Your genome is the totality of your genetic instructions, embodied in your DNA, of which each cell carries a copy. (Two copies, but let’s ignore that.) How big is your genome? Often, the answer is given in terms of base pairs — the rungs of the ladder of double-stranded DNA. For humans, it’s 3 billion base pairs. This, however, isn’t really a size. That 3 billion base pairs of DNA, laid out in a line, would be about one meter long. Each of your cells fits this into its nucleus, a space a few microns (millionths of a meter) wide. How impressive a fact this is depends on physics: is DNA stiff or pliant? The answer, which I won’t elaborate on in this short post in our series of biophysical questions (here’s post #1), is “stiff” — so stiff that it would take some effort to compress 3 billion base pairs into a blob smaller than a few hundred microns. Your cells supply this effort, deliberately packaging their DNA, winding it around protein spools and then into fibers that are further organized. How this organization occurs is a biophysical topic of considerable current investigation, not just because it’s intrinsically interesting, but because it has consequences for how cells, and whole organisms, function. The packaging of DNA affects the accessibility of genes for reading by cellular machineries. It seems, in fact, that organisms subtly control aspects of packaging, for example at different stages of development.
There’s much more that one can say about the sizes of genomes. (Why, for example, are birds’ genomes so small?) I’ll stop here, though. The topic of DNA packaging, and a more thorough explanation of what it means for DNA to be “stiff,” is in my pop-science biophysics book. Here are the usual links: My description, Publisher, Amazon.)
Today’s illustration: This is not what DNA packaging looks like.
— Raghuveer Parthasarathy; August 16, 2022