Chromatin and Chromosome Structure

DNA-Protein Association: DNA does not exist as a "naked" molecule within the cell under physiological conditions. It is always found in combination with proteins.
Definition of Chromatin: Chromatin is the complex formed by the association of DNA and proteins ( $\text{DNA} + \text{protein} = \text{chromatin}$ ).
Histones: These are a specific class of proteins that interact with DNA to provide the first level (order) of chromatin structure.

Micrococcal Nuclease Digestion Experiment:
- Process: Nuclear DNA (associated with proteins) is treated with Micrococcal Nuclease, an enzyme that digests DNA. Following digestion, the proteins are eliminated.
- Analysis: The resulting DNA fragments are analyzed using Agarose Gel Electrophoresis.
- Results: The electrophoresis reveals a "ladder" of DNA fragments. These fragments correspond to specific lengths: $200$ , $400$ , $600$ , $800$ , and $1000\,bp$ (base pairs). This indicates that the DNA is protected by proteins at regular intervals of approximately $200\,bp$ .
- Notation of fragments: The fragments are often denoted as $1n$ , $2n$ , $3n$ , $4n$ , $5n$ , $6n$ , and $7n$ , where $n \approx 200\,bp$ .
Electron Microscopy (EM):
- EM imaging of chromatin fibers reveals a structure described as "beads on a string."
- The scale of these fibers is approximately $50\,nm$ .

Nucleosome Core Particle:
- Consists of DNA wrapped around an octameric histone core.
- The core is composed of two molecules each of histones $H2A$ , $H2B$ , $H3$ , and $H4$ .
- The DNA length wrapped around this core is approximately $147\,bp$ .
Chromatosome:
- Defined as the nucleosome core particle plus Histone $H1$ .
- The total DNA length in a chromatosome is approximately $166\,bp$ .
Linker DNA:
- The segment of DNA between nucleosomes is called linker DNA, measuring approximately $50\,bp$ .
Interval Summary:
- The fundamental repeat unit (Nucleosome) involves approximately $200\,bp$ of DNA.
- The association of DNA with histones results in a $6$ -fold shortening (compaction) of the DNA molecule.

1st Order / Level: Nucleosomes ("Beads-on-a-String").
- Structure: DNA double helix ( $2\,nm$ ) wrapped around histone octamers.
- Fiber thickness: $11\,nm$ (or $10\,nm$ ).
2nd Order / Level: Chromatin Fiber (Solenoid Form).
- Achieves a $50$ -fold shortening of DNA.
- Thickness: $30\,nm$ fiber.
- Models: Two equally likely models for this level include the "solenoid" model and the "zigzag" model.
3rd Order / Level: Looped Form.
- Further folding of the chromatin fiber mediated by scaffolding proteins (non-histone proteins).
- Loops are anchored to a protein scaffold.
4th Order / Level: Highly Condensed Chromatin (Heterochromatin/Chromosome).
- Highest level of compaction observed in mitotic chromosomes.
- Chromatid width: $700\,nm$ .
- Duplicated chromosome width: $1400\,nm$ .

DNA Double Helix: Diameter $2\,nm$ ; Packing ratio $1$ .
Beads-on-a-String: Diameter $11\,nm$ ; Packing ratio $6$ --$7.
Solenoid ( $30\,nm$ ): Packing ratio $40$ .
Loops ( $50$ turns per loop): Diameter $0.25\,\mu m$ ; Packing ratio $680$ .
Miniband ( $18$ loops): Diameter $0.8\,\mu m$ ; Packing ratio $1.2 \times 10^4$ .
Chromosome (stacked minibands): Diameter $0.84\,\mu m$ ; Packing ratio $1.2 \times 10^4$ .

Challenge to the $30\,nm$ Fiber: Recent analysis (e.g., Eltsov et al., PNAS 2008) suggests that $30\,nm$ chromatin fibers may not exist in situ within mitotic chromosomes.
Topologically Associated Domains (TADs): Current research suggests chromatin is organized into TADs.
- Small Scale ( $10\,bp$ to few $kb$ ): Compaction into nucleosomes.
- Intermediate Scale ( $10\,kb$ to few $Mb$ ): Local regulation via chromatin loops and TADs.
- Large Scale ( $100\,Mb$ to $3000\,Mb$ ): Regulation of contact frequency between individual chromosomes within "Chromosome Territories."

Euchromatin:
- Relatively decondensed and distributed throughout the nucleus.
- Transcriptionally active: This is where genes are transcribed and DNA is replicated.
Heterochromatin:
- Very highly condensed state, resembling mitotic chromatin.
- Transcriptionally inactive.
- Accounts for approximately $10\%$ of all chromatin.

Cell Cycle and Packing:
- Interphase: Chromatin is largely decondensed.
- M Phase (Mitosis): Chromatin condenses through Prophase, Metaphase, Anaphase, and Telophase.
Metaphase Chromosome:
- Maximum compaction. DNA is so compact that it cannot be used as a template for transcription.
- Consists of exact copies called Sister Chromatids connected at a Centromere.
Karyotype: A visual representation of the complete set of chromosomes in a cell (e.g., $22$ pairs of autosomes and $1$ pair of sex chromosomes).
- Clinical Relevance: Abnormalities like Trisomy ( $3$ copies of a chromosome, such as Trisomy $21$ ) can be identified via karyotyping.

Telomeres:
- Do NOT encode for any gene product.
- Contain repetitive sequences (Telomeric Repeats).
- Structure includes a T-loop and the Shelterin complex (including protein factors $TRF1$ and $TRF2$ ).
Centromeres:
- Do NOT encode for any gene product.
- Act as the attachment site for spindle fibers via a specialized protein structure called the Kinetochore.
- Yeast Research: Plasmids containing only an Autonomously Replicating Sequence ( $ARS$ ) missegregate; the addition of a Centromere sequence ( $CEN$ ) ensures regular segregation into daughter cells.
- Structure Variability: Centromere sequences vary (e.g., simple $CDE I/II/III$ in S. cerevisiae vs. complex satellite DNA and transposons in Drosophila).
- CENP-A: All centromeres contain a variant of Histone $H3$ called $CENP-A$ (Centromeric $H3$ ). It is essential for the epigenetic inheritance of centromere identity during DNA replication.