Nucleus Flashcards
What is the history of the nucleus?
A prokaryote organism was invaded by another, non-nucleated cell, setting up an endosymbiotic relationship, with the invading cell’s outer membrane forming the nuclear envelope (Pederson, 2011)
What is the nucleus?
Double membrane-enclosed organelle found in eukaryotic cells
Contains most of the cell’s genetic material and is the control centre of the cell
Can be multinucleated - 2 nuclei, 1 macronucleus and 1 micronucleus
Macronucleus governs all the general cellular activities such as water balance
Micronucleus is responsible for sexual reproduction
What is significant about leukocytes?
Most nuclei are spherical but in leukocytes we can find multilobed nuclei - polymorphonuclear (K Shrivastav, 2020)
What does the nucleus do?
Nucleus maintains the integrity of DNA and controls the activities of the cell by regulating gene expression and mediating the replication of DNA during the cell cycle
Separates nuclear and cytosolic enzymes – concentrated where needed to protect DNA from harsh cytosolic environment
Separates site for transcription from translation in the cytoplasm
What are some components of the nucleus?
Nuclear Envelope Nucleolus Nuclear Matrix Nuclear Lamina Chromatin Nuclear pore complex Sub nuclear bodies
How can we visualise the nucleus?
Fluorescence Micrograph with markers for NPC, nucleolus and chromatin
Resolution limit of 200 nm (wavelength of visible light)
Used Confocal Microscopy
Electron micrograph - spatial resolution of 3-5 nm
Describe the nuclear envelope?
Nuclear envelope is a double lipid bilayer membrane that encloses the entire organelle
Outer membrane is continuous with the RER (similar structure with bound ribosomes)
The inner nuclear membrane contains many inner nuclear membrane proteins, mostly associated with the nuclear lamina
There is a perinuclear space between the two membranes 20-40 nm (K Shrivastav, 2020)
What does the nuclear envelope do?
Role - a physical barrier, separating the contents of the nucleus (nucleoplasm) from the chemical reactions in the cytoplasm
This allows reactions synthesising genetic material and RNA before replication
What is significant about the nuclear envelope?
The nuclear envelope disappears during prophase of mitosis into fragments (depolymerisation and phosphorylation)
It is reassembled as two new envelopes (repolymerisation and dephosphorylation) (BSCB)
Nuclear envelope is not a spherical coat - the surface invaginates to form deep or shallow tubules in the nucleoplasm
This could be to bring nuclear pores closer to deeper parts of the nucleoplasm (BSCB)
Describe the nucleolus?
Non-membrane bound dynamic element within the nucleus
It disappears in late prophase and reappears in telophase (during cell division)
The number of nuclei differ per nucleus changes
Structure consists of:
Fibrillar centers - contains rRNA (in the form of partly condensed chromatin)
Fibrillar component - they surround the fibrillar centers, which contain RNA molecules in the process of transcription
Granular regions - outer most regions having mature ribosomal precursor particles
(K Shrivastav, 2020)
Describe the nuclear matrix
A nuclear scaffold as it has a network of protein filaments providing a structural framework for the organisation of chromatin
It facilitates transcription and replication
It has a similar structure to the nuclear lamina except the nuclear matrix extends within the nucleoplasm
We can isolated the nuclear matrix from the nuclei (nucleolus) using: non-ionic detergents, nucleases and high salt buffers
(K Shrivastav, 2020)
Describe the nuclear lamina?
A dense (30 -100nm thick) fibrillar network inside the nucleus Composed of intermediate filaments and full of membrane associated proteins (lamin proteins) Contains lamin A,C and B and is associated with emerin proteins
What does the nuclear lamina do?
Nuclear lamina regulates important cellular events such as DNA replication and cell division (not just mechanical support)
Participates in chromatin organization and it anchors the nuclear pore complexes embedded in the nuclear envelope - provides great stability
Similar to the cytoskeleton
This controls what happens to gene expression as it is involved in how compact the DNA is and therefore what can be expressed
What are laminopathy disorders?
Group of rare genetic disorders caused by mutations in genes encoding proteins of the nuclear lamina
Leads to defects in filament assembly and/or attachment to the nuclear envelope
Jeopardizes the nuclear envelope’s stability in physically stressed tissues such as muscle fibres, bone, skin and connective tissue
Two types: Emery-Dreifuss muscular dystrophy and Hutchison-Gilford progeria syndrome (HGPS)
First reports of laminopathies in the late 1990s
We can see abnormal looking nuclei
Large variety of clinical symptoms including skeletal and/or cardiac muscular dystrophy, lipodystrophy, neuropathies and progeria (premature aging)
No cure and treatment is largely symptomatic and supportive
Describe the laminopathy disorder: Emery-Dreifuss muscular dystrophy?
A condition that mainly affects skeletal muscles and cardiac muscle
Earliest features are joint deformities called contractures, restricting the movement of joints
Most affected individuals also experience slowly progressive muscle weakness and wasting
Almost all have heart problems, due to abnormalities of the electrical signals that control the heartbeat (cardiac conduction defects)
