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For example I know that after transcription a mature mRNA is sent outside of the nucleus through the nuclear pore and then it goes to ribosome. But as for the free floating ribosomes that are out somewhere in the cytoplasm, how does the mRNA get to those organelles? Is there something else that attaches to the mRNA to help it? Even then I don't understand what force is helping a large molecule (mRNA) find and even larger macromolecular assembly (the ribosome).

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  • $\begingroup$ I have changed your title to remove the anthropomorphic implications. $\endgroup$ – David Jan 22 '18 at 15:02
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Once a eukaryotic mRNA enters the cytoplasm, in most cases it is probably the cytoplasmic concentration of the eukaryotic translation initiation factors that determines whether or not a specific mRNA is translated on the ribosomes. The concentration of ribosomes in the cytoplasm tends to be quite high, and I think the same is true for the eIF subunits. The short answer to the first part of your question is therefore: diffusion and the law of mass action. The last sentence of your question is unclear (to me) and should probably form the basis for two separate questions.

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This is fairly often question for molecular biology

how does the mRNA know exactly how to get to those organelles

The secret to understand it, is that nothing knows where to go. Different molecules have different affinity to each other. Molecule A likes molecule B, but not molecule Y. Inside the cell, thermal fluctuations causes disorder that drives molecule A to bump into all kinds of molecules, including Bs, Zs, Ys, and others. However, when stumble upon molecule B, molecule A will very likely form some sort of stable complex with it. One example, is ribosome and mRNA.

When mRNA and ribosome join into complex, their inner parts can be considered in the same fashion: they will jiggle a lot around, but most likely jiggle will produce polypeptide, not, let's say DNA, or not cause mRNA to fall away from the ribosome.

So, you can say that mRNA tries to go everywhere it can, without any clear direction, apart from the fact that it will most likely form something stable (i.e. useful) with ribosome. Or maybe it will be mRNA degradation protein, such as ribonuclease.

PS: "will very likely form some sort of stable complex" means that in some reasonable time mRNA will find and bind to ribosome. However, it might take billions of years for ribosome to bind DNA and produce some sort of polypeptide.

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