A Short Course on Biosemiotics: 4. Time in living systems

Alexei A. Sharov
Laboratory of Genetics
National Institute on Aging (NIA/NIH)
Baltimore, USA
Presented in the Embryo Physics Course, May 2, 2012


According to physics, time and space are universal categories, and change is a trajectory in time and space. However, in biology there is an alternative concept of time that goes back to Aristotle: time is made of changes. Thus, time is not universal but specific for each living system. To study time we need tools to model changes in systems during evolution and development. Development follows the logic of embryogenesis and understanding of this logic helps to predict possible evolutionary changes. Living systems not only follow the flow of time, they make their own time by reorganizing the rates and directions of their functions. Animals can model their environments and  use these models to optimize their actions. Models of the environment may include time and space components. Biological time often includes qualitative steps (checkpoints) that ensure the completion of earlier functions before the start of the next set of functions. Organisms develop master clocks (e.g., circadian rhythm) as a tool to organize a large number of other functions. Photoperiodism allows organisms to adjust their activities to the seasonal changes. Time and life are inseparable; time without life is an abstraction.




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  1. Alexei Sharov says:

    Here are answers on some questions asked yesterday during my talk on Time in Living Systems:
    (1) The question on embryological checkpoints is indeed very interesting. One of the checkpoints explored in entomology is so called “pupation window model”. One of the papers is here http://www.ncbi.nlm.nih.gov/pubmed/11931252. However the concept was developed about 30 years ago. The idea is to find conditions when insect larvae should pupate, and optimize these conditions to get highest fitness in variable environments.
    (2) Cell cycle genes in mammals. I have this reference: Whitfield et al. Identification of genes periodically expressed in the human cell cycle and their expression in tumors. Mol Biol Cell 2002;13:1977–2000. The quality is moderate, it is based on cDNA arrays. Another study http://www.ncbi.nlm.nih.gov/pubmed/21661451 again used cDNA arrays. I hope somebody will repeat it with better arrays.