Circadian Clock Essay

The rotation of earth around its own axis is responsible for cyclic changes in environment basically by causing changes in variation in light and temperature. Circadian clocks present in almost all organisms ranging from bacteria to Humans helps in anticipating these changes and adapting accordingly (1). Though the biological processes present in circadian cycle may change according to the ecological niche of the species but the molecular basis shows that these are evolutionarily related. The coordinated feedback activity of transcriptional /translational loops present in various pacemaker cells is responsible for the activity of circadian clocks (2). In Drosophila Melanogaster, this molecular clock is located in a circadian network of 150 neurons in adult brain and coordinated activity of whole network is important for responses under different environmental stimuli while under constant conditions the circadian locomotors activity depends on 8 neurons which are located on the accessory medulla on each side of the adult brain called small and large lateral ventral neurons (sLNvs and ILNvs respectively) (3, 4). As these all express Pigment Dispersing

This model was proposed by Pittendrigh and Daan (1976) which discribes a morning oscillator (M) which locks on to dawn and evening oscillator (E) which locks on to dusk. The observations which led to this model were occurance of bimodal bouts of activity in a variety of organisms under periodic L/D cycle, and of splitting of morning and evening components of activity. It was assumed that clock cells in animals weere distributed bilaterally in the brains of animals serve as M and E oscillators. A study by Iglesia et al (2000) supported this claim in which unilateral lesions of suparchiasmatic nucleus (SCN), completely completely or partially compressed one of the split components of activity in study