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How Do mRNA and Other Molecules Get Around in Cells

  1. Dec 1, 2016 #1

    Drakkith

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    In order for a cell to function, chemical signals and other types of molecules need to get from one area of the cell to another. mRNA needs to get from where it is created to DNA, and then from DNA to the ribosomes and other locations of the cell. Chemical signals need to get from one areas of the cell to another in order to keep the signaling pathways functioning. Sugars need to get from outside the cell to the inside, while waste products need to get back out.

    But how do they get around? Is it simple diffusion with some "pumps" or "gates" to get in or out of certain areas of the cell? Or are there other factors working here? I know things like vacuoles exist, but then how does those get around?

    I'd appreciate any information and links. Even suggestions for books.

    Thanks in advance.
     
  2. jcsd
  3. Dec 1, 2016 #2

    Ygggdrasil

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    The cytoskeleton provides intracellular "train tracks" for the transport of certain molecules and structures inside of the cell. Microtubules are cytoskeletal elements that have a (+)-end and a (-)-end. The microtubules are arranced such that the (+)-end is near the cell periphery while the (-)-end is near the nucleus. Motor proteins move along microtubles in a specific direction (kinesins generally move toward the (+)-end and dyenins generally move toward the (-)-end), so if the cell want to send something to the cell periphery, it attaches it to a kinesin, and if it wants to send something to the nucleus, it hooks it onto a dyenin.

    For more info see:
    https://www.ncbi.nlm.nih.gov/books/NBK21710/
     
  4. Dec 2, 2016 #3

    BillTre

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    Besides moving things around in the cytoplasm with motor proteins moving on the cytoskeleton, there are other processes that can either move or control where things. You mentioned some of these in your post.

    Membranes provide a separation between the cytoplasm and the outside of the cell. Internal membranes separate the cytoplasm from membrane limited confined spaces that are inside the cell. Biological membranes can pinch off and fuse together based on their biophysics. The lumen contents of different spaces separated by a membrane from the cytoplasm can mix together if they fuse. Different proteins are associated with different membranes in different parts of the cell. This gives different membranes a variety of different properties. Biological control of membrane fusion, pinching off bits and movement is exerted by the different specific proteins in a very detailed way.

    Like other things in the cytoplasm, membrane vesicles can be transported around by the motor proteins on things like microtubules.

    There is a lot of specificity of what and how things get transported across membranes in different locations based on the membrane proteins present.

    There are also mechanisms in how ribosomes make proteins that target the proteins to the cytoplasm or to membranes or across membranes to lumenal spaces.

    There are also different cytoplasmic regions, such as: apical vs. basal surfaces of a cell, or in neurons axons vs. dendrites, presynaptic vs. post-synaptic areas, etc. where different set of proteins will end up. These regions are probably molecularly distinguished in some way.
    Phenomena such as differential adhesion between the proteins and other cytoplasmic components can result in sorting out of parts by different adhesion. More multiple and more complex specificities that can lead to a sequential stacking of molecular components to auto-assemble larger scale structures. This is similar to the reassembly of molecular components in some biochemistry experiments.
    Binding between two proteins could also change their conformations and result in a change in behavior, like, staying stuck together.
    Adhesion and binding of a substrate to a receptor are similar and related in many ways.

    The structure of neurons (one of my favorite cells) accentuate these issues. Motor neurons (with their cell body and cell body in the spinal cord) project a thin little axon out to the muscles where they make synapses. In a giraffe foot muscle the synapses can be a few meters away from the nucleus where DNA transcription (DNA --> RNA) happens. Either proteins produced in the cell body have to be transported to the distant synaptic areas or the RNA and ribosomes have to be transorted there to make protein on site.

    Here is another https://www.amazon.com/Molecular-Biology-Cell-Bruce-Alberts/dp/0815341059 which covers these issues.
     
    Last edited by a moderator: May 8, 2017
  5. Dec 3, 2016 #4

    Drakkith

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    Thanks for the links, Bill! Same to you, Ygggdrasil!
     
  6. Dec 12, 2016 #5

    Andy Resnick

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    2016 Award

  7. Dec 12, 2016 #6

    BillTre

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    @Andy Resnick, I love pictures like those.

    This paper from 1984 has some 3D (as stereo-pairs) high voltage electron microscopy shots of cytoplasm which show a similar packing of components. Not colored though.
    The samples were frozen not fixed (fixation immobilizes proteins by chemical alterations usually resulting in cross-linking to between amino groups of precipitation of proteins, fixation can cause artifacts), viewed in thick sections (for EM), and two pictures were taken at different angles by tipping the sample stage.

    A lot of people don't realize this.
     
  8. Dec 12, 2016 #7

    Drakkith

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    I certainly didn't!
     
  9. Dec 12, 2016 #8

    Drakkith

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