Podcast
Questions and Answers
What are the two main categories that epigenetic mechanisms are divided into?
What are the two main categories that epigenetic mechanisms are divided into?
Transcriptional and Post-transcriptional
Which of the following is not a type of covalent histone modification?
Which of the following is not a type of covalent histone modification?
Histone methylation is irreversible and stable.
Histone methylation is irreversible and stable.
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What is the attachment of a methyl group (CH3) to cytosine in CpG dinucleotides called?
What is the attachment of a methyl group (CH3) to cytosine in CpG dinucleotides called?
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What are the main types of DNA methyltransferases?
What are the main types of DNA methyltransferases?
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Epigenetic changes are always permanent and irreversible.
Epigenetic changes are always permanent and irreversible.
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Epigenetic changes can alter the DNA sequence.
Epigenetic changes can alter the DNA sequence.
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What is the process by which one X chromosome in female mammals becomes inactive called?
What is the process by which one X chromosome in female mammals becomes inactive called?
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What is the name of the condensed structure formed during X chromosome inactivation?
What is the name of the condensed structure formed during X chromosome inactivation?
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What is the name of the specific gene responsible for initiating and regulating X inactivation?
What is the name of the specific gene responsible for initiating and regulating X inactivation?
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What term is used to describe the suppression of an expressible gene?
What term is used to describe the suppression of an expressible gene?
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Genomic imprinting is an example of Mendelian inheritance.
Genomic imprinting is an example of Mendelian inheritance.
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Which of the following syndromes is associated with a deletion in the Small nuclear ribonucleoprotein associated protein N gene's promoter and ICR on the paternal chromosome?
Which of the following syndromes is associated with a deletion in the Small nuclear ribonucleoprotein associated protein N gene's promoter and ICR on the paternal chromosome?
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Which imprinted gene, primarily active in brain cells, is affected by a deletion in its ICR, leading to Angelman syndrome?
Which imprinted gene, primarily active in brain cells, is affected by a deletion in its ICR, leading to Angelman syndrome?
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Rubinstein-Taybi syndrome is a genetic disorder associated with alterations in the epigenetic histone code.
Rubinstein-Taybi syndrome is a genetic disorder associated with alterations in the epigenetic histone code.
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Fragile X syndrome is caused by errors in the DNA methylation mechanism that affect repeat sequences.
Fragile X syndrome is caused by errors in the DNA methylation mechanism that affect repeat sequences.
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Which gene is responsible for mRNA transport and undergoes alterations when CGG repeats in its 5'UTR region exceed 200 copies, contributing to Fragile X syndrome?
Which gene is responsible for mRNA transport and undergoes alterations when CGG repeats in its 5'UTR region exceed 200 copies, contributing to Fragile X syndrome?
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Study Notes
Epigenetic Mechanisms
- Epigenetics examines changes in gene expression without altering the DNA sequence.
- Epigenetic regulation is crucial for development and differentiation.
- Epigenetic mechanisms can be broadly categorized into those that directly influence gene expression (transcriptional) and those that indirectly impact it (post-transcriptional, mRNA silencing).
Mechanisms Directly Controlling Gene Expression
- DNA Methylation: Methylation occurs when a methyl group attaches to cytosine in CpG dinucleotides. This is a common modification, and its impact often results in gene silencing.
- Histone Modifications: These involve covalent changes to histone proteins (e.g., acetylation, methylation, deimination, phosphorylation). Acetylation generally promotes gene expression, while methylation and deimination can either activate or repress it, depending on the specific residue and context.
- Chromatin Modifications (Non-Covalent): These encompass interactions with small non-coding RNAs (e.g. miRNA, siRNA), histone exchanges, histone deposition, interactions with other agents (like viruses), and chromatin repair.
Mechanisms Indirectly Controlling Gene Expression
- Post-transcriptional mechanisms primarily involve the inhibition of protein synthesis through the interactions of non-coding RNAs with mRNA.
DNA Modifications
- DNA Methylation: A significant type of DNA modification, often linked with gene silencing. This occurs at CpG islands, which are short sequences of cytosine and guanine nucleotides.
- CpG Islands: Regions rich in CpG sequences typically associated with actively transcribed genes. In contrast, regions with limited CpG content are often associated with silenced genes.
Histone Modifications
- Histone Acetylation: Acetylation of lysine residues on histones generally promotes gene activation by reducing the DNA-histone attraction. Enzymes like histone acetyltransferase (HAT) introduce acetyl groups
- Histone Methylation: The addition of methyl groups to histones has variable effects. Mono-, di-, or tri-methylation often alter the binding of proteins to genes, affecting transcriptional activity. Histone methyltransferases (HMTs) add the methyl groups.
- Histone Deimination: This involves the conversion of arginine to citrulline reducing the positive charge. This alteration usually promotes chromatin relaxation and gene expression, but in specific contexts, it can contribute to chromatin compaction.
- Histone Phosphorylation: Phosphates bind to histones through specific amino acids (serine, threonine, tyrosine). This process is reversible, and phosphorylation can influence gene expression depending on the context.
- Histone Ubiquitination: The attachment of ubiquitin in a complex process involving enzymes (E1, E2, and E3), can have diverse outcomes. In essence, it typically leads to chromatin relaxation and gene activation.
- Histone Sumoylation: Sumoylation, the attachment of small ubiquitin-like modifier (SUMO) proteins, is a modification mostly linked to gene repression.
Epigenetic Mechanisms and Cancer
- Epigenetic changes, such as DNA methylation and histone modifications, play significant roles in cancer development. They often impact tumour suppressor genes (silencing them) or oncogenes (activating them) leading to uncontrolled cell division and tumor growth.
Treatment and Epigenetic Regulation
- Therapies targeting epigenetic changes are gaining prominence. Current approaches include DNA methylation inhibitors (acting on DNA methyltransferases) and histone deacetylase (HDAC) inhibitors, that help regulate gene expression.
Epigenetic Events
- Epigenetic mechanisms regulate gene expression or suppression without modifying the DNA's underlying sequence and play a vital role in several biological processes, including genomic imprinting, gametogenesis, and cellular rejuvenation. Genomic imprinting is a specific type, where the activity of a gene is determined by its parental origin. Notably, some imprinted genes have distinct methylation patterns (correlated to either a maternal or paternal allele's characteristics).
Dosage Compensation (X Chromosome Inactivation)
- Dosage compensation ensures an equal expression level of X chromosomes between males and females. In female mammals, one X chromosome becomes inactive in each cell to equalize the expression level with males. This process is a crucial example of epigenetic control.
X Chromosome Inactivation
- During X chromosome inactivation, a highly condensed structure known as the Barr body forms due to DNA condensation, leading to a silencing of many of the genes carried on the inactive X chromosome.
Xist and Tsix
- Xist and Tsix genes play key roles in the X-chromosome inactivation process; Xist RNA is a key player in silencing the genes on the inactive X chromosome
Genetic Imprinting
- Imprinting is a critical epigenetic mechanism with impact on the expression of genes determined by their parental origin, resulting in differential gene expression depending on whether a gene is inherited from the mother or father.
Fragile X Syndrome
- Errors in DNA methylation can accompany repeat disorders, including fragile X syndrome, characterized by a variety of intellectual and behavioral alterations.
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Description
Explore the fascinating world of epigenetics, where gene expression changes occur without alterations to the DNA sequence. This quiz delves into mechanisms like DNA methylation and histone modifications that regulate gene expression during development. Test your knowledge on how these processes play a crucial role in gene silencing and expression.
