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I am a mechatronics engineer who stopped learning biology after high school - but this is bothering me.

mRNA is, if I recall correctly, created in the nucleus of the cells and migrates out of the nucleus inside the cytoplasm where it will be translated by ribosomes.

mRNA vaccines inject mRNA molecules inside the body, which are apparently transported by their nanolipidic particle coating across the cell membrane, correct?

I understand that the DNA→mRNA transcription is probably not reversible, but I do not understand how the mRNA is only able to cross the nucleus-cytoplasm membrane in one direction.

Care to explain? Bonus kudos for clarifications on the rest of my doubts.

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    $\begingroup$ mRNA vaccines are not "transported" by their lipid coating (no active transport involved). It's just that lipids are attracted to the nonpolar plasma membrane and expelled from polar water. $\endgroup$
    – Damocle Damoclev
    Jan 3, 2022 at 16:21

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Nuclear pores control what gets in and out of the nucleus. In general, mRNAs are only allowed out, they don't go back in. Reverse transcriptases, of course, will put mRNA back into DNA, but only some viruses, like HIV, have those enzymes.

https://portlandpress.com/biochemj/article-abstract/477/1/23/221793/Into-the-basket-and-beyond-the-journey-of-mRNA?redirectedFrom=fulltext

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  • $\begingroup$ Interesting, thanks! Do you mean that, unless a person is infected by rare specific viruses, no reverse transcriptase is present in the body and therefore the mRNA cannot reenter the nucleus? $\endgroup$
    – Mister Mystère
    Aug 19, 2021 at 18:02
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    $\begingroup$ It is very hard to categorically say something is impossible in biology; there are always exceptions. But your cells are full of your mRNA all the time. When you get infected with most viruses, like COVID, there is foreign mRNA in your cells. In general, this does not cause permanent change to your DNA, let alone clinically pathogenic change. $\endgroup$
    – swbarnes2
    Aug 19, 2021 at 23:22
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    $\begingroup$ Just note that reverse transcriptase works in the cytoplasm to create a DNA copy of an RNA, so it does not allow mRNA transport to the nucleus, it just creates DNA which could then be transported to the nucleus. $\endgroup$
    – Armand
    Aug 19, 2021 at 23:58
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    $\begingroup$ I think this is the beginning of a good answer, but it doesn't directly address OP's question: "but I do not understand how the mRNA is only able to cross the nucleus-cytoplasm membrane in one direction". Yes, nuclear pores control the flow of mRNA out of the nucleus, but they also allow things into the nucleus (like proteins). What is the mechanism by which mRNAs are exported but not imported? $\endgroup$
    – acvill
    Aug 20, 2021 at 16:38
  • $\begingroup$ @acvill: this would indeed be particularly interesting to know! $\endgroup$
    – Mister Mystère
    Jan 5, 2022 at 16:54

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