Homework - Nucleus

Question 1

Describe the structure of the nucleus

Answer 1

The nucleus consists of four parts: Nuclear envelope, chromatin, nucleolus and nuclear matrix.

• Nuclear Envelope
  A double membrane structure surrounding the nucleus, with the outer membrane often continuous with the Endoplasmic reticulum. Protein and lipids are the macromolecules which comprise the outer membrane of the nuclear envelope and are continuous with the ER. Proteins (lamins) are the macromolecules which comprise the lamina which lines the inner surface of the nuclear envelope. This nuclear lamina supports the nuclear envelope, gives it shape and stability and provides a structural link between the nuclear envelope and chromatin.
• Chromatin
  Thread-like structures composed of DNA tightly wound around proteins which condense into visible chromosomes during cell division
• Nuclear Matrix
  Also called the nucleoplasm, is the fluid inside the nucleus in which chromatin is suspended.
• Nucleolus
  A dense region (the most prominent substructure) within the nucleus where ribosomes (rRNA) undergo transcription, processing and assembly.

Question 2

Describe the four (4) parts which comprise the nuclear envelope

Answer 2

The nuclear envelope consists of two concentric membranes enclosing the peri nuclear space and a nuclear lamina lining the inner surface. Nuclear pore complexes are embedded within the nuclear envelope, allowing molecules to move between the nucleus and cytoplasm.

• Outer Nuclear Membrane
  Outermost layer of the nuclear envelope, continuous with the ER and often having ribosomes attached to it.
• Inner Nuclear Membrane
  The inner layer of the nuclear envelope which directly faces the nuclear matrix and is associated with the nuclear lamina.
• Perinuclear space
  The fluid-filled space between the outer and inner nuclear membranes.
• Nuclear lamina
  A protein network that lines the inner nuclear membrane, providing structural support to the nucleus and playing a vital role in maintaining the shape of the nucleus and anchoring chromatin fibers.

Question 3

What is the mechanism of karyophilic proteins transport into the nucleus

Answer 3

Karyophilic proteins are transported into the nucleus through nuclear pore complexes (NPCs) in a process called nuclear import. Molecular transport through the nuclear pore complexes is accommodated by passive transport, active transport and bidirectional transport.

• There are short amino acid sequences on karyophilic proteins that direct selective nuclear import called nuclear localization signals (NLSs)
• Importin is a carrier molecule that mediates the transport of karyophilic proteins by acting as an adapter molecule between the cargo protein and the carrier protein.
• There is a complex that forms between the karyophilic protein and cytoplasmic factors which binds to the nuclear pore before the protein enters the nucleus called nuclear pore-targeting complex.
• This process of translocation through the nuclear pore complex is dependent on ATP hydrolysis

Question 4

What are the functions of lamina

Answer 4

The functions of the nuclear lamina include:
- provide structural support and integrity to the nucleus
- plays a vital role in various nuclear activities including DNA replication, chromatin organization, gene regulation, etc.
- maintains nuclear positioning within the cell by connecting to the cytoskeleton and acting as an interface between the nucleus and the cell’s structural framework
- lines the inner surface of the nuclear envelope
- composed of lamins
- supports the nuclear envelope by giving it shape and stability
- provides a structural link between chromatin and the nuclear envelope
- regulates its integrity by phosphorylation and dephosphorylation
- phosphorylation of the lamins triggers the disassembly of the nuclear lamina which in turn causes the nuclear envelope to break up into vesicles; dephosphorylation of the lamins helps to reverse the process
- the old nuclear envelope reforms the new nuclear envelope