Viruses encode their genomes in RNA or DNA, which they pack into a tiny space. For example, Varicella zoster, the virus that causes chickenpox and shingles, stuffs a double-stranded DNA molecule about 40,000 nm in length into a shell about 200 nm in diameter. This is even more impressive than it may seem because DNA and RNA are rather stiff molecules, and so resist being bent, and are highly charged, and so repel themselves strongly if confined. As a result of the viral packaging, the genomic contents are highly pressurized. We might suspect that, like a champagne bottle with the cork lopped off, opening a hole in the viral shell will cause the encapulated genome to burst out. This is, in fact, true! Beautiful experiments by William Gelbart and colleagues at UCLA in the early 2000’s achieved such controlled openings and ejections of DNA, and moreover tuned the external pressure impinging on the viruses to tune the degree of ejection [1]. I won’t describe the experiments further, both because I wrote a description in my pop-science biophysics book — coming out in Italian in March! — and because the goal of this series is to highlight questions rather than answers. I’ll merely point out that the measured internal pressures are tens of atmospheres!
Viruses open themselves up when they attach to a cell they’ll infect. Is the explosive ejection of DNA sufficient to deliver the viral genome into a target cell that is itself densely packed with stuff? The answer seems to be “sort of” — biophysical calculations suggest that the DNA packing forces suffice to deliver about 20-50% of the genome [2,3]; motor proteins do the rest. As far as I know, the mechanics of ejection into cells has never been measured; I can’t think of how it would be done. But 20 years ago, the mechanics of ejection into simple liquids had never been measured, and I’m sure that new methods as clever as those devised in the past await invention.
Today’s illustration
A virus infecting a cell.
— Raghuveer Parthasarathy October 27, 2023
References
[1] A. Evilevitch, L. Lavelle, C. M. Knobler, E. Raspaud, W. M. Gelbart, Osmotic pressure inhibition of DNA ejection from phage. Proc. Natl. Acad. Sci. USA. 100, 9292-9295 (2003). https://www.pnas.org/doi/10.1073/pnas.1233721100
[2] I. J. Molineux, D. Panja, Popping the cork: mechanisms of phage genome ejection. Nat Rev Microbiol. 11, 194-204 (2013). https://www.nature.com/articles/nrmicro2988 — A thorough and fascinating review of the biophysics of viral genome ejection.
[3] C. São-José, M. de Frutos, E. Raspaud, M. A. Santos, P. Tavares, Pressure Built by DNA Packing Inside Virions: Enough to Drive DNA Ejection in Vitro, Largely Insufficient for Delivery into the Bacterial Cytoplasm. Journal of Molecular Biology. 374, 346-355 (2007). https://doi.org/10.1016/j.jmb.2007.09.045
The Eighteenth Elephant 
Congratulations, so cool to see your book will be coming out in Italian! I’ll look for it in a bookstore here.
(I love that you call those experiments beautiful, that you don’t feel the need to conceal an aesthetic kind of admiration as well as (I assume) a utilitarian one.)
Cheers, Gina P
F. Regina Psaki Professor Emerita, Romance Languages Dept. of Romance Languages 1233-University of Oregon Eugene OR 97403-1233 DREAMERS: I support all students regardless of immigration status or country of origin. I support Dreamer students. Remember: When interacting with faculty, staff, and offices around campus, you are never required to reveal your status.
Thanks, Gina! Yes, I certainly mean beautiful in an aesthetic sense. It’s quite common in the natural sciences to refer to experiments or theories as beautiful and especially “elegant,” but this tends to get excised from textbooks or formal documents.