Epigenetic Regulation of Gene Expression

Epigenetic Regulation of Gene Expression

Introduction to Epigenetics

  • Epigenetic Regulation of Gene Expression: Refers to the mechanisms that regulate gene expression without altering the underlying DNA sequence.

  • Importance of Gene Expression Regulation: Essential for expressing various traits of life in organisms. The flow of information from genes to proteins must be tightly regulated.

  • Key Quote: "With the tools and the knowledge, I could turn a developing snail's egg into an elephant. It is not so much a matter of chemicals because snails and elephants do not differ that much; it is a matter of timing the action of genes." - Barbara McClintock

Learning Goals

  • Explore the concept of epigenetics and its role in gene expression regulation.

  • Understand the processes from transcription initiation to post-translational protein modification.

Required Reading

  • Lecture Notes: Essential information.

  • Top Hat Chapter: Complementary, but not necessary for understanding core concepts.

  • Additional Resources:

    • Video: [YouTube link](http://www.youtube.com/watch?v=eYrQ0EhVCYA&index=9&list=PL4zWoYeELBdOn0QI1swSb_iRvte- -w1vwL)

Vocabulary Definitions

  • Chromatin: The nucleoprotein material that makes up chromosomes, consisting of DNA and proteins.

  • Nucleosome: The structural unit of chromatin, consisting of DNA wrapped around histone proteins.

  • Autosomal Genes: Genes found on autosomal chromosomes, not sex-linked.

  • Monoallelic Genes: Genes that express from only one of the two alleles in diploid cells.

  • Bi-allelic Genes: Genes where both alleles in diploid cells are expressed.

Lecture 1: Epigenetic Regulation of Gene Expression

Key Concepts

  • Definition of Epigenetics: The study of heritable changes in gene expression that do not involve changes to the underlying DNA sequence.

  • Chromatin Structure:

    • Components: DNA and histone proteins.

    • Smallest Repeating Unit: Nucleosome.

Types of Heterochromatin

  • Constitutive Heterochromatin: Permanently condensed regions of DNA that are generally not expressed.

    • Examples: Centromeres and telomeres.

  • Facultative Heterochromatin: Regions that can condense and relax depending on cell conditions and tissue types.

    • Example: X chromosome inactivation, where one X chromosome in females is inactivated to balance gene dosage between sexes.

Chromatin Structure and Gene Expression

  • Chromatin can exist in closed (condensed and tightly wrapped) and relaxed (loosely wrapped) conformations.

  • Impact on Transcription Rate: Open conformations allow transcription factors to access DNA, increasing transcription rates.

Epigenetic Marks

  • Different types of epigenetic marks:

    • Methylation of DNA

    • Modification of histones (e.g., acetylation, methylation)

  • Modification of Histones: Histone tails are the part of the histones that get modified with epigenetic marks affecting gene expression.

DNA Methylation

  • Regulates Gene Expression: Methylation typically silences genes.

  • Site of Methylation: Occurs primarily at cytosine residues, particularly those followed by guanine (CpG sites).

  • Enzyme Involved: DNA methyltransferases catalyze the transfer of methyl groups to DNA.

Mitotic Heritability

  • Definition: The ability of cells to maintain DNA methylation patterns during cell division.

  • Significance: This ensures that daughter cells inherit the same gene expression patterns as their progenitors.

  • Mechanism: DNA methyltransferase enzymes help propagate the methylation patterns to daughter cells.

The Epigenome

  • Definition: The complete set of epigenetic modifications on the genetic material of a cell.

  • Methylation Patterns: Vary between different chromatin types:

    • Heterochromatin: Typically exhibits high levels of methylation.

    • Euchromatin: Lower levels of methylation, more actively transcribed regions.

Significance of DNA Methylation

  • Intergenic Areas and Repetitive Sequences: Methylation in these regions plays a crucial role in genome stability.

  • Gene Promoter Methylation: Methylation of gene promoters leads to transcriptional silencing, preventing gene expression.

  • Role of Methyl-CpG-Binding Proteins: These proteins recognize methylated DNA and recruit repressive complexes to silence transcription.

Lecture 2: Epigenetic Phenomena and Reprogramming

Epigenetic Phenomena Discussed

  • Two main examples of epigenetic phenomena studied:

    • X chromosome inactivation

    • Genomic imprinting

X Chromosome Inactivation

  • An example of facultative heterochromatin, where one X chromosome is randomly inactivated in female mammals to equalize gene dosage between males and females.

Gene Expression Types

  • Monoallelic vs Bi-allelic Genes:

    • Monoallelic Genes: Only one allele expresses while the other remains inactive.

    • Bi-allelic Genes: Both alleles express in diploid organisms.

    • Similarities and Differences: Both are important for genetic diversity but differ in expression patterns.

Genomic Imprinting

  • Definition: A form of gene regulation where genes are expressed in a parent-of-origin-specific manner.

  • Paternal vs Maternal Imprinting:

    • Paternally Imprinted Genes: Genes that are silenced when inherited from the father.

    • Maternally Imprinted Genes: Genes that are silenced when inherited from the mother.

  • Location of Imprinted Genes: Imprinted genes are not restricted to a single region but are scattered throughout the genome.

  • ICR (Imprinting Control Regions): Sequences that control the expression of imprinted genes; methylation of the ICR regulates accessibility of the associated genes.

Embryonic Development and Gamete Fusion

  • The fusion of paternal and maternal gametes is essential for proper embryonic development; it allows for the expression of both alleles, which is critical for normal development.

Significance of Imprinting Reprogramming

  • Refers to the resetting of imprints during gametogenesis and early embryonic development, which is crucial for ensuring the correct expression of imprinted genes during development.

Review Questions

  1. The conformation of the chromosome when genes are active is:

    • a. Tightly wrapped

    • b. Relaxed and loosely wrapped

    • c. Bare DNA not attached to histones

    • Answer: b. Relaxed and loosely wrapped

  2. Which of the following chemical groups are used to modify the histones?

    • a. Acetyl group

    • b. Methyl group

    • c. All of the above

    • Answer: c. All of the above

  3. The chromosome 15 maternal and paternal pair for a child with Prader-Willi Syndrome shows:

    • a. Maternally Imprinted

    • b. Paternally Imprinted

    • Answer: b. Paternally Imprinted

  4. Difference between the genome and the epigenome:

    • Genome: The complete set of an organism's DNA.

    • Epigenome: The complete set of epigenetic modifications that influence gene expression.

  5. In the described scenario with Sarah and Max, if Sarah's child carries the disease-causing allele, would she express the monoallelic gene from her maternal chromosome?

    • Answer: This question requires knowledge of the specific imprinting pattern and whether the gene is maternally or paternally imprinted, affecting its expression in the child.