Objectives

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Objectives

• To know about how nucleus is organized in a
  eukaryotic cell
• To understand how nucleus regulates the movement
  of molecules in and out of the region
• To discuss the complications and advantages of
  having partially enclosed region for holding the
  genetic materials

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Nucleus

• The site within a eukaryotic cell where
• Chromosomes are localized and replicated
• DNA is transcribed
• Repository and control centre of most of the
  cells genetic information

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• Nuclear envelope
• Trilamellar appearance 2 membranes a
  perinuclear space
• Outer nuclear membrane
• Continuous with endoplasmic reticulum
  (perinuclear space is continuous with ER lumen)
• Studded on its outer surface with ribosomes
• Intermediate filaments anchoring nucleus to other
  organelles / plasma membrane

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• Inner nuclear membrane
• Rests on a network of supporting fibers (nuclear
  lamina)
• Nuclear pores
• Small cylindrical channels fusion of inner and
  outer membranes direct continuity between
  cytosol and nucleoplasm
• Lined with an intricate protein structure
  (nuclear pore complex NPC)

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• Octagonal arrangement of subunits protrude on
  both cytoplasmic and nucleoplasmic sides
• Proteins extend into perinuclear space anchor
  the pore complex
• Transporter moves macromolecules across fibers
  extend from the rings into cytosol and
  nucleoplasm (forming a basket)

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Transportation

• IN Enzymes / proteins for chromosome
  replication and DNA transcription
• OUT RNA molecules / partially assembled
  ribosomes for protein synthesis
• To protect newly synthesized RNA from being acted
  upon by cytoplasmic organelles / enzymes before
  it has been fully processed

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• Passive Transport
• 8 separate aqueous diffusion channels (periphery
  of the pore complex between the spokes) / 1
  channel (center of transporter)
• Freely permeable to ions and small molecules (lt10
  nm)

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• Active Transport
• Large proteins and RNA ( 26 nm)
• Requires energy specific binding of the
  transported substance to membrane proteins
• IN
• Amino acid sequences enabling the protein to be
  recognized and transported into nucleus by NPC
  (nuclear localization signals NLS)

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• Cytoplasmic receptor proteins (importins) bind to
  NLS mediate movement of proteins toward nuclear
  pores
• Binding of importins-NLS complex to cytoplasmic
  side of NPC
• The complex is transported into nucleus by
  transporter (center of the pore)
• GTP-hydrolyzing protein (Ran)

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• OUT
• Ribonucleoprotein complexes
• Amino acid sequences targeting the complexes for
  export through NPC (nuclear export signals NES)
• Exportins bind to NES mediate the complexes to
  transport out through NPC (mechanisms NLS
  importins)

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• Nucleoplasm
• The interior space of nucleus
• Nuclear matrix
• A protein network maintaining the shape of
  nucleus
• To provide an organizing scaffold for chromatin
  fibers
• Organizing DNA for orderly replication and
  transcription
• Guiding the newly formed mRNA to nuclear pores
  transportation to cytoplasm

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• Nuclear lamina
• A thin dense meshwork of fibers lining the inner
  surface of inner nuclear membrane
• To support nuclear envelope
• As attachment sites for chromatin

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• Nucleolus
• The ribosome factory
• A membrane-free organelle consisting of DNA that
  is being transcribed into rRNA (fibrils) rRNA
  molecules being packed with proteins imported
  from cytoplasm (granules)

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• A stretch of DNA carrying multiple copies of rRNA
  genes (nucleolus organizer region NOR)
• Multiple copies are grouped into gt 1 NORs reside
  on gt 1 chromosomes
• A nucleolus may contain rRNA genes derived from gt
  1 NORs

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• Size of nucleolus is correlated with its level of
  activity
• Cell approaches division ? chromatin condenses
  (as compact chromosomes) ? shrinkage ? nucleoli
  disappear
• Mitosis is ending ? chromatin uncoils ? NORs loop
  out again ? rRNA synthesis resumes ? near the tip
  of 10 chromosomes ? 10 NORs ? 10 tiny nucleoli
  (in human) ? nucleoli enlarge ? fuse into a
  single large nucleolus