moleculoar concept!! silly, but driving me crazy! - er12

[ArchivalUser]

where does ribosomal SUBUNITS assembly occur in the cell?
nucleolus or nucleus?

i got confused:

the 5s rRNA is synthesized in the nucleus, then transported into the nucleolus..
the 45s rRNA ( that will give rise to 5.8, 18, 28s rRNA) is synthesized in the nucleolus..

so? the assembly of the SUBUNITS will occur in the nucleolus, right?

[ArchivalUser]

the reason i was focusing about ribosomal SUBUNITS assembly, is that in HY molecular bio, they have a picture explaining in it the various steps of ribosomal synthesis, and in that pic, they focused a looot, on the idea that the FINAl assembly of the subunits INTO FORMING MATURE RIBOSOMES occur in the cytoplasm!!

i.e: after the large and the small subunits have been assmebled somewhere in the nucleus or nucleolus, they are transported INTO the cytoplasm, where they will be assembled to form the ribosomes...

so according to HY, mature ribosomes assembly occurs in the cytoplasm... but the subunits assembly occurs either in nucleus or nucleolus ( it didnt state which exactly, though i lean more towards the nucleolus!!)

[ArchivalUser]

The 45S sununit assembles in the nucleolus. The 5S assembles outside the nucleolus but still in the Nucleus. The the 5S joins the 45S together inside the nucleolus, along with some other ribosomal proteins. They form a complex..this complex will the split into the 60S and 40S subunits. Mind you..these are still seperate and not together yet. Up to this step ..everthing has taken place within the boundries of the nucleus. Now these 2 subunits will pass thru the nucleopores (unattached) and go into the cytoplasm and organize as as one complex called the ribosome. So the ultimate assembly of the "ribosome" takes place in the cytolplasm, but the assembly of the "subunits" takes place in the nucleus (5S) and nucleolus (45S), with the formation of the large complex in the Nucleolus.
I think to get your answer of what takes place where, you would need to pay attention to the question and see exactly what they are asking for.

[ArchivalUser]

there's no q usmleiosis.. i was just thinking about that myself.. i mean when i read in HY that the ribosomal assembly occurs in the cyto (as u said), i just asked myself, what about the subunits!?
anyways....

ur post was very accurate..
but: u were referring to the 5s and the 45s as ''subunits'', which, i think, is wrong..

the subunits are 40s and 60s... so, we cant use the word ''subunit'' until the 5s ( that was formed in the nucleus, then transported into the nucleolus) is joined by the 45s and the ribosomal proteins: INSIDE the nucleolus to form a complex... now that ''complex'' will then split to give rise to the 40s and the 60s subunits..

so based on this, i assume that the ''subunits: 40s and 60s'' are assembled in the nucleolus.. ( since the ''complex'' is formed in the nucleolus)

[ArchivalUser]

yes exactly...sorry for the minor error there..but glad you got it now.

[ArchivalUser]

nucleolus= has ribosomal DNA which is transcribed by RNA pol I into rRNA( the most abundant=45SrRNA)- the big piece 45SrRNA is cleaved into nucleolus in 28S, 18S, 5.8S=production and maturation of ribosome and release into cytoplasma. 18S forms 40S subunit and 28S, 18S, 5.8S+5S( from nucleoplasma) form 60S subunit

nucleoplasma has RNA pol II, transcribing DNA, euchromatin from there, into hnRNa, then maturation in mRNA and releasing into cytoplasma

nucleoplasma also has RNA pol III, making tRNA,snRNA, 5SrRNA from DNA.......so the smallest particles.


this is what i understood.

[ArchivalUser]

yup, thx for the help...

[ArchivalUser]

thanks mira.. thats basically what we are saying...