Chromatin Structure and Function Quiz

Cell Nucleus

• The interphase nucleus is a membrane-bound compartment in eukaryotic cells.
• It houses the cell's genetic material (DNA).
• DNA replication and RNA transcription occur here.

Learning Objectives

• Define "interphase nucleus"
• List components of a eukaryotic interphase nucleus
• Differentiate between euchromatin and heterochromatin
• Diagram DNA packaging into chromosomes and describe the contents of nucleosomes, fibers, and chromosomes
• Define the functions and parts of the nucleolus
• Describe protein and ribosome transport across the nuclear pore complex
• Define karyolysis, pyknosis, and karyorrhexis
• Identify structures (nucleus, nucleolus, heterochromatin, euchromatin) in a stained tissue section.

The Nucleus

• The interphase nucleus contains chromatin, nucleolus, nuclear envelope, and nucleoplasm.
• It is a membrane-bound compartment holding genetic information for eukaryotes.

Components of the Nucleus

• Chromatin: Made of DNA and histones (proteins). Histones organize DNA into nucleosomes.
• Nucleolus: Contains ribosomal RNA (rRNA) genes, RNA, proteins, and regulatory cell cycle proteins. It's the site of rRNA synthesis.
• Nuclear Envelope: Composed of two membranes; inner and outer nuclear membranes separated by a perinuclear space. The outer membrane is continuous with the endoplasmic reticulum. Nuclear pores are found on both membranes.

Chromatin

• Chromatin is DNA and proteins
• Histones organize DNA into structural units called nucleosomes
• Nucleosomes are assembled into chromatin.
• Chromatin further organizes into chromosomes.

Chromatin cont.

• Under a microscope, extended chromatin looks like beads on a string.
• These beads are nucleosomes.
• Each nucleosome is a DNA segment around eight histone proteins.
• Chromatin folds into 30-nm fibers in non-dividing cells.

Summary of the function of the Nucleus

• Houses genetic material to regulate cellular activities.
• Creates ribosomal subunits for protein production.

Euchromatin vs. Heterochromatin

• Euchromatin: Decondensed, light-appearing. More active gene expression. Found in actively transcribing cells like liver cells.
• Heterochromatin: Condensed, dark-appearing. Less active, transcriptionally inactive regions. Present in resting cells.

Which is Where?

• Heterochromatin is concentrated in perinucleolar and nuclear periphery areas.
• Nucleolus-associated domains (NADs) are enriched with heterochromatin.
• Lamina-associated domains (LADs) are enriched with heterochromatin.

Heterochromatin cont.

• Heterochromatin is divided into constitutive and facultative types.
• Constitutive heterochromatin is unchanging throughout the cell cycle.
• Facultative heterochromatin is transcriptionally inactive in certain stages or tissues.

Barr bodies

• Barr bodies are a form of facultative heterochromatin, representing an inactive X chromosome in female cells (46,XX)
• Visualized as a dense structure in some cell types.

Nucleolus

• The nucleolus is a generally spherical, highly basophilic subdomain within the nucleus.
• It is the non-membrane region of the nucleus where ribosomoal RNA (rRNA) genes are found.
• The nucleolus is the primary site for ribosome subunit production, which is vital for protein synthesis.

Nucleolus cont.

• The nucleolus has 3 regions: fibrillar center (NO), dense fibrillar component (PF), granular component (PG).
• The fibrillar center contains rRNA genes.
• The dense fibrillar component has rRNA processing occurring.
• The granular component is where ribosome subunits are assembled.

Ribosomal Assembly

• Ribosomal proteins are synthesized in the cytoplasm.
• mRNA for these proteins cross the nuclear envelope to the nucleolus.
• Ribosomal proteins and rRNA combine in the nucleolus to create ribosomal subunits.
• Subunits leave the nucleolus via nuclear pores to the cytoplasm where they function in translation.

Section of A Single Ribosome

• A ribosome consists of a large and small subunit.
• rRNA and structural proteins combine to form the subunits.

The Nuclear Envelope

• The nuclear envelope consists of two continuous membranes, the outer and inner nuclear membranes.
• The outer membrane is continuous with the rough endoplasmic reticulum (RER).
• The nuclear pore complex (NPC) regulates molecular transport between the nucleus and cytoplasm.

The Nuclear Pore Complex (NPC)

• NPC is a multi-protein complex, the nucleoporins (NUPs), forming a central pore between the cytoplasmic and nuclear rings.
• NPCs act as selectively permeable gates.

Nuclear Lamina

• The nuclear lamina is a fibrous protein layer inside the inner membrane of the nuclear envelope.
• It reinforces the nuclear envelope and organizes the chromatin.
• Lamins are intermediate filament proteins that influence transcription factors and chromatin structure..

Laminopathies

• Laminopathy diseases result from mutations in lamin genes.
• These conditions affect skeletal and cardiac muscles, fat distribution, and other tissues.
• They are classified into muscular dystrophy, partial lipodystrophy, neuropathy types.

Transport of molecules across the nuclear pore complex (NPC)

• Proteins requiring nuclear entry have a nuclear localization signal (NLS).
• This signal binds to importin proteins which facilitate transport through NPCs.
• Ribosomes that need to leave the nucleus have a nuclear export signal (NES).
• Proteins involved in export use exportin proteins to facilitate transport.
• Small molecules like water, ions cross by simple diffusion.