Commentors on a recent post here began arguing the side issue of bacterial immortality (derived from the question of whether the last universal common ancestor of all living organisms is extinct or whether it, in some way, is still with us today). Though the argument, as presented in the comments, was largely semantic, the central point was summed up by Don Cox in the statement: “When a cell divides (or multiplies) into two identical cells, you cannot say that one or other is an offshoot“. This is a common textbook representation of binary fission as practiced by most bacteria and many eukaryotes*. As with many textbook representations, it is highly likely to be wrong.
*Or most, depending (again) on your choice of semantics. After all, you yourself multiply by binary fission, at least at the internal level.
Any cell, whether bacterial, eukaryote or what have you, is constantly bombarded throughout its life by inimical factors. Toxic substances build up, whether ingested from the outside world or produced as by-products of the cell’s own metabolism. DNA and other vital cellular structures become damaged or otherwise functionally altered (for instance, by the process of DNA methylation). As this damage builds up, the cell ‘ages’ and, if the damage becomes too great, dies. One of the reasons cells multiply in the first place is to counter-act the build-up of these inimical factors. For instance, replication of a chromosome will produce a daughter chromosome in which the damage to the original has been repaired (I’m over-simplifying matters, of course, but you get the idea).
The nice thought experiment regarding immortal bacteria is pretty much destroyed by reality. This is a post detailing just how different daughter cells may be from the progenitor.
In fact, there is evidence that producing a fit daughter cell and one that is less fit may actually result in better survival chances hat having two mediocre daughter cells.
As is often the case, reality is more interesting that conjecture.