Nucleolus & Transcription

[futuredoctor10]

I know the nucleolus is the site of rRNA synthesis and ribosome assembly.

1. Is the nucleolus also the site of RNA synthesis in general (for mRNA or tRNA for example)?

2. If not, I assume that means DNA undergoes transcription to RNA in the nucleus (OUTSIDE the nucleolus), while DNA --> rRNA specifically occurs in the nucleolus?

3. How does DNA with rRNA-coding segments go from the nucleus to the nucleolus to be transcribed? I am guessing some special polymerases might ensure only rRNA-coding segments are transcribed in the nucleolus?

**This might be beyond the scope of the MCAT, esp question 3 but just checking!

**Edit: This should be in the Study Q&A, mods feel free to move 🙂

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[bodonid]

I know the nucleolus is the site of rRNA synthesis and ribosome assembly.

1. Is the nucleolus also the site of RNA synthesis in general (for mRNA or tRNA for example)? NO

2. If not, I assume that means DNA undergoes transcription to RNA in the nucleus (OUTSIDE the nucleolus), while DNA --> rRNA specifically occurs in the nucleolus? Pretty much

3. How does DNA with rRNA-coding segments go from the nucleus to the nucleolus to be transcribed? I am guessing some special polymerases might ensure only rRNA-coding segments are transcribed in the nucleolus? No. The nucleolus isn't some special compartment, and is NOT an organelle. It looks different only because the rRNA genes happen to be there. No translocation is necessary because the rRNA genes are, by definition, in the nucleolus. Make sense?
**This might be beyond the scope of the MCAT, esp question 3 but just checking!
**Edit: This should be in the Study Q&A, mods feel free to move 🙂

responses in bold ^^^^ (if you are still confused about #3, yes, there are multiple rRNA genes in different places in the genome, and they "somehow" coalesce after mitosis. If there is a detailed mechanism, I don't know it and you don't need to. But it is important to know that the nucleolus isn't a defined organelle and doesn't even exist in some cells. Some cells have multiple nucleoli. The only thing you really need to know is that rRNA is synthesized there and it looks different because the chemical composition is different than the rest of the nucleus. Sorry if my post is confusing, hopefully someone will be more concise.)

 

[futuredoctor10]

OK thanks that makes sense! I knew the nucleolus was not an organelle and not membrane-bound, but I didn't know how rRNA could be selectively transcribed in that area.

But your post helps; so I think the main thing to know is only rRNA transcription happens there, not mRNA and tRNA transcription.

 

[Vihsadas]

Moving to Study Q&A forum

 

[StIGMA]

Technically the nucleolus IS an organelle. Organelles do not have to be membrane bound. They are simply macro-structures ie: cytoskeleton, centrioles, flagella, cajal body, etc. Subnuclear structures are commonly referred to as organelles, check out pubmed.

The nucleolus IS a defined structure with myriad functions. It is involved in a huge amount of RNA splicing activity as well as subnuclear trafficking, RNA modification and ribonucleoprotein assembly. It is very dynamic, as it dissasembles for cell division, etc., but it is more than just a site of rRNA biogenesis. The nucleolus has a defined structure composed of specific proteins to construct a scaffold, namely fibrillar centers, dense FC's, and granular components.

Different RNA polymerases transcribe different rRNA genes also, so there is some specificity there as well (Pol I and III).


These are just scratching the surface, too... check out a review. ie:"Nucleolus: the fascinating nuclear body." Histochem Cell Biol (2008) 129:13–31.


Try not to think of anything in a cell as simple 🙂

 

[futuredoctor10]

Haha cool, thanks!

 

[bodonid]

Technically the nucleolus IS an organelle. Organelles do not have to be membrane bound. They are simply macro-structures ie: cytoskeleton, centrioles, flagella, cajal body, etc. Subnuclear structures are commonly referred to as organelles, check out pubmed.

http://en.wikipedia.org/wiki/Organelle

the accepted definition of an organelle requires it to be surrounded by a lipid bilayer. Terms become useless when they don't have a concrete meaning, IMO. Or at least useless for high-yield study.

Edit- it is obvious that you know more than me, mr. MDPHD, so I would like to stress the IMO part. I am just going after what could possibly be tested on the MCAT.

You bring up another good MCAT factoid, though: tRNA is transcribed by RNA Polymerase III along with ssrRNA, whereas most rRNA is transcribed by Pol I and mRNA by Pol II.

 

[bodonid]

And let me take another stab at the original question. I have no evidence to support this claim, but based on absolutely nothing, I would guess that the proteins and molecules that make up the nucleolus find the rRNA genes themselves via affinity and scaffolding rather than the rRNA genes being "translocated", as you put it, to the nucleolus. So the nucleolus "gets set up" after mitosis by proteins around the preexisting rRNA genes.

Man, I'm not very good at this. That wasn't very clear either.

 

[futuredoctor10]

Actually it was! I think your explanation/theory is right, bodonid. It makes sense that the proteins/molecules find the rRNA genes rather than the other way around. 🙂