Nucleus Flashcards
Chromosomes visibility
Chromosomes are only visible during mitosis, not interphase. During interphase, DNA exists as chromatin.
Chromatin Structure
Chromatin is a complex DNA, proteins, and a small amount of RNA. It consists of repeating units called nucleosomes and achieves high packaging rations, allowing DNA to fit within the nucleus.
Chromatin compaction model
Interphase chromatin is loosely organized, while mitotic chromatin is highly compact.
ZigZag model: nucleosomes stack in a zigzag pattern, with histone H1 acting as a linker histone to stabilize the structure and contribute to its compaction. This model is often supported by in vitro experiments that show short stretches of nucleosomes can form such zigzag structures.
Heterochromatin Vs. Euchromatin
Heterochromatin is transcriptionally inactive, is more densely packed, marked by specific histone modifications that restrict gene expression, and can be spread and limited by boundaries or insulators. Euchromatin is less condensed and generally associated with active gene expression.
Histone Modifications and the Histone Code””
Histone tails undergo post-translational modifications (e.g., methylation and acetylation), which are read by specific protein complexes to alter chromatin structure and regulate gene expression.
Reader complexes interpret modifications, and writer and eraser complexes propagate or remove these marks.
Chromatin organization and chromosome territories
Chromosomes occupy distinct regions within the nucleus, evidenced by techniques like FISH (fluorescence in situ hybridization).
Chromatin loops can extend from a chromosome territory when a gene is actively transcribed, highlighting gene positioning’s role in regulation.
Gene Localization and Expression
The localization of genes within the nucleus can change depending on their activity. Inactive genes may be positioned at the nuclear periphery, while active ones are often found in transcriptionally active domains.
Nuclear Matrix and Experiments supporting it
A nuclear matrix is a scaffold-like structure providing spatial organization within the nucleus. Evidence from techniques like chromosome conformation capture (3C) and FISH (fluorescence in situ hybridization) demonstrates chromatin domains’ structural compartmentalization and localization.
Position Effect Variegation (PEV) vs. Insulators
PEV: gene silencing due to proximity to heterochromatin.
Insulators: act as barriers, preventing the spread of heterochromatin into active regions.
