Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why proteins and DNA aren't organelles?

  1. Jan 31, 2017 #1
    Hi, when I look up organelles listed, DNA and proteins are not in that list. How come?
    Aren't they "specialized subunit within a cell that have specific functions" too?

    Then I see ribosome made it to the list. Why is that protein more special than others? Why not put RNA Polymerase in there? That one makes the ribosome! :)
    Last edited: Jan 31, 2017
  2. jcsd
  3. Feb 1, 2017 #2
    Hi ikakeov:
    I am not an expert in this field, but I think I may be able to offer some help.

    An organelle is an organization of multiple components, not a single molecule. The combination can perform functions that the components alone cannot.

    That is my interpretation of the definitions below.
    A differentiated structure within a cell, such as a mitochondrion, vacuole, or chloroplast, that performs a specific function.

    Hope this helps.

  4. Feb 1, 2017 #3
    I agree with that statement. I interprete organelle as the organ of the cells, maybe the DNA and RNA is the part of the cells but it has nothing to than carry the template and transcript and translate it to form a new cells. When the organelle are needed for doing 3 major procesess (producing,transferring and secretion) so the ribosom is included for eucaryiot

    Correct me if i'm wrong and sorry for bad grammar and language
  5. Feb 1, 2017 #4

    jim mcnamara

    User Avatar

    Staff: Mentor

    Edit: @icakeov oops - noticed this: ribosomes are RNA structures, not protein.

    Organelle is a definition. Definitions change when there is something to be gained, aside from additional confusion for everyone.
    There is also an historical component to microscopy. Structures got named and defined long before anyone knew much about function.

    As to your question: proteins are everywhere in a cell, most are not identifiable in the wild (i.e., floating around in the protoplasm). So you could literally have many thousands of them, sort of like "stealth organelles". It is a little like asking why aren't molecules organelles?

    DNA is already part of an organelle - the nucleus. During mitosis it is what makes up part of a chromosome. Other molecules, like histones, are also bound to the DNA in the chromosome. In fact molecular binding to DNA is a large part of epigenetics, because changes to DNA by methylation, for example, "turns off" gene in in the section of the DNA affected.

  6. Feb 1, 2017 #5
    Hi Rafa:
    I think all you said is OK except for the ribosome. Ribosomes are not restricted to eucaryotes. All living cells have them. They are the agent that performs the translation of transfer RNA to make amino acid sequences which make proteins.

  7. Feb 1, 2017 #6


    User Avatar
    Science Advisor
    2017 Award

    Remember that organelle is a way for biologists to artificially categorize things inside the cell, and the decision to call something an organelle is to some extent arbitrary (similar to actual organs in the human body, where scientists can decide that a certain structure is actually an organ after all). There are many different definitions of organelle, and opinions may vary on what is or is not an organelle. For example, one of the major cell biology text books, Molecular Biology of the Cell, defines organelles as membrane-bound structures, which would exclude ribosomes. However, that definition would also exclude structures such as the nucleolus which also lack membranes.

    Personally, I would not classify ribosomes as organelles, but I have definitely seen them described as such. My guess is that some people classify ribosomes as organelles because you can clearly see them in electron micrographs of the cell.
  8. Feb 1, 2017 #7
    Hi @Ygggdrasil:

    Do you know whether or not the people who classify ribosomes as organelles also classify chromosomes similarly, since chromosomes are more visible in electron micrographs than ribosomes?

  9. Feb 1, 2017 #8
    Thanks everyone for comments and corrections.
    Sounds like the organelle classification challenge is somewhat similar to species classification challenge.
    I am guessing the attempt originally stemmed from wanting to classify structures that are in "size"/"structure" on a level between the basic biomolecules and the cell itself?

    p.s. New organ! Whohoo!!
  10. Feb 1, 2017 #9


    User Avatar
    Science Advisor
    2017 Award

    I have not seen chromosomes classified as organelles (for example, Wikipedia lists ribosomes among the structures considered organelles, but does not list chromosomes). This may be because compacted chromosomes are not constitutively present throughout the cell cycle.
  11. Feb 1, 2017 #10
    Does that mean that organelles are also classified based on whether they have a functional role during the cell cycle?
  12. Feb 1, 2017 #11
    All organelles have a function, but having a function is not sufficient for being classified as an organelle. For example the fiber bundle within a nucleus that separates the two sets of chromosomes during mitotic cell division is not classified as a organelle. My guess is that this is because it is almost entirely one kind of molecule, so it does not qualify as an organization of different components. Also is is part of an organelle, rather than directly a distinct part of a cell.

  13. Feb 1, 2017 #12


    User Avatar
    Science Advisor
    2017 Award

    No, I was just saying that because mitotic chromosomes exist only during a small fraction of the cell cycle, so if you were to look at a field of cells, very few would show the presence of mitotic chromosomes (cells spend most of their time in interphase). Note: the nucleus disassembles during mitosis, so it is also not present constitutively. Of course, since cells spend most of their time in interphase, cells will have a nucleus for the majority of their existence.
  14. Feb 1, 2017 #13
    Understood! Thanks :)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted