Table 2

Theories of ageing (modified from ref 22, 27, 36)

Type of theory Explanation
Genetic-molecularCodon restriction: accuracy of DNA-mRNA impaired
Error theories: accuracy of mRNA-proteins impaired
Gene regulation: postreproductive changes in gene expression
Somatic mutation: radiation damage to DNA
CellularWear and tear: with use the body wears out and dies
Age pigments: lipofuscin deposit as a primary cause of ageing
Free radicals: lead to cell damage and thus ageing
Cross linking: increased cross linkage of DNA leading to irreversible damage to DNA and enzymes
EvolutionaryWide range of species-species variation in lifespan
Disposable somaBalance between energy investment in reproduction and maintenance
System levelNeuroendocrine control: a “biological clock” controls development through neural and hormonal mechanisms
Immune control: thymus gland as an “immunological clock” leading to cell destruction due to failure to recognise self
Age changesChanges occurring throughout the whole lifespan, often involving deterioration of an anatomical/physiological nature