5.2 Fission


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    The most important low-copy number component in your cell is its genome; without the instructions, nothing gets built. Reflecting its importance, cells have evolved complex mechanisms to coordinate DNA replication and segregation in time as well as space. The details are beyond our scope here, but we can touch on some general structural principles.

    If your cell divides by splitting in the middle, what is the easiest way to get one complete genome copy to each daughter? (Assume your cell has a single chromosome like most bacteria and archaea.) Why not just tether each copy of the chromosome to an opposite pole of the cell? Nearly all bacteria, and many archaea, have a protein called ParB (for Partitioning) that recognizes a specific sequence (ParS) on the chromosome, creating a molecular handle. In Caulobacter crescentus like this, ParB also binds a scaffolding protein at the pole called PopZ, hooking the handle to the pole. The PopZ scaffold is not highly ordered, so we see it as a diffuse blob of DNA and protein, noticeable because it excludes other large protein complexes like ribosomes. Several species of bacteria use PopZ or other hub-organizing proteins to tether a genome copy, as well as other low-copy-number things like chemosensory machinery, to the cell pole. Other species use a different mechanism involving many copies of a dynamic protein called ParA that bind and release ParB, ratcheting the ParS handle of the chromosome across the cell. Other low-copy-number components, like the carboxysomes and storage granules you just saw, also use ParA for segregation.

    Remember that your cell’s chromosome is colossal, so getting the ParS handle to one side is only part of the battle. An army of other proteins work to condense the chromosome to a more manageable volume and wrangle with the division machinery to make sure no straggling loops get caught off-sides.