The Nucleus Flashcards
What is the role of the nucleus?
- Maintains integrity of DNA
- Regulates gene expression
- Mediates replication of DNA
- Separates nuclear and cytosolic enzymes
- Separates sites for transcription and translation
What is the nuclear envelope?
Double membrane that encloses entire organelle
What is the nuclear membrane?
- Double lipid bilayer - separates contents of nucelus from cytoplasm.
- Inner nuclear membrane connected to nuclear lamina
- Outer nuclear membrane continuous with Rough ER
What are nuclear lamina?
- dense (30-100nm) network inside nucleus
- intermediate filaments and membrane associated proteins
What is the role of the nuclear lamina?
- regulates DNA replication
- regulates cell devision
- chromatin organisation
- anchors NPC in nuclear envelope
What are laminopathies
Genetic disorders caused by mutations in genes encoding proteins of the nuclear lamina
What are some common clinical symptoms of laminopathies?
- skeletal dystrophy
- cardiac muscle dystrophy
- lipodystrophy
- progeria
What happens in Emery-Dreifuss muscular dystrophy
- affects skeletal & cardiac muscle
- contractures in joints
- cardiac conduction defects
What happens in Hutchinson-Gilford progeria syndrome (HGPS)
- accelerate aging
- point mutation LMNA gene
- translated lamin A lacks 50aa
- LAD50 mutants incorporates abnormally into lamina
What does the LAD50 mutants in HGPS cause?
- thickening of lamina
- loss of peripheral heterochromatin
- increased DNA damage
How is genetic material organised in the nucleus?
individual patches - chromosome territories
How much DNA does a human cell contain?
approx. 2m
How is DNA organised for most of the cell cycle?
euchromatin - looser nucleosome, ‘beads on a string’, transcription factors can bind
heterochromatin - compact nucleosome, fibres
What is euchromatin?
active DNA, open form, transcription factors bind
What is heterochromatin?
inactive DNA
- constitutive - never expressed, around centromere
- facultative - differentially expressed (development/stress)
What do insulator elements do in the nucleus?
organise chromatin fiber - establish separate compartments of higher-order chromatin structure
How do proteins & mRNA transverse the nuclear membrane?
nuclear pore complex
What size of proteins is NPC freely permeable to
<40 kDa
larger need active mechanism & nuclear transport factors
What is the NPC?
- 125 mDa
- embedded into both nuclear membranes
- cylinder
- 145nm diameter, 80nm long
- 69nm channel
- mammalian NPCs composed 30-60 nucleoporins
Organisation of the NPC?
8 composite protein rings, cytoplasmic surface and at inner surface
rings connected by spoke proteins
How do proteins get into the nucleus?
- NLS recognised by importin complexed with Ran
- importin binds cytoplasmic filaments
- translocation, binding pore proteins
- GEF - GDP on Ran changes to GTP
- Importin-Ran/GTP complex re-exported
- GAP hydrolyess to Ran-GDP
What are the NLS binding sites of importin alpha?
- monipartite NLS
- Bipartite NLS
- usually lysinse or arginine rich
What are Nuclear export signals? (NES)
- Leucine rich
- recognised by receptors, exportins
- presence of RNA-GTP
- GTP hydrolysis, dissociation of target protein
How can disease be associated with NPC?
- change in NUPs
- nuclear/cytoplasmic accumulation of material
- Nup214 mutation - accumulation of protein in nucleus
- detrimental upregulation of NF-kB, cancer
What happens in Triple A syndrome?
- mental retardation
- adrenal insufficiency
- muscle control of heart/oesophagus
- accumulation of ALADIN
What does ALADIN (Triple A syndrome) affect the import of?
- Aparataxin
- DNA ligase I
- Ferritin heavy chain
normally repair DNA under oxidative dress
How do viruses associate with the nuclear pore?
- small viruses able to cross without capsid disassembly
- Large viruses dock to pore via importin B or Nup214
How does bulk mRNA exit the nucleus?
RNA bound by 1000s of protein - hTREX
marks mRNA ready to be transported
TAP and p15 are exportins
What does mRNA nuclear export look like?
- elongates into rod and passes through pore centre
- material rounds into spherical particle on cytoplasmic side
What is the consequence of hTREX mutations?
- high grade tumours (aggressive)
- enhances formation of R loops
- DNA-RNA hybrid formed
- R loop stops transcription, increases DNA damage
- chromosomal lagging
