Notes on Epigenetic Regulation of Gene Expression

Focus of this lecture segment is on epigenetic regulation of gene expression in eukaryotes.
Emphasizes the importance of multiple levels of gene regulation in eukaryotic organisms, providing various opportunities to modify gene expression outcomes.

Epigenetics: Refers to changes in DNA structure that modify gene expression without altering the underlying DNA sequence itself.
- Etymology: The prefix "epi-" means "above" or "on top of," indicating modifications that do not change the genetic code itself.
- Epigenetic changes can influence cellular behavior and the expression levels of different genes in various cell types.

Reversibility: Unlike genetic mutations, epigenetic modifications are easily reversible and can adapt in response to environmental factors.
Inheritance: Epigenetic changes can be inherited from one cell to another during mitotic division, suggesting a form of cellular memory persists through cell generations.

DNA structure plays a significant role in transcription and gene expression.
- During different phases of the cell cycle, the DNA is more loosely packed in interphase and compacts during the M phase (mitosis).
Importance of naked DNA understanding for gene functionality is misleading; DNA is typically associated with proteins that alter its packaging.

Nucleosomes: The basic unit of DNA packaging consisting of DNA wrapped around a complex of eight histone proteins.
- DNA is wrapped around these histones to form nucleosomes, creating a structural motif in chromatin that influences accessibility.
- The nucleosome can form a more compact structure, known as the 30 nanometer chromatin fiber.
The flexibility of DNA structure oscillates between 30 nanometer fibers and nucleosome configurations, regulating accessibility for transcription factors and RNA polymerase.

Compacted DNA is less accessible to transcription factors, hindering transcription.
Modifications to nucleosome positioning can expose specific DNA segments to enable transcription.
Key regulatory proteins include histones that direct chromatin structure and transcription accessibility.

Histone proteins feature protruding amino acids known as histone tails, subject to various chemical modifications that influence transcription activity.
- Modifications can include:
- Methylation: Addition of methyl groups (e.g., single or multiple methyl groups).
  - Can positively or negatively impact transcription based on the context of modification.
- Phosphorylation: Addition of phosphate groups to histone tails.
Understanding specific modifications and their effects is complex and context-dependent, influencing overall transcription rates.

Methylation: Involves adding methyl groups to bases in the DNA sequence, notably at cytosines adjacent to guanines, forming CpG islands.
- Five-methylcytosine: A methyl group added to the fifth carbon of cytosine within a CpG island.
- Methylated CpG islands, typically found in gene promoter regions, are associated with transcription repression as they attract proteins that compact DNA structure.
Conversely, unmethylated CpG islands allow for transcriptional activity in promoters.

Early embryonic cells display mostly open chromatin, minimal methylation, and higher levels of gene expression.
As development progresses, different cell types emerge, showing increases in repression (i.e., compaction) of certain genomic regions while maintaining accessibility for genes relevant to specific cellular functions (e.g., muscle cells expressing specific proteins).

Abnormal chromatin remodeling can reactivate gene expression patterns typically silenced in specific cell types.
Example: Telomerase, an enzyme usually only expressed in stem cells, can be reactivated in cancer cells, allowing them to continue dividing beyond normal limits.

Accessibility of DNA is a fundamental factor influencing transcription viability; tighter compaction corresponds with decreased transcription rates.
Key regulatory mechanisms involve histone modifications and methylation of promoter regions.
This segment concludes the discussion on the importance of structural DNA modifications in the regulation of gene expression in eukaryotic cells, paving the way for future detailed explorations in subsequent lectures.