BIOB11H3 Lecture Notes - Lecture 8: Nuclear Pore, Nuclear Membrane, Intermediate Filament
BIOB11
LEC 8 - Nuclear and Chromatin Structure
January 26, 2017
→ Structure of the Nuclear Envelope
● b/w prokaryotes and eukaryotes ONLY eukaryotes have nucleus
○ Most eukaryotic organisms have larger genomes → present as double-stranded DNA arranged in
linear chromosomes
● Chromatin: DNA-protein complexes that large genomes are packaged into
● Contents of nucleus present in viscous, mass of material enclosed by a complex nuclear envelope
○ Forms boundary b/w nucleus and cytoplasm
● Within a typical (non mitotic) nucleus:
○ Chromosomes → exist as highly extended nucleoprotein fibers (chromatin)
○ One/more nucleoli → irregularly shaped electron-dense structures
■ Function in synthesis of ribosomal RNA and assembly of ribosomes
○ Nucleoplasm → fluid substance in which solutes of the nucleus are dissolved
● Nuclear Envelope
○ Has 2 cellular membrane arranged parallel to one another and separated by 10-50 nm
■ together→ have > 60 transmembrane proteins + species that link the outer nuclear
membrane w/ elements of the cytoskeleton
○ Inner and outer nuclear membranes fused at sites → form circular pores that contain complex
proteins
○ Outer membrane studded w/ ribosomes
■ Continuous with membrane of the rough Endoplasmic Reticulum (RER)
■ Space b/w the membranes continuous
with ER lumen
→ Figure 12.5 The Cell Nucleus
● Electron micrograph of interphase HeLa cell nucleus
● Heterochromatin evident around the entire inner surface of
nuclear envelope
● Clumps of chromatin can be seen scattered throughout
nucleoplasm
● Nuclear Lamina 1
○ Thin filamentous network that binds integral membrane proteins to inner surface of nuclear
envelope
○ Function: site of attachment for chromatin fibers at nuclear periphery (outside nucleus)
○ Poorly understood role in DNA replication and transcription
○ Composed of polypeptides called lamins
■ Members of superfamily of polypeptides that assemble into intermediate filaments of the
cytoplasm
find more resources at oneclass.com
find more resources at oneclass.com
→ Figure 12.7b
● Electron micrograph of a freeze-dried, metal shadowed nuclear envelope
● Lamina appears as continuous meshwork consisting of filaments oriented roughly perpendicular to
e/o
● The inset (square) shows where nuclear pores have been mechanically removed
→ Nuclear Pore Complex and Its Role in Nucleocytoplasmic Trafficking
● Nuclear Pore: gateways across the barrier (nuclear envelope)
● Nuclear envelope is the hub of activity for the movement of RNAs and proteins in both directions b/w
nucleus → cytoplasm
● Replication and transcription of genetic material within nucleus requires many proteins that are synthesized
in the cytoplasm and transported across the nuclear envelope
● mRNAs, tRNAs, and ribosomal subunits manufactured in the nucleus must be transported through the
nuclear envelope from nucleus → cytoplasm
● Nuclear Pore COmplex (NPC)
○ basket-shaped structure within nuclear pores
○ Sits on nuclear envelope → projecting into both cytoplasm and nucleoplasm
○ NPC is a huge supercomplex (much larger than ribosome)
■ Exhibits a octagonal symmetry → eightfold repetition
○ Contain about 30 diff proteins → nucleoporins
■ Each nucleoporin is present in multiple copies of eight →keeping w/ octagonal symmetry of
the structure
○ At heart of NPC → central channel which is surrounded by ring of nucleoporins whose
rearrangement can change the diameter of the opening
○ Not a static structure → its nucleoporins are replaced with new copies over time
○ F-G repeats
■ Sit among nucleoporins
■ Aa sequence largely consists of F (phenylalanine) and G (glycine) repeats
■ Repeats clustered in a specific region of each molecule called → FG domain
● Disordered structure → extended and flexible organization
■ FG repeat-containing nucleoporins thought to line central channel of NPC w/ their
filamentous FG domains extending into heart of channel
● FG domains form hydrophobic meshwork that blocks free diffusion of larger
macromolecules b/w nucleus and cytoplasm
● Nuclear Localization Signal (NLS)
○ Stretch of aa near C-terminus of nucleoplasmin
○ This sequence enables protein to pass through nuclear pores and enter the nucleus →
“PASSPORT”
○ “Classical” NLSs consist of one or two short stretches of (+) charged aa
○ [ if NLS is fused to a nonnuclear protein and injected into cytoplasm → modified protein becomes
concentrated in nucleus ]
● Family of proteins that functions as mobile transport receptors
○ Importins
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Structure of the nuclear envelope b/w prokaryotes and eukaryotes only eukaryotes have nucleus. Most eukaryotic organisms have larger genomes present as double-stranded dna arranged in linear chromosomes. Chromatin: dna-protein complexes that large genomes are packaged into. Contents of nucleus present in viscous, mass of material enclosed by a complex nuclear envelope. Chromosomes exist as highly extended nucleoprotein fibers (chromatin) One/more nucleoli irregularly shaped electron-dense structures. Function in synthesis of ribosomal rna and assembly of ribosomes. Nucleoplasm fluid substance in which solutes of the nucleus are dissolved. Has 2 cellular membrane arranged parallel to one another and separated by 10-50 nm together have > 60 transmembrane proteins + species that link the outer nuclear membrane w/ elements of the cytoskeleton. Inner and outer nuclear membranes fused at sites form circular pores that contain complex proteins. Continuous with membrane of the rough endoplasmic reticulum (rer) Space b/w the membranes continuous with er lumen. Electron micrograph of interphase hela cell nucleus.
