6: Nucleus

Question 1

Nucleus architecture

Answer 1

• double membrane
  
  • 10-15nm between membranes
• outer membrane continuous with ER
  
  • a lot of movement between the two
• outer membrane contains many ribosomes

Question 2

Nuclear envelope

Answer 2

• is DYNAMIC!!!
• disintegrates during prophase
• reforms during telophase
• contains nuclear pore complexes (NPCs) that mediate movement through the envelope
• acts as a gate keeper
• 3-4 thousand pores on each nucleus
  
  • pores have 8 fold symmetry

Question 3

Nuclear lamina

Answer 3

• exists just below the envelope
• mechanical support
• made of IF lamins A B and C
• right angled meshwork
• phosphorylation by kinases in prophase causes disassembly and envelope disintegrates
• dephosphorylation by phosphatases in telophase causes assembly and envelope reforms

Question 4

DNA packaging

Answer 4

• 1 meter of DNA fits into each cell so is highly organized
• DNA wraps around histones to form a nucleosome
• 5 histones: H1, H2A, H2B, H3, H4
  
  • DNA is slightly acidic, histones enriched in basic AAs
  • highly conserved except H1
  • 2 copies of each histone (except H1) forms Octomer
• DNA wraps around octomer twice
  
  • then leaves small spacer before next nucleosome

Question 5

Histone H1

Answer 5

• sits on outside of octomer
• only one per nucleosome
• bigger and less curved
• sometimes called linker histone because it binds to the linker DNA between nucleosomes

Question 6

Chromatin

Answer 6

• nucleosomes are the basic unit of chromatin
• condenses to 30nm
• chromatin loops are held together by cohesin rings
• chromatin loops fold onto themselves to make euchromatin and heterochromatin

-chromatin = complex of DNA + protein

Question 7

Euchromatin

Answer 7

• loosely packed

- available to be transcribed

Question 8

Heterochromatin

Answer 8

• densely packed
• silent, repressed DNA
  
  • not being transcribed
• often distributed close to nuclear envelope

Question 9

Nucleoli

Answer 9

• ribosome factories
• rRNA is synthesized in the nucleolus and spliced into correct sized pieces
• nucleoli also fall apart during cell division, so no ribosomes are made during this time
• cell synthesizes all necessary proteins in advance of the onset of mitosis

Question 10

Nuclear pore complex transport

Answer 10

-movement of nuclear proteins being imported into the nucleus in a linear manner

Question 11

NPC structure

Answer 11

• 120nm in diameter
• proteins called nucleoporins
  
  • 30 unique nucleoporins
• 8-fold symmetry
• nuclear basket inside nucleus
  
  • more rigid than fibres
• transporter (FG nucleoporins)
  
  • phenylalanine and glycine
  • small molecules can diffuse through
• cytoplasmic fibres on outside

Question 12

Nuclear localization signals

Answer 12

• 8-30 AA in length
• strong positive charges (lys, Arg)
  
  • often contains prolines

-importin must bind to a protein with NLS
-to get out of nucleus must have nuclear export signal which will bind exportin
-

Question 13

NLS mechanism

Answer 13

1. Importin binds to protein with NLS
2. Once bound, the complex binds to a cytoplasmic fibre and fed through transporter
3. Ran-GTP binds to the complex to release cargo protein
4. Importin:Ran-GTP complex exit nucleus through transporter
5. Ran will hydrolyze GTP to release importin

Question 14

GEFs and GAPs

Answer 14

• GEFs: guanine exchange factor
  
  • will put on a GTP
  • activate protein
  • GEF for Ran = RCC1 (lives in nucleus)
• GAPS: GTPase activating proteins
  
  • help hydrolysis go faster
  • deactivate protein
  • GAP for Ran = RanGAP1 (lives in cytoplasm)