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The nucleus

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  • leafers tree style avatar for user brenasuarus
    At , I'd like to ask a clarifying question/make a clarifying point. Aren't two distinct things happening at the nucleolus?
    1) rRNA is transcribed, and then it is exported to the cytoplasm after modifications.
    2) The proteins coded by the rRNA are brought back into the nucleus from the cytoplasm and assembled in the nucleolus, forming ribosomes. i.e. rRNA is never TRANSLATED in the nucleus, correct? I just want to be sure that there's no translation occurring in the nucleus, as this is a big distinguishing point.
    (12 votes)
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    • blobby green style avatar for user Sam Bradbrook
      You are correct in that no translation occurs in the nucleus. But the protein needed for partial ribosomal assembly in the nucleolus is translated in the cytoplasm from mRNA, not rRNA. rRNA is only part of the ribosome structure and can help catalyze peptide bond formation. Might be useful for you to review the video on non-protein gene products.
      (6 votes)
  • winston baby style avatar for user Subiksha
    At , can't the ribosomes produce the proteins inside the nucleus itself? Why does it have to go out to the cytoplasm, produce protein there, which gets trafficked back into the nucleolus? Thanks!
    (3 votes)
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    • leaf blue style avatar for user Kevin D. Fettel
      You're partially correct. The process is incredibly complex.

      Ribosomal proteins, for instance, are made in the cytosol, imported into the nucleus—where they assemble with newly made ribosomal RNA into particles—and are then exported again to the cytosol as part of a ribosomal subunit.

      These ribosomal subunits then come together to form a ribosome in the cytoplasm.
      (10 votes)
  • female robot grace style avatar for user OpenMinded737
    "The ribosomes also have proteins in them, and these proteins are produced in cytoplasam, and are trafficked back into the nucleus, where they can be assembled, and then make the return trip back to the cytoplasam" was the speaker drunk? or is producing and assembling two different meanings? this is a mcat question, lol.
    (2 votes)
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    • piceratops ultimate style avatar for user Darmon
      Your confusion is understandable; the speaker provided an unnecessarily convoluted description of ribosome formation. A more clear explanation: ribosomes are composed of both proteins and RNA (specifically ribosomal RNA, which is also termed rRNA.) The necessary proteins are synthesized by existing cytoplasmic (not organelle-bound) ribosomes, in the same manner as any other protein. These proteins are transported to the nucleolus, where they are subsequently combined with the resident rRNA to produce new ribosomes. After assembly, the newly-formed ribosomes are transported out of the nucleolus to their final destinations, which either could be the cytoplasm or the rough ER.

      I hope that resolved your question! :)
      (5 votes)
  • blobby green style avatar for user Orissa Agnihotri
    What is the benefit of having the rough ER continuous with the outer membrane of the nucleus?
    (2 votes)
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    • blobby green style avatar for user maryoh
      For one thing it anchors the structures; also it is more efficient for the mrna to travel directly to ribosome, then protein to travel from ribosome all the way through protein modifications steps and exocytosis in one path rather than requiring transporters to take it to different parts of the cell.
      (2 votes)
  • blobby green style avatar for user jasmine
    How many channels are there through the nuclear envelope ?
    (1 vote)
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  • blobby green style avatar for user Charlie Cameron
    What is the difference between the main functions of messenger RNA, and ribosomal RNA?
    (1 vote)
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  • blobby green style avatar for user Charlie Cameron
    How are the physical features, and functions of the inner membrane, and the outer membrane?
    (1 vote)
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  • blobby green style avatar for user 26john.mutter
    Whats the quesion
    (1 vote)
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  • duskpin seedling style avatar for user Khan2.0
    why do different cells have different models?
    (1 vote)
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  • leafers seed style avatar for user Matthew Pinckard
    At , What are the differences between cytoplasm and nucleoplasm?
    (1 vote)
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Video transcript

- [Voiceover] Here's a diagram of a eukaryotic cell. Let's look a little bit closer at the structure of one of the most important organelles in the eukaryotic cell, the nucleus. So here we have a depiction of the nucleus. And the most important function of the nucleus is to contain the genetic material of the cell. But what is the structure? Let's look at the outside of the nucleus first. The nucleus is surrounded by two separate membranes, an outer membrane and an inner membrane. Let's look at this structure up close. So, we have an outer membrane, and we have an inner membrane. These two membranes separate the nucleus from the cytoplasm, which is the liquid-filled space that makes up the majority of the cell, and all of the non-compartmentalized parts of the cell. Essentially, the parts of the cell that aren't enclosed within an organelle. On the inside of the nucleus is what's called the nucleoplasm. In the nucleoplasm is the fluid inside the nucleus. And so these two, the cytoplasm and nucleoplasm, aren't continuous. However, a lot of the times, the stuff needs to be transported between the nucleoplasm and the cytoplasm. So think the interior of the nucleus is where MRNA is produced, but MRNA needs to get into the cytoplasm where it can be translated into protein. And also, proteins in the cytoplasm need to be able to get into the nucleus, such as polymerases, which convert DNA into messenger RNA. So this transport of things like MRNA out of the nucleus, and protein into the nucleus, happens at a special complex called a nuclear pore. And so there's another nuclear pore over here in my blown-up diagram of the outer and inner membranes. What's interesting about the nuclear pore, is that it spans both membranes, the outer membrane and the inner membrane, so that compounds in the cytoplasm can be transported into the nucleus, and compounds in the nucleus can be transported out. But now, the cell has gone to a lot of trouble to create these two lipid membranes to isolate the nucleus from the rest of the cell. So the nuclear pore is actually very selective in what can be transported inside and out. And so the nuclear pore actually recognizes special signals on different proteins, and only with the presence of these signals, can proteins be transported into and out of the nucleus through the nuclear pore. So the nucleus' primary function is to contain the cell's DNA, or chromosomes. But when you look at the nucleus up close, you see there is a very densely compacted area. And this area is called the nucleolus. Just as the nucleus is kind of the center of the cell, the nucleolus is the center of the nucleus. And now, but why is the nucleolus so much more compact and dense than the rest of the nucleus? Well, that's because the nucleolus has a very important function. And that is, the site of ribosome assembly. So the nucleolus is densely packed with regions of DNA that produce not messenger RNA, but ribosomal RNA. Which is the RNA that makes up the majority of the ribosome. And it is at the site of the nucleolus where this ribosomal RNA is assembled with the proteins that also make up the ribosome, into a fully-formed ribosome. And the ribosomes then, can be trafficked through the nuclear pore, out into the cytoplasm. The ribosomes also have proteins in them, and these proteins are produced in the cytoplasm, and are trafficked back into the nucleus where they can be assembled, and then make the return trip back into the cytoplasm. So there's lots of stuff moving back and forth in the nuclear pores. But it's very important for the compartmentalization of the nucleus from the rest of the cell. To move into our next discussion, the nucleus is intimately associated with another organelle, the endoplasmic reticulum, which you can see in the faded green outline. An interesting topological feature of the nucleus, is that the outer membrane, remember which we've drawn up in the top left, is actually continuous with the membranes that make up the endoplasmic reticulum. So I'll just kinda draw that. This outer membrane just turns around and actually becomes the membranes of the endoplasmic reticulum. And I won't draw it all in. So the interior space of the endoplasmic reticulum, is continuous with the interior space of between the outer and inner nuclear membranes. And one final term used to describe the nucleus, is the nuclear envelope. And the nuclear envelope just refers to the combination of the inner and outer membranes along with the nuclear pores. So these three structures together comprise the nuclear envelope, which encloses the nucleus.