Lecture 23: Nucleus, Nuclear Import, Nucleolus Flashcards
The nucleus function
- Storage, replication and repair of genetic material
- Expression of genetic materials: transcription of mRNA, tRNA and rRNA, and RNA splicing
- Ribosome sbiosynthesis
Storage of DNA is aided by
Histones which supercoil the DNA into a chromatid
What causes DNA damage?
Replication errors, cellular metabolism, chemical exposure, ionization radiation, UV light exposure
During transcription…
MRNA, tRNA, rRNA and many more are transcripted. The control of transcription is the promoter region,
RNA splicing
Cutting out the introns (non coding regions of RNA) and fudge together the Exons (coding regions of RNA) through the method of spliceosomes
Transcription occurs in the…
Nucleus
Coding RNA vs Noncoding RNA
Coding - mRNA, types of RNAS that code for proteins
Noncoding - mRNA and tRNA, types of RNAs that DO NOT code for proteins
Noncoding RNA takes place in the…
Nucleolus, transcribed from DNA
The nucleus structure
- nuclear envelope: nuclear membrane, nuclear pores, nuclear laminate
- Nuclear content: chromatin, nucleoplasm, nucleolus
The nuclear envelope structure
- 2 parallel phospholipid bilayers separated by 10-50 nm space
- outer nuclear membrane (ONM): binds ribosomes and is continuous with RER
- inner nuclear membrane (INM): has integral proteins and connects to the nuclear lamina
Nuclear envelope functions
- separates nuclear content from cytoplasm
- separates transcription and translation processes
- selective barrier that allows limited overeat of molecules between nucleus and cytoplasm - through nuclear pores
The nuclear lamina functions
- supports the nuclear envelope, structural support
- attachment sites for chromatin
What is the nuclear lamina?
- thin mesh work of IF proteins: Lamins
- nuclear lamina is bound to inner membrane of the nuclear envelope by integral membrane proteins
Plant cells do have a nuclear lamina but…
Not made of lamin protein
Nuclear pores
- gateways between cytoplasm and nucleus
- 3000-4000 pores
- found where inner and outer membrane fuses
- contains complex protein structure that involve arrangement of different type of proteins
Nuclear pore complex (NPC)
- located within nuclear pore
- composed of nuceoporins (NUPs) - proteins
- octagonal symmetry
- projects into cytoplasm and nucleoplasm
- supramolecular complex
- passive diffusion for smaller molecules: rapid, 100 molecules/min/pore
- regulated movement of larger molecules: slow 6 molecules/min/pore
Nuclear import: Nuclear localization signal (NLS)
Targeting sequence from AA that regulates the movement of proteins into the nucleus. Several positively charge AA within the sequence
How does the NLS target proteins to nucleus?
- Protein with NLS (cargo) interacts with Importin protein in cytoplasm
- Cargo complex interacts with FG-NUPs at NPC and enter nucleoplasm
- Rap-GTP (G-protein) interacts with importin; cargo dissociates and stays in nucleoplasm
- Ran-GTP/Importin complex edits nucleus through NPC
- GTP hydrolysed to GDP. Importin released in cytoplasm to find new cargo
- entire process is controlled
Nuclear import and export are important for the following proteins to function
- structural proteins
- DNA packaging proteins
- proteins for DNA replication, repair and transcription
- proteins for RNA processing and export
- proteins for ribosomes synthesis and export
Nucleolus
- largest structure in nucleus
- function: to make ribosomes
- not membrane bound
- consists of two major components: small ribosomal subunits which read RNA and large subunits which join AA to form polypeptide chain
