ribosomes--where are they assembled???

[mytoechondriac]

Hi!!

I'm trying to figure out how ribosomes are made. I know that rRNA is transcribed in the NUCLEOLUS, and my EK Bio book says that "ribosomal subunits are assembled at the nucleolus." OK, fine. But then it has a question about where rRNA is translated, and it says rRNA is translated in a place that is NOT THE NUCLEOLUS(presumably, the cytoplasm, where most translation takes place). So is this telling me that the rRNA is translated in the cytoplasm, the polypeptides of the ribosome are made in the cytoplasm, and then the RIBOSOMAL SUBUNITS go BACK to the nucleolus to be assmbled? It doesn't seem logical or likely to me. Can someone please help me out on this. Thanks.!!

---

Members don't see this ad.

 

[seraph524]

Yep. the rna is made in the nucleolus, while the ribosomal proteins are made in the cytoplasm. The proteins are then sent into the nucleus through nuclear pores to be assembled with the rna to make mature ribosomes, which then gets exported back into the cytoplasm. funky aint it?

 

[mytoechondriac]

Yep. the rna is made in the nucleolus, while the ribosomal proteins are made in the cytoplasm. The proteins are then sent into the nucleus through nuclear pores to be assembled with the rna to make mature ribosomes, which then gets exported back into the cytoplasm. funky aint it?

I thought that was it was saying... but it just seemed odd and counterintuitive... but this clears it all up. THANK YOU!!

 

[supafield]

Ribosomes aren't completely finished in the nucleus though... the ribosome will become fully mature in the cytoplasm, which is thought to be so the ribosome doesn't start translating RNA in the nucleus.... or at least what TPR tells me

 

[seraph524]

Ribosomes aren't completely finished in the nucleus though... the ribosome will become fully mature in the cytoplasm, which is thought to be so the ribosome doesn't start translating RNA in the nucleus.... or at least what TPR tells me

Yup, the large and small subunits will not assemble until they are in the cytoplasm.

 

[Farcus]

well technically they can't make anything even while inside the nucleus since ribosome only work when its binded to post transcribed mRNA. mRNA binds to the small ribosome unit then the big one comes and finish the forming of ribosomes. Because there are no post transcribed mRNA in the nucleus nothing would happen. Thats how I see it, correct me if I"m wrong.

 

[cornpops21]

well technically they can't make anything even while inside the nucleus since ribosome only work when its binded to post transcribed mRNA. mRNA binds to the small ribosome unit then the big one comes and finish the forming of ribosomes. Because there are no post transcribed mRNA in the nucleus nothing would happen. Thats how I see it, correct me if I"m wrong.

This is correct.

In eukaryotic initiation of translation, the small subunit is bound weakly to the methionine-tRNA and is recruited to the 5' cap. Then the small subunit/met-tRNA complex slides along the mRNA trying to find the first available AUG codon and binds to it tightly. Once this happens, the initiation factor IF2 (which is bound to the small subunit) undergoes GTP hydrolysis, which causes binding of the large subunit.So translation is initiated from the first AUG encountered from the 5' cap. This is known as the Sliding Clamp Hypothesis.

And of course, this is all occurring in the cytosol where all translation of proteins takes place.

 

[superduper12]

well technically they can't make anything even while inside the nucleus since ribosome only work when its binded to post transcribed mRNA. mRNA binds to the small ribosome unit then the big one comes and finish the forming of ribosomes. Because there are no post transcribed mRNA in the nucleus nothing would happen. Thats how I see it, correct me if I"m wrong.

I thought post transcriptional processing of mRNA occured in the nucleus...isn't it the same for the rRNA...or does post transcriptional processing for rRNA occur in the cytoplasm?

 

[superduper12]

So I just wanted to make sure I understand this:

- In the nucleolus DNA is transcribed to rRNA.
- This rRNA then leaves the nucleolus.
- Post transcriptional processing of rRNA occurs in cytoplasm.
- Then the modified rRNA is translated in the cytoplasm to become polypeptides, which are the ribosomal subunits.
- These ribosomal subunits go back into the nucleolus to be assembled with another rRNA chain/ protein? (don't get this part)
- The new subunits then leave the nucleus to become fully mature in the cytoplasm.

I don't know...can someone explain this step by step.