Questions and Answers
What physiological mechanism leads to the tortoise-shell coat coloration in female cats?
Random X-inactivation results in patches of cells expressing different color alleles.
What reversal occurs during germ cell formation concerning X-chromosome inactivation?
The inactive X-chromosome becomes active in haploid oocytes.
How does X-chromosome inactivation exemplify epigenetic inheritance?
X-chromosome inactivation is maintained through cell divisions without changing the DNA sequence.
Define genomic imprinting in the context of gene expression.
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What is the consequence of an imprinted gene being epigenetically silenced?
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What role does epigenetic marking play during gamete production?
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Explain how an imprinted gene is established during gamete formation.
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Why are male cats typically solid in color, unlike female cats?
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What specific gene-determining feature is responsible for different expressions of X-linked genes following X-inactivation?
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List one example of how genomic imprinting can affect development.
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Study Notes
Epigenetics and Imprinting
- Epigenetic marks are established during gametogenesis and maintained through DNA replication in somatic cells of the embryo and throughout the life of the organism.
- The preceding generation's imprint is erased in the germ line, and a new imprint is re-established according to the sex of the organism.
Imprinted Genes
- Imprinted genes are genes whose expression depends on their parent of origin.
- The gene for insulin-like growth factor-2 (Igf2) is an imprinted gene that is required for prenatal growth in mice.
- Only the paternal copy of the Igf2 gene is transcribed, and paternal gene mutations result in stunted mice, while maternal gene defects have no effect.
Imprinting Mechanisms
- Imprinting mechanisms include DNA methylation, histone modifications, higher-order chromatin structures, and noncoding RNA, which change gene expression without altering the DNA sequence.
Epigenome
- The epigenome is the collection of all epigenetic marks on the DNA in a single cell.
- Epigenomic maps are diagrammatic representations of gene expression, DNA methylation, and histone modification status of a particular genomic region.
X-Chromosome Inactivation
- In female mammalian embryos, one X chromosome in each cell becomes highly condensed into heterochromatin, resulting in X-inactivation and the formation of a Barr body.
- Dosage compensation mechanisms are used to equalize the dosage of X chromosome gene products between males and females.
- X-inactivation is a transcriptional inactivation of one X chromosome in female somatic cells.
X-Inactivation Center (XIC)
- The X-inactivation center is a regulatory element that initiates X-inactivation from the middle of the X chromosome.
- XIC encodes an RNA called XIST RNA, which is necessary for X-inactivation and coats the inactive X chromosome, facilitating the spread of heterochromatin formation along the entire X chromosome.
Heterochromatin Formation
- Heterochromatin formation involves the coating of the inactive X chromosome with XIST RNA, specific histone variants, and histone modifications, making the chromosome resistant to transcription.
X-Chromosome Inactivation Timing
- X-chromosome inactivation occurs after several thousand cells have formed in the embryo, resulting in groups of cells in which either the paternal or maternal X chromosome is silenced.
- This result is distributed in small clusters in adult animals.
Clonal Inheritance
- The condensed inactive X chromosome is inherited in a clonal manner in female mammals, resulting in patches of cells with distinct phenotypes, such as the tortoiseshell coat coloration in female cats.
Epigenetic Inheritance
- X-chromosome inactivation is an example of epigenetic inheritance, where the phenotype does not depend on the DNA sequence.
- Once an X-chromosome is inactivated, it remains silent over many cell divisions during DNA replication and mitosis.
Reversal of X-Chromosome Inactivation
- X-chromosome inactivation is reversed during germ cell formation, resulting in haploid oocytes containing an active X chromosome that expresses X-linked gene products.
Genomic Imprinting
- Genomic imprinting is the regulation of genes whose expression depends on whether they are maternally or paternally inherited.
- Imprinted genes are epigenetically silenced genes that play a crucial role in normal development.
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Description
Learn about the establishment and maintenance of imprinted states during gametogenesis and DNA replication, and how they impact gene expression in organisms. Explore an example of an imprinted gene, Igf2, and its role in prenatal growth.
