heterochromatin?
What are some differences between euchromatin and
heterochromatin?
Describe the composition and structure of the
nucleosome
The Correct Answer and Explanation is:
Correct Answer:
Differences between euchromatin and heterochromatin:
| Feature | Euchromatin | Heterochromatin |
|---|---|---|
| Structure | Loosely packed | Densely packed |
| Staining | Lightly stained with DNA dyes | Darkly stained |
| Transcriptional activity | Transcriptionally active | Transcriptionally inactive or silent |
| DNA accessibility | DNA is accessible to enzymes | DNA is largely inaccessible |
| Location in nucleus | Found throughout nucleus | Found near the nuclear envelope |
| Function | Active in gene expression | Involved in structural support and gene regulation |
Composition and structure of the nucleosome:
A nucleosome is the basic structural and functional unit of chromatin in eukaryotic cells. It plays a critical role in the packaging of DNA into the nucleus and in gene regulation.
Structure:
A nucleosome consists of:
- A histone octamer, which includes:
- 2 copies each of the core histones: H2A, H2B, H3, and H4
- 146 base pairs of DNA wrapped around this histone core in approximately 1.65 left-handed turns.
- Linker DNA (about 10–80 base pairs long) connects one nucleosome to the next.
- Histone H1 binds to the linker DNA and helps compact the nucleosomes into higher-order structures (e.g., the 30-nm fiber).
Detailed Explanation (300+ words):
Eukaryotic DNA is tightly packaged inside the nucleus through a series of hierarchical levels of organization. One of the most important levels of this organization is the nucleosome, often described as “beads on a string” under the microscope.
Each nucleosome is composed of a core particle made up of a histone octamer. The octamer is formed by pairing two copies each of histone proteins H2A, H2B, H3, and H4. These histones are small, positively charged proteins that bind strongly to the negatively charged phosphate backbone of DNA. Approximately 146 base pairs of DNA wrap around this octamer, forming a supercoiled structure. This packaging helps in shortening the DNA length, allowing it to fit into the limited nuclear space.
Between each nucleosome is a stretch of linker DNA, and histone H1 binds to this region to stabilize the nucleosome and assist in forming more compact chromatin structures. When nucleosomes coil further, they form chromatin fibers, which can either exist in the form of euchromatin or heterochromatin.
Euchromatin is the less condensed form of chromatin, making it more accessible to RNA polymerase and other transcription machinery. This is where active genes are typically found. In contrast, heterochromatin is tightly packed and often contains genes that are silenced or not expressed. It is further classified into constitutive heterochromatin (permanently silent regions like centromeres) and facultative heterochromatin (regions that can switch between active and inactive states).
Understanding nucleosomes and chromatin states is fundamental to epigenetics and gene regulation, as changes in chromatin structure can influence gene expression without altering the underlying DNA sequence.
