The nucleus Flashcards
Main purpose of the nucleus
- separate transcription (inside the nucleus) from translation (outside the nucleus)
- protects and stores DNA from toxic substances in the cytoplasm
Nuclear envelope
2 parallel phospholipid bilayers surrounding the nucleus and contains a double membrane, nuclear pores and nuclear lamina
- selective barrier and allows limited movement of molecules between nucleus and cytoplasm
- Small molecules and ions can passively diffuse through pores in the envelope but large proteins and RNA require active transport
Nuclear contents
chromatin (DNA + histones), nucleoplasm, nucleolus
Outer nuclear membrane vs inner nuclear membrane
ONM binds to ribosomes and is continuous with rough ER
INM bears integral proteins, connects to nuclear lamina
- intermembrane space is continuous with the ER lumen
Nuclear Lamina
- thin meshwork of filamentous proteins
- bound to the inner membrane of the nuclear envelope by integral membrane proteins
- provides structural support for the NE
- attachment sites for chromatin
- next to the nucleoplasm leaflet of the inner nuclear membrane
Intermediate filaments of nuclear lamina
- Lamins come together to form intermediate filaments
- composed of charities in the cytoplasm
- composed of lamins are unique to the nucleus
Nuclear pores
- serve as gateway between cytoplasm and nucleoplasm
- 3000 - 4000 pores per nucleus
- have complex protein structure (nuclear pore complex)
Nuclear Pore Complex
- composed of nucleoporins - projects into cytoplasm and nucleoplasm
- these proteins are hydrophobic and restrict movement in and out of the complex
- function: passive diffusion, regulated movement of larger molecules
- octagonal symmetry
Nuclear localizing signal
Several positively charged amino acids within the protein sequence
- regulated movement of proteins into the nucleus requires an NLS
- With NLS, When the protein is generated it is properly targeted to the nucleus
How does NLS target proteins in the nucleus?
- Protein with NLS cargo interacts with importing protein in cytoplasm
- Cargo/importin complex interacts with FG-NUPs at nuclear pore and enters nucleoplasm
- Ran-GTP interacts with importin; cargo dissociates and stays in nucleoplasm
- Ran-GTP/ importin complex exits nucleus through NPC
- GTP hydrolysed to GDP. Importin released in cytoplasm to find new cargo
Ran-GTP
- ran-GTP is a G protein
- changes the confirmation of importin so it releases its cargo (in the nucleus)
- ## releases importin in the cytoplasm
Why is nuclear import and export critical to cell function?
- nucleocytoplasmic trafficking is required for things to get in such as nucleotides, structural proteins, DNA packaging proteins (histones) and more proteins
The nucleolous
- dense structure within the nucleus
- where ribosome synthesis occurs
- functions to synthesize rRNA and rRNA processing
- assembles 60s and 40s subunits but do not synthesize the complete ribosome
40s and 60s subunits
Ribosomes contain two different subunits, both of which are required for translation. The small subunit (40S) decodes the genetic message and the large subunit (60S) catalyzes peptide bond formation.
- each subunit consists of a variety of independent proteins as well as structural RNAs
