Chromatin and DNA Packaging

What is chromatin, and what are the differences between euchromatin and heterochromatin?

Chromatin is the combination of DNA and proteins (histones) that packages long DNA molecules into compact, dense structures. Euchromatin is lightly packed, rich in genes, and most often under active transcription because DNA is accessible to enzymes. Heterochromatin is tightly packed and consists mainly of genetically inactive sequences, preventing enzyme access. Heterochromatin can be constitutive (always tightly packed, very gene-poor, and never accessible) or facultative (can switch back and forth between states, allowing gene silencing; Barr bodies are an example).

What is the folded fiber model, and why is it considered incorrect?

The folded fiber model proposes that DNA winds and folds into a tightly packed structure. The model is described in the notes as having no proteins and random folding, which is why it is considered obviously wrong.

What is the nucleosome model, and what is a nucleosome?

The nucleosome model is the most commonly accepted model for DNA packaging. A nucleosome is the simplest packaging structure of eukaryotic chromatin. Nucleosomes create localized areas of transcription and provide a structure better suited for protein biosynthesis.

What are the core histones, and what are their characteristics?

The core histones are H2A, H2B, H3, and H4. Each consists of approximately 120 amino acids. They are highly conserved during evolution, including between organisms such as humans and yeast. Together, they form the core particle. They are highly basic and positively charged, with approximately 25% lysine and arginine.

What is linker histone H1, and how does it differ from the core histones?

Linker histone H1 consists of approximately 200 amino acids. Unlike the core histones, it is not highly conserved and has tissue-specific expression. It is loosely associated with the core particle. H1 is not part of the core particle but is part of the nucleosome.

How is the nucleosome formed, and what roles do histone tails and protein domains play?

Histone tails link negatively charged DNA to the positively charged histone core. A protein domain is a part of a protein responsible for a specific process. H1 is associated with the nucleosome but is not part of the core particle. The 10 nm fiber is the first step in chromatin packaging.

What is the 30 nm fiber, and how does supercoiling form the solenoid?

The solenoid is formed by helical coiling of 10 nm fibers. It consists of approximately six nucleosomes, and histone H1 is responsible for packaging. Supercoiling reduces the length of DNA by approximately seven times, producing the 30 nm fiber.

How does histone H1 work to regulate nucleosome packaging and gene expression?

Histone H1 binds to two distinct DNA regions: linker DNA and a portion of the 146-base-pair core. Its interaction with linker DNA pinches nucleosomes together and helps regulate how tightly DNA is wrapped, influencing gene expression.

How does the zigzag model differ from the solenoid model, and what do experiments suggest?

The bend in the solenoid model is extremely tight and is not flexible enough to bend between nucleosomes. The zigzag model goes back and forth across the solenoid arrangement, with straight linker DNA connecting opposite nucleosomes while the middle is blocked. Experimental results show that both solenoid and zigzag topologies may be present simultaneously in chromatin fibers.

What happens during higher-order coiling, and what is the 300 nm fiber?

Higher-order coiling produces the 300 nm fiber and chromatin loops made up of solenoids. These loops are built around a scaffold of topoisomerase II. This is identified in the notes as the compaction level of euchromatin and the last level at which DNA can be accessed.

What happens during final chromatin condensation, and what forms the 700 nm fiber?

Final condensation produces the 700 nm fiber, which has a spiral scaffold composed of topoisomerase II and approximately 15 non-histone proteins. This is identified in the notes as the compaction level of heterochromatin.

What is the most compact form of chromatin, and how does it relate to metaphase chromosomes?

After final condensation, chromatin forms a metaphase chromosome, which is the most compact form of chromatin.

When does real chromatin condensation begin during the cell cycle?

Real chromatin condensation begins during prophase.