MadSci Network: Zoology


Date: Thu Aug 12 17:00:22 1999
Posted By: Andrea Bixler, staff (postdoctoral associate), biology, UM-St. Louis
Area of science: Zoology
ID: 930746764.Zo

I guess I would argue that all organisms have some sort of 
activity/inactivity cycle, but it wouldn't necessarily be comparable to 
what you're familiar with in humans or raccoons or even plants (which have 
a light/dark cycle because they conduct photosynthesis, obtaining energy 
from the sun during the day and transforming it into sugars at night).  In 
microorganisms, there could be activity and inactivity based on 
photosynthesis (just as in plants), the cell cycle, or spore production.  
The cell cycle is the process all cells go through, and includes a period 
of growth, when the cell is basically enlarging itself and replicating its 
DNA, and a period of mitosis or cell division, during which the cell splits 
into two "daughter" cells.  So microorganisms could appear to have a cycle 
of activity (mitosis) and relative inactivity.  
The other aspect of microorganisms that you could consider an 
active-inactive cycle is when they produce spores.  A lot of fungi and 
bacteria reproduce by spores, which are sort of like the seeds produced by 
flowering plants, in the sense that they can remain dormant for a while, 
and a new organism grows from them.  But spores can remain inert 
for up to hundreds of years while environmental conditions are tough.  
Then, after a nice rain shower, or some other required environmental 
change occurs, the spore becomes active and grows into a new organism.  So 
this could look like an activity-inactivity cycle, with the spore being the 
inactive phase and the adult organism the active phase.  This is similar to 
your and my activity cycle in that it is maintained by environmental 
factors, but these are factors much less predictable than whether the sun 
will come up tomorrow.

To answer the other part of your question about the life span of 
microorganisms, I can either play biologist or philosopher.  Some people 
argue that microorganism are immortal, because they simply make exact 
copies of themselves over and over.  Barring mutations (mistakes in the DNA 
replication), a bacterium that you put in a test tube today could copy 
itself endlesslesly and produce identical "offspring" for centuries, thus 
appearing to be immortal.  I would argue, however, that the bacterium's 
life span lasts from one mitosis to another; in other words, from the time 
its "parent" cell divided to produce it, until the time it divides to 
produce its own daughter cells.  That life span could be just a few 

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