of DNA methylation.
19. Hypermethylation of which region can silence a gene?
A. Enhancer
B. Promoter
C. Exon
D. Introns
Answer: B
Explanation: Hypermethylation of promoter regions blocks the binding of transcription factors, silencing gene expression.
20. Which class of proteins binds methylated DNA?
A. Histone deacetylases
B. Methyl-CpG-binding domain proteins
C. DNA polymerases
D. RNA helicases
Answer: B
Explanation: Proteins with methyl-CpG-binding domains (e.g., MECP2) specifically recognize and bind methylated DNA, mediating repression.
21. DNA methylation is often analyzed using which assay?
A. ELISA
B. Restriction enzyme digestion
C. Southern blotting
D. Mass spectrometry
Answer: B
Explanation: Methylation-sensitive restriction enzyme digestion differentiates between methylated and unmethylated DNA.
22. The silencing of genes by DNA methylation is critical for:
A. DNA replication
B. Developmental gene regulation
C. Enhancing DNA repair
D. Activating transcription
Answer: B
Explanation: DNA methylation is crucial for regulating genes during development and maintaining cellular identity.
23. Environmental factors influence DNA methylation by:
A. Altering DNA sequences
B. Modifying epigenetic marks
C. Enhancing histone synthesis
D. Blocking RNA polymerase
Answer: B
Explanation: Environmental factors like diet and stress can influence epigenetic marks, including DNA methylation.
24. Unmethylated CpG islands are often found in:
A. Inactive genes
B. Actively transcribed genes
C. Telomeric regions
D. Centromeres
Answer: B
Explanation: Unmethylated CpG islands in promoter regions are associated with active gene transcription.
25. Methylation of DNA is associated with:
A. Active chromatin
B. Heterochromatin formation
C. Increased gene transcription
D. RNA splicing
Answer: B
Explanation: DNA methylation leads to the formation of heterochromatin, a compact chromatin state associated with gene silencing.
26. DNA methylation during embryonic development ensures:
A. Random gene expression
B. Differential gene silencing
C. Increased chromosomal recombination
D. Activation of all alleles
Answer: B
Explanation: During embryonic development, differential DNA methylation patterns ensure specific gene silencing and activation.
27. Which epigenetic modification is reversible?
A. DNA mutations
B. DNA methylation
C. Chromosomal translocations
D. Gene deletions
Answer: B
Explanation: DNA methylation is an epigenetic modification that is reversible through active or passive demethylation processes.
28. Aberrant methylation is commonly observed in which disorder?
A. Cardiovascular diseases
B. Neurodegenerative diseases
C. Cancer
D. Viral infections
Answer: C
Explanation: Aberrant DNA methylation patterns, including hypermethylation of tumor suppressor genes, are a hallmark of many cancers.
29. Which cellular process is NOT regulated by DNA methylation?
A. Gene silencing
B. X-chromosome inactivation
C. Histone synthesis
D. Imprinting
Answer: C
Explanation: DNA methylation regulates processes like gene silencing, X-chromosome inactivation, and imprinting but not histone synthesis.
30. CpG sites are often underrepresented in the genome due to:
A. Active transcription
B. Spontaneous deamination of methylated cytosines
C. High mutation rates in guanines
D. DNA replication errors
Answer: B
Explanation: Methylated cytosines in CpG sites can undergo spontaneous deamination, converting them to thymine and reducing CpG site representation.
These MCQs cover key concepts related to DNA methylation and gene silencing, providing a comprehensive understanding of the topic.
