Lecture 2

• two membranes + nuclear lamina = nuclear envelope
• Inner nuclear membrane defines nucleus
• Outer nuclear membrane continues with rough endoplasmic reticulum
• Inner and outer membrane are each phospholipid bilayers separated by perinuclear space
• Nuclear envelope is basally supported by the nuclear lamina
• Meshwork of filaments located adjacent to the inside face of the inner nuclear membrane
• Comprised of a class of cytoskeletal proteins known as intermediate filaments (nuclear lamina)
• Nuclear membrane: Cage or lattice-like network that interconnects with nuclear pores
• Nucleoli is a clearly defined structure in nucleoplasm
• Does the Nucleolus have a defined membrane?

  No
• What is the site of ribosome biogenesis

  Nucleolus/Nucleoli
• What is formed around regions of DNA encoding rRNA

  Nucleolus/Nucleoli
• 80% of RNA in the Nucleolus/Nucleoli is what kind of RNA?


  rRNA
• Does nuclear bodies have a membrane

  No
• What genetically defines structures formed as a result of making ribosomes

  Nucleolus/Nucleoli
• Formation of nuclear bodies may enhance process efficiency and facilitate regulation
• Nuclear bodies: Concentrated regions of protein and RNA
• Chromatin structure is dynamic
• Structure of chromatin determines gene expression
• Nucleosome=DNA + histone complex
• What does chromatin do

  Packaging 2m of DNA within nucleus
• In chromatin histone tails extend from nucleosome can be targets of several post-translational modifications
• In unacetylated chromatin, chromatin highly condenses (transcriptionally inactive) heterochromatin
• In acetylated chromatin, chromatin is less condenses (transcriptionally active) heterochromatin
• Histone post-translational modifications represent histone code, determines gene expression
• Proteins modify histones control chromatin structure and access of DNA to replication, transcriptional and repair machinery
• Transcriptional machinery

  1. Activators bind to DNA to recruit chromatin remodelling complexes to open up chromatin structure
  2. Also recruit protein bridge (mediator) to help recruit transcription factors to a promoter sequence
  3. Mediator complexes facilitates assembly of preinitiation complex that includes loading a RNA polymerase on DNA
  4. After initiation transcription if PAUSED by an elongation factor complex
  5. Elongation pause is relieved by phosphorylation and remodelling of the elongation factors by a cdk/cyclin pair
• 5S


  Transcribed into nucleoplasm by RNA Pol III and diffuses into nucleolus
• rRNA

  First transcribed by RNA Pol I as a large transcript (pre-rRNA) that is then processed to mature rRNA found in ribosomes
• 60S and 40S ribosomal subunits

  • Undergo a quality control check prior to export in cytoplasm (only correctly assembled go through)
• Ribosome biogenesis

  Making the machines for translation
• Passage through nuclear pores

  Mediated by nuclear export adaptors
• 80S

  Final assembly into functional translation machinery occurs in cytoplasm
• Nuclear Transport: The Nuclear Pore Complex (NPC) spans both nuclear membranes
• The human NPC is big
• Passive diffusion of small molecules

  Can occur through the NPC
• The NPC pore is large enough to fit larger ribosomal subunits, larger molecules needs guidance
• The NPC is the only way in or out of the nucleus
• Only 30 different proteins (nucleoporins (Nups)) make up the NPC
• Nups


  • Nucleoporins (Nups)