Unit 3: Cytoskeleton, Chromatin and Cell Cycle

1. Structure and Functions: Microtubules, Microfilaments and Intermediate filaments
2. Chromatin: Euchromatin and Heterochromatin
3. Chromatin: Packaging (nucleosome)
4. Cell cycle and its regulation

1. Structure and Functions: Cytoskeleton

The Cytoskeleton is a dynamic network of protein filaments that provides structural support, facilitates movement, and organizes the cytoplasm. It has three main components.

[Image of the three types of cytoskeleton filaments]

Feature	Microtubules	Microfilaments (Actin Filaments)	Intermediate Filaments
Protein Subunit	Tubulin (alpha and beta)	Actin (G-actin and F-actin)	Various proteins (e.g., Keratin, Lamins, Vimentin)
Structure	Hollow, unbranched tubes. Thickest.	Solid, intertwined strands. Thinnest.	Tough, fibrous, rope-like. Intermediate size.
Key Property	Dynamic instability (rapidly assembles and disassembles).	Dynamic; involved in contraction with myosin.	Very stable and permanent.
Primary Functions	Maintain cell shape (resist compression). Form mitotic spindle for chromosome movement. Form core of cilia and flagella. Act as "tracks" for organelle transport (motor proteins).	Muscle contraction (with myosin). Form cleavage furrow in cytokinesis. Amoeboid movement (pseudopods). Maintain cell shape (tension-bearing cortex).	Purely structural: bear tension, provide mechanical strength. Anchor nucleus and other organelles. Form nuclear lamina (supports nuclear envelope). Component of desmosomes (cell-cell adhesion).

2. Chromatin: Euchromatin and Heterochromatin

Chromatin is the complex of DNA and histone proteins that forms the chromosomes within the nucleus. During interphase (the non-dividing state), chromatin exists in two forms:

Feature	Euchromatin	Heterochromatin
Packing	Loosely packed, "beads-on-a-string" appearance.	Tightly condensed and coiled.
Staining	Stains lightly.	Stains darkly.
Genetic Activity	Genetically active. The DNA is accessible for transcription (gene expression).	Genetically inactive. The DNA is condensed and inaccessible for transcription.
Location	Found throughout the nucleoplasm.	Often at the periphery of the nucleus and at centromeres/telomeres.

3. Chromatin: Packaging (nucleosome)

A cell's DNA must be tightly packed to fit inside the nucleus. This packing is achieved through multiple levels of coiling, starting with the nucleosome.

Nucleosome: The fundamental, repeating unit of chromatin packaging.

Structure of a Nucleosome

Histone Octamer: A core "spool" made of 8 histone proteins (two each of H2A, H2B, H3, and H4). Histones are positively charged.
DNA: The negatively-charged DNA wraps around this histone octamer approximately 1.65 times.
H1 Histone: A "linker" histone that binds to the DNA where it enters and exits the spool, helping to pull nucleosomes together.

Levels of Packaging:

Nucleosomes ("Beads-on-a-string"): First level of compaction.
30-nm Fiber (Solenoid): The nucleosomes are further coiled.
Looped Domains: The 30-nm fiber forms loops that attach to a protein scaffold.
Metaphase Chromosome: The most condensed form, visible during cell division.

[Image of the levels of Chromatin Packaging from DNA to Chromosome]

4. Cell cycle and its regulation

The Cell Cycle

The cell cycle is the ordered series of events that a cell goes through from its formation until its own division.

[Image of the Cell Cycle showing G1, S, G2, and M phases]

Interphase (The "growing" phase): The longest part of the cycle.
- G1 (Gap 1) Phase: The cell grows, carries out normal metabolic functions, and prepares for DNA replication.
- S (Synthesis) Phase: DNA replication occurs. The cell's chromosomes are duplicated.
- G2 (Gap 2) Phase: The cell continues to grow and makes final preparations for division.
M (Mitotic) Phase:
- Mitosis: Nuclear division (Prophase, Metaphase, Anaphase, Telophase).
- Cytokinesis: Cytoplasmic division.

Cell Cycle Regulation

The cell cycle is tightly controlled by a molecular control system involving checkpoints and regulatory proteins.

Checkpoints: "Stop" signals that pause the cycle to ensure all processes are complete and correct before proceeding.
- G1 Checkpoint ("Restriction Point"): The most important. Checks for cell size, nutrients, and growth factors.
- G2 Checkpoint: Checks if DNA replication is complete and if any DNA is damaged.
- M (Spindle) Checkpoint: Checks if all chromosomes are properly attached to the mitotic spindle.
Key Regulatory Molecules:
- Cyclins: Proteins whose concentration rises and falls in a cyclical pattern.
- Cyclin-Dependent Kinases (Cdks): Enzymes that are activated when bound to a cyclin. The active Cdk complex then drives the cell cycle forward by phosphorylating target proteins.

Exam Tip: The Cdk-Cyclin complex is the "engine" of the cell cycle. Cdks are always present, but are inactive. Cyclins are the "key" that turn the Cdk engine on, and their levels must rise and fall to ensure the cycle proceeds in the correct order.