Aging/senescence- introduction

Despite the obvious disadvantages of aging, it is not opposed by natural selection probably because it can be beneficial to the species by avoiding overcrowding and promoting further evolution, thus increasing the fitness of subsequent generations. There are many theories of aging which can be broadly divided into stochastic and non-stochastic theories of aging. The former views aging as a phenomenon resulting from random events leading to cellular damage whereas the latter considers aging a programmed or a predetermined phenomenon occurring in all organisms within a particular time-frame.

Senescence can be conditional or replicative. When a bacterial culture is exposed to stressful conditions such as starvation for prolonged periods, they gradually accumulate oxidative damages and their ability to recover from the damages may also be lost and the cells may finally undergo death. This conditional senescence (Nystrom 2003) is different from the replicative senescence in that the latter results from the sequential loss of fitness following multiple rounds of replication.

Whereas aging is visible and very evident in higher eukaryotic organisms, it is also observable in simple eukaryotes like the fruitfly Drosophila or yeast Saccharomyces. However, senescence in prokaryotes like bacteria was not known till recently. For more than a century, bacteria were considered to be functionally immortal organisms because of their symmetrical division. It was believed that a parent bacterium split symmetrically into two equal daughter cells and that both the cells receive damaged and new cellular constituents equally. Thus, bacteria were considered to be organisms immune to natural aging and death, even though they can be killed by many external agents like starvation or other stressors.

Before moving into bacterial senescence, I will discuss the aging model in simple eukaryotes.

Next- Replicative senescence in Saccharomyces cerevisiae

Nystrom, T. (2003). Conditional senescence in bacteria: death of the immortals. Mol Microbiol 48(1), 17-23