7 - The Nucleus Flashcards
what is a nucleosome
A nucleosome is when DNA is wrapped around a histone protein
what is chromatin
chromatin a series of repeated nucleosomes
where does transcription occur and translation occur
transcription occurs in the nucleus
translation in ribosomes of the cytoplasm
what are the 2 processes of modification that mRNA needs to undergo after transcription
- adding a 5’ (prime) cap
- 3’ polyadenylation ( adding a poly-A tail)
what do post transcription modifications help with?
- stabilising the mRNA
- The binding of mRNA to ribosomes and in initiating translation
- export from the nucleus
describe the structure of a nucleus
- Double membrane nuclear envelope with peri nuclear space between
- double membrane forms regions of ER
- nuclear pore complexes
- nuclear lamina which anchors chromatin to envelope
- chromatin
- Nucleolus
- ribosomes on ER
Describe Nuclear pore complexes
- go through both levels of the nuclear envelope
- made from 30 different proteins called nucleoporins
- each pore is made from 8 identical proteins
- has cytosolic fibrils extending the cytoplasmic side
- has a nuclear basket on the nucleus side
- each NPC has 500-1000 proteins in so very large
Proteins larger than 60000 Daltons cannot diffuse through nuclear pore complexes so how are they transported into the nucleus
- large proteins have nuclear localisation signals on their end terminus
- this is a special signal sequence of amino acids that let the protein import into the nucleus
what is the mechanism behind how larger proteins enter the nucleus through a nuclear pore complex
- Protein with nuclear localisation signal gets recognised by the nuclear import receptors (importins)
- importins then bind to the nuclear localisation signal
- the complex moves to the nuclear pore complex and binds to the cytosolic fibrils via FG repeats
- the complex then moves through the NPC repeatedly unbinding and binding to the next FG complex which line the inner cavity of the NPC
- The complex eventually reached the nucleus
What is a GTP binding protein?
A protein that binds to GTP and also when activated acts as a GTPase hydrolysing GTP into GDP and Pi
what is GAP?
- GTPase Activating Protein
- This binds to GTP binding protein and induces GTP hydrolysis
What is GEF?
- Guanine nucleotide Exchange factor
- Binds to GTP binding protein and causes it to release GDP in exchange for a new GTP
How does the nuclear import protein release the protein into the nucleus?
Ran-GTP binds to the nuclear import protein causing a conformational change in the importin releasing the protein into the nucleus
How is the importin/nuclear import protein recycled back into the cytoplasm and thus how is Ran-GTP cycled in this mechanism
- Ran-GTP binds to the importin in the nucleus causing it to release its protein
- The Ran-importin complex travels along FG repeats back into the cytoplasm
- A Ran binding protein then seeks out a Ran anf binds to Ran in the Ran-importin complex, this causes the importin to dissociate from Ran-GTP being free for another use
- A Ran GAP protein then hydrolyses the GTP into GDP and Pi causing the Ran binding protein to dissociate from Ran-GDP
- Ran-GDP can freely move back into the nucleus through a Nuclear pore complex
- Ran GEF regenerates Ran-GTP by exchanging the GDP for a GTP molecule
How does mRNA change change just before being exported to the nucleus
before export mRNA is complexed with numerous proteins being converted to mRNP
what is mRNP
mature mRNA with a 3’ Poly-A tail and 5’ Cap binding complex
How is mRNP exported from the nucleus
- Nuclear export receptors bind to the adaptor proteins, which bind to the 5 prime cap binding complex
- This then travels along the FG repeats into the cytoplasm
describe the nucleolus
- not membrane bound
- distinctive because its dark and packed with RNA’s and proteins
- site of ribosome synthesis
- constantly making ribosomes to keep up with protein demand
Describe ribosomes
- 1 Large subunit, 60s, 49 proteins and 3 rRNA’s
- 1 Small subunit, 18s, 33 proteins and 1 rRNA’s
Large subunit made from 5.8s, 28s, 5s rRNA
Small subunit made from 18s rRNA
there is extremely high demand for ribosomal synthesis, how does the body deal with this
- 4 rRNA’s are encoded by more than 200 rRNA genes across many chromosomes
where and how are the 4 rRNA’s transcribed in the cell?
- 18s, 5.8s, 28s rRNA’s transcribed by RNA polymerase 1 in the nucleolus
- 5s rRNA is transcribed by RNA Polymerase 3 in the nucleoplasm
Describe how DNA becomes 90s-rRNA
- the rRNA gene on DNA is transcribed into 45s pre-rRNA
- Modification occurs where ribosomal proteins enter the nucleus from the cytoplasm through the NPC.
- these proteins bind the the 45s pre-rRNA in the nucleolus forming 90s pre-rRNA
How does 90s pre-rRNA become 3 rRNA’s
- snoRNP’s are small nuclear RNA’s with proteins bound
- snoRNP’s are complimentary to the target regions of 90s pre-rRNA
- the proteins bound on snoRNP’s are enzymes which modify the 90s pre-rRNA in 3 ways
- The ITS and ETS regions of 90s pre-rRNA get cleaved to end up with pure rRNA
Which methods do SnoRNP’s use to modify 90s pre-rRNA
1) methylation of hydroxy groups on ribose
2) Conversion of uridine to pseudouridine
3) Cleavage of 45s pre-RNA
explain how pre-RNA is cleaved into 3 rRNA’s and which rRNA are they
- 18s, 5.8s, 28s
- Exoribonucleases on the snoRNP’s cleave off the ETS (externally transcribed spaces) which are regions in the pre-RNA outside of the regions that are rRNA
- Endoribonucleases on the snoRNP’s cleave off the ITS (internally transcribed spaces) which are regions that dont code for rRNA between rRNA coding regions
where do the large and small subunits of the ribosomes finally combine to become a full ribosomes
In the cytoplasm after each subunit is separately transported out of the nucleus
which type of rRNA is not transcribed in the nucleolus
5s rRNA, which is transcribed in the nucleolus
