section_path: Biological basis
section_name: Biological basis
paragraph_text: Biological basis
  
  In the 21st century, researchers are only beginning to investigate the biological basis of ageing even in relatively simple and short-lived organisms, such as yeast. Little is known of mammalian ageing, in part due to the much longer lives of even small mammals, such as the mouse (around 3 years). A model organism for the study of ageing is the nematode C. elegans – having a short lifespan of 2–3 weeks – enabling genetic manipulations or suppression of gene activity with RNA interference, and other factors. Most known mutations and RNA interference targets that extend lifespan were first discovered in C. elegans.
  
  The factors proposed to influence biological ageing fall into two main categories, programmed and error-related. Programmed factors follow a biological timetable that might be a continuation of inherent mechanisms that regulate childhood growth and development. This regulation would depend on changes in gene expression that affect the systems responsible for maintenance, repair and defense responses. Factors causing errors or damage include internal and environmental events that induce cumulative deterioration in one or more organs.