Overview
This lecture explains how epigenetic mechanisms such as chromatin remodeling, histone modification, and DNA methylation regulate gene expression in eukaryotic cells without changing the underlying DNA sequence.
Chromatin Structure and Gene Access
- Eukaryotic DNA is wound around histone proteins to form nucleosomes, which control gene accessibility.
- Nucleosomes can slide along DNA, exposing or hiding regions for transcription as needed.
- Tight nucleosome packing prevents transcription factor binding and turns gene expression off.
- Loosely packed nucleosomes expose DNA and allow transcription factors to initiate gene expression.
Histone Modification
- Histone proteins have "tails" that can be chemically modified by adding phosphate, methyl, or acetyl groups.
- Acetylation reduces positive charge on histones, loosening DNA binding and increasing gene access.
- These chemical modifications do not change the DNA sequence but alter chromatin structure and gene expression.
DNA Methylation and Epigenetic Silencing
- DNA methylation adds methyl groups to cytosines in CpG islands, often found in gene promoter regions.
- Methylated DNA in promoter regions typically silences genes and prevents transcription.
- Highly methylated DNA regions with deacetylated histones are transcriptionally inactive and tightly coiled.
- Some methylation patterns are inherited (genomic imprinting) and can be influenced by environmental factors.
Dynamics and Effects of Epigenetic Changes
- Epigenetic changes are heritable but can be reversed and may persist through multiple cell divisions or generations.
- Chromatin remodeling and DNA methylation work together to regulate whether a gene is open for transcription or silenced.
Key Terms & Definitions
- Epigenetic Change — Heritable change in gene expression not caused by alterations in DNA sequence.
- Chromatin Remodeling — The dynamic modification of chromatin architecture to allow or restrict gene access.
- Histone — Protein around which DNA is wrapped, forming nucleosomes.
- Nucleosome — Structural unit of chromatin, composed of DNA wrapped around histone proteins.
- Acetylation — Addition of an acetyl group, usually increases gene expression by loosening DNA-histone interaction.
- Methylation — Addition of a methyl group to DNA, often silencing gene expression.
- CpG Island — DNA region rich in cytosine and guanine nucleotides where methylation commonly occurs.
- Genomic Imprinting — Epigenetic phenomenon where certain genes are expressed in a parent-of-origin-specific manner.
Action Items / Next Steps
- Review how histone modifications and DNA methylation alter gene expression.
- Watch provided video on epigenetic gene regulation for further understanding.
- Prepare for questions on mechanisms controlling transcriptional access and gene silencing.