Table of Contents
- Introduction
- What is Euchromatin?
- Euchromatin Structural Composition
- Euchromatin Appearance
- Euchromatin Functions
- Different Genomic Regions of Euchromatin
- Regulation of Euchromatin
- Conclusion
Introduction
- Chromatin is a term describing the association of DNA and histone proteins in chromosome formation.
- Chromatin composition: proteins (50-60%), DNA (30-40%), and RNA (1-10%).
- Chromatin structure resembles 'beads on a string,' with DNA threads wrapped around histone proteins, observed during mitosis/meiosis.
- Chromatin fibers are stainable, allowing easy identification of cell replication stages based on chromatin bands.
- Functions of chromatin:
- Packaging genetic material to fit inside the nucleus.
- Regulating gene expression via DNA replication, transcription, chromosome segregation, and recombination.
- Classification of chromatin fibers based on structure, compaction, and location in a chromosome:
- Euchromatin: Associated with active transcription of genes.
- Heterochromatin: Composed of less accessible chromatin fibers, associated with gene silencing.
- Centromeric chromatin: Involved in spindle binding during chromosomal segregation.
What is Euchromatin?
- Euchromatin is chromatin fibers containing transcriptionally active genes.
- It has wider spaces between nucleosomes (DNA + histone protein = repeating units of chromatin structure), hence also called Open Chromatin.
- Euchromatin fibers facilitate active transcription by allowing histone modifications and higher accessibility for transcription machinery.
- Euchromatin consists of more nucleosome beads with specific nucleosomal positioning.
- The exact positioning of nucleosomes aids in transcription regulation, with histone proteins precisely recognizing DNA sequence motifs.
- According to the article "Finishing the euchromatic sequence of the human genome," 92% of the human genome comprises the euchromatin structure.
Euchromatin Structural Composition
- Euchromatin consists of repetitive units of nucleosomes, each approximately 11 nm in diameter, with DNA fibers wrapped around histone proteins.
- The spacing between individual nucleosomes in euchromatin is wider, allowing transcriptional protein complexes easier access to DNA motifs.
- DNA helices are condensed, with less than two helical turns wrapping around the histone proteins.
- Approximately 200 base pairs of DNA coil around the histones in each nucleosome.
- The DNA that links two nucleosomes is known as Linker DNA, containing around 0 to 80 base pairs.
- The histone protein core comprises four pairs of polypeptide chains: two pairs each of H2A, H2B, H3, and H4, forming histone octamers.
- Histones possess 'tail structures' that vary in amino acid sequences and act as control switches through methylation and acetylation processes, swapping between different chromatin conformations.
- Methylation of the fourth lysine in the histone tail induces euchromatin conformation, making it a marker for observing euchromatin structures in experiments.
Euchromatin Appearance
- Under magnification, euchromatin appears as ‘beads on a string’ in chromosomal structures.
- In electron and optical microscopy, euchromatin fibers stain lighter than heterochromatin due to their wider nucleosome spacing and more open structure.
- The G-banding (Giemsa) staining technique is commonly used to differentiate euchromatin from heterochromatin.
- In this technique, euchromatin appears in a lighter shade, while heterochromatin appears darker due to its more compact nucleosomal structures.
Euchromatin Functions
- Euchromatin consists of loosely bound nucleosomes and exposed DNA helical sequences that carry active genes accessible for transcription by polymerases (DNA and RNA) and other regulatory protein complexes.
- The presence of euchromatin in a nucleus indicates transcriptionally active cells.
- Cells use the transformation mechanism between euchromatin and heterochromatin to regulate gene expression and replication levels.
- Cells control chromatin structure as needed; euchromatin structure is observed when genes are active or turned on.
- In housekeeping genes, euchromatin is always present because these genes are constantly transcribed for basic cell survival.
- Many environmental interactions lead to post-translational modifications of histone octamers, causing alterations in chromatin structure with few or no changes to the DNA sequence.
- Accelerated aging can be caused by epigenetic alterations that increase euchromatin expression in the nucleus.
Different Genomic Regions of Euchromatin
Euchromatin contains active genes and includes various genetic sequences such as gene bodies, promoters, and enhancers, all of which promote transcription.
Transcriptionally Active Gene Bodies
- Gene bodies in euchromatin contain introns and exons from start to end.
- Active histone modifications maintain gene bodies in an open chromatin state, allowing easy access for polymerases.
- Gene bodies in highly expressed genes are enriched with DNA methylation to prevent intragenic transcription initiation.
- Nucleosomal positioning weakens along the length of active gene bodies, promoting transcription.
Promoters
- Promoters are regions in a gene with specific sequences or motifs recognized by polymerases to initiate transcription.
- Located upstream of the 5’ end of the coding or sense strand, where polymerase recognizes the sequence to start transcription.
- Typically, promoters are 100-1000 base pairs in length.
- Promoters show characteristic motifs of GC-rich DNA sequences; vertebrate promoters have 70% GC nucleotide base pairs.
- Examples include the TATA box and E-box (sequence CACGTG).
Enhancers
- Enhancers are DNA sequences that regulate transcription by binding to transcription factors.
- They can be located upstream or downstream from the respective gene and at various distances from promoters.
- In an open state, enhancers may be far from the coding strand, but in a folded state, they can be near the transcription start site.
- The orientation of the enhancer sequence (forward or reverse) does not affect gene transcription.
Regulation of Euchromatin
Euchromatin structure is regulated by various processes, including methylation, acetylation, and phosphorylation. These regulatory modifications occur on the histone proteins through the action of enzymes involved in post-translational modifications. Enzymes act on the N-terminal tails of nucleosomal histones, leading to either the formation of an open euchromatin structure or a closed heterochromatin form.
Acetylation and Methylation
- Histone acetylation results in the formation of euchromatin, whereas histone methylation leads to the formation of heterochromatin.
- Acetylated histones become more negatively charged, disrupting the interaction between the DNA helix and histones, making the DNA more accessible to polymerases and transcription factors.
- Acetylation typically occurs on the lysine residues in the N-terminal tails of histone proteins, further enhancing access for transcription factors.
Phosphorylation
- Euchromatin structures are also regulated by the addition and removal of phosphate groups by kinase and phosphatase enzymes.
- Phosphate groups are added to the serine, tyrosine, or threonine residues on nucleosomal euchromatin.
- The addition of phosphate groups increases the negative charge of DNA, causing the helix to relax and open up, thereby facilitating transcription.
Conclusion
Chromatin is categorized into euchromatin and heterochromatin based on nucleosomal positioning in eukaryotic cells. Euchromatin is more transcriptionally active and less condensed compared to heterochromatin, which contains repressed DNA sequences. Euchromatin consists of repetitive units of DNA helix wrapped around histone proteins in a more open state, allowing easy transcription. Histone modifications, such as methylation and acetylation, specifically mark euchromatin to facilitate gene expression. Environmental factors can alter chromatin structure, causing transformations between euchromatin and heterochromatin. Increased presence of euchromatin is associated with an enhanced aging process, indicating higher transcription and gene expression levels.
