Nucleus Flashcards
1
Q
What is the nucleus
A
- A memebrane bound organelle in eukaryotic cells containing DNA
- And the cells control centre
2
Q
Define eukaryote
A
- An organism whose cells have a memebrane-bound nucleus
- Including animals, plants, fungi and other unicellular organisms
3
Q
What is part of the structure of the nucleus
A
- Contains DNA arranged in chromosomes
- Surrounded by the nuclear envelope/membrane
- Nuclear lamins support the nuclear membrane
4
Q
Define DNA
A
- A polymer composed of two polynucleotide chains that coil around each other to form a double helix
- Carries genetic instructions for development, functioning, growth, and reproduction
5
Q
What is the nuclear envelope/membrane
A
- It is made up of a phospholipid bilayer that surround the nucleus
- Which encloses the genetic material
- Outer membrane is continuous with rough ER
- Inner membrane contains proteins that act as anchoring sites for chromatin and for the nuclear lamina
- The space inbetween the membranes is called perinuclear space, where proteins made get transported to
6
Q
What is nuclear lamina
A
- It is in the nuclear side of the inner nuclear membrane
- Meshwork of interconnected protein subunits called nuclear lamins
- The lamins are intermediate filament proteins plymerised into 2D lattice
- Gives shape and stability to envelope
- attached to nuclear pore complexes NPC and integral proteins on the inner membrane
7
Q
Why does the nuclear envolope contain pores
A
- To control the movement of substances in and out of the nucleus
- E.g. RNA is selectively transported into the cytoplasm
- Proteins are selectively transported into the nucleus
- Regulating gene expression
8
Q
Why is the nucleus important
A
- Separates fragile chromosomes from cell contents – crucial for proper function of cell
- DNA replication, transcription and RNA processing - all in the nucleus
- Separates RNA transcription in the nucleus from translation machinery in the cytoplasm
9
Q
What is the nucleolus
A
- When one or more nucleoli are found inside the nucleus
- Most prominent in cells that are synthesising large amounts of protein
- Sites at which ribosomes are reassembled and ribosomal RNA is transcribed
10
Q
What happens to the nuclear envelope during mitosis
A
- Nucleus disassembles, lamina depolymerises & NPCs disperse in cytosol (phosphorylation of lamins and NPC proteins)
- Some NPC proteins bound to nuclear import receptors – important in reassembly of NPCs at end of mitosis
- Nuclear envelope membrane proteins disperse throughout ER membrane
- Later in mitosis, nuclear envelope reassembles close to surface of daughter chromosomes (dephosphorylation of lamins)
10
Q
What is around and inside nuclear pores
A
- Nuclear basket
- Inside are proteins called neoporins that create eightfold rotaional symmetry
- Inside the neoporins is filled with numerous repeats of unstructured phenylalanine-glycine proteins that create a mesh inside the NPC
- The mesh acts as a sieve - restricting diffusion of large macromolecules but smaller ones can pass through
11
Q
How do large macromolecules go through NPCs
A
- They need import or export signals
- Other proteins contain both nuclear localisation signals (NLS) and nuclear export signals (NES)
- Relative rates of import and export determine steady-state localisation
12
Q
What are nuclear localisation signals
A
- Responsible for the selectivity of active nuclear import process
- Most commonly - 1 or 2 short sequences rich in lysine and arginine (+ charge) in aa - sequence varyies for different proteins
13
Q
What are nuclear import receptors
A
- They are karyopherins (proteins that are involved in transporting molecules between the cytoplasm and the nucleus)
- Nuclear localization signals must be recognized by nuclear transport receptors (import or export receptors) to get in or out
- Receptors can use adaptor proteins that form an import receptor/NLS bridge (for proteins that dont fit into an import receptor)
- Have FG binding sites
14
Q
What happens in the NPC when cargo has to go through
A
- Nuclear import receptors recognise nuclear localisation signals on the cargo proteins and bind (or use an adaptor protein to bind)
- The FG (phenylalanine and glycine) binding sites on the import receptors bind to FG repeats found in the NPC mesh
- This binding disrupts interactions between the repeats dissolving the mesh
- Allowing the receptor-cargo complex to diffuse into the NPC pore
15
Q
What are nuclear export signals
A
- Same as nuclear localisation signals but required for exporting cargo
16
Q
What direction do import and export receptors move cargo proteins
A
- Import receptors bind their cargo molecules in the cytosol, release them in the nucleus, and are then exported to the cytosol for reuse
- Export receptors bind cargo in nucleus, release them in cytosol, then go back into nucleus
17
Q
What does Ran-GTP and Ran-GDP do
A
- Ran-GTP binds to the nuclear import receptor in the nucleus (after delivering cargo) to allow the receptor to go back into the cytosol for reuse
- Once back in the cytosol Ran-GTP is hydrolysed into Ran-GDP (by Ran GAP) and as a result dissociates from the receptor
- Then the import receptor is ready for another cargo protein to join
18
Q
What deos a DNA nucleotide consist of
A
- Nitrogenous base
- Five carbon sugar (deoxyribose)
- Phosphate group
19
Q
Which bases are prymadines
A
- Cytosine
- Thymine
- Uracil
20
Q
Which bases are purines
A
- Adenine
- Guanine
21
Q
What are the differences between RNA and DNA
A
- Ribose sugar vs deoxyribose sugar
22
Q
How do nucleotides join together
A
- By consensation reactions
- Creating phosphodiester bonds
- Between 5’ triphosphate and 3’ hydroxyl group on adjacent carbons
23
Q
Characteristics of double helix
A
- Nucleotides are covalently linked
- Hydrogen bonds between bases
- Strands run anti-parallel
24
Base pairing
- 2 H bonds between A and T
- 3 H bonds between G and C
- Purine is paired with a prymidine because it is most energetically favourable
25
What are chromosomes
- 23 pairs of chromosomes
- Single linear DNA molecule + proteins fold the DNA into compact structure
26
What is a centromere
- The middle of a sinlge chromosome and where sister chromatids join together
- Keeps chromosomes properly aligned during cell division
27
What are telemeres
- Repetitive stretches of DNA located at the ends of linear chromosomes
- Protect the ends of chromosomes to keep them from unravelling
- Telomeres lose a bit of their DNA every time a cell divides. When all of the telomere DNA is gone, the cell cannot replicate and dies
- WBC and other rapidly dividing cells have an enzyme (Telomerase) that prevents their chromosomes from losing their telomeres – cells live longer
28
What is heterochromatin
- Chromatin regions that are condensed during interphase and transcriptionally active
- Found in the nucleus
29
What is euchromatin
- Chromatin regions that are decondensed and DNA sequences are being transcribed into RNA
30
What are histones and what do they do
- It is a complex of eight histone
proteins
- A histone octamer forms a protein core around which the
double-stranded DNA is wound
31
What are looped domains
- Folded chromatin fibre are attached to proteins forming a chromosome scaffold
- Looped domains can unwind for gene expression or for histone-modifying emzymes, chromatin remodelling complexes or RNA polymerase
32
What structures and functions do we have to protect our DNA
1. Telomeres - prevent degredation
2. Nuclear pores - controls/monitors what passes through the nucleus
3. Nuclear localisation signal - contols what cones in and out of nucleus
4. Condensation of chromatin - controls what section of DNA gets transcribed
33
What structure is responsible for making ribosomes
Nucleolus
