lengths in different seasons of the year. The light-sensitive proteins in the eye called, rhodopsins (rhodopsin 1 and 6), are crucial in activating the M and E oscillations. When environmental light is detected, approximately 150 neurones (there are about 100,000 neurones in the Drosophila brain) in the brain regulate the circadian rhythm. The clock neurones are located in distinct clusters in the central brain. The best-understood clock neurones are the large and small lateral ventral neurons (l-LNvs and s-LNvs) of the optic lobe. These neurones produce pigment dispersing factor (PDF), a neuropeptide that acts as a circadian neuromodulator between different clock neurones. Drosophila circadian keeps time via daily fluctuations of clock-related proteins which interact in what is called a transcription-translation feedback loop. The core clock mechanism consists of two interdependent feedback loops, namely the PER/TIM loop and the CLK/CYC loop. The CLK/CYC loop occurs during the day in which both Clock protein and cycle protein are produced. CLK/CYC heterodimer act as transcription factors and bind together to initiate the transcription of the per and tim genes by binding to a promoter element called E box, around mid-day. DNA is transcribed to produce PER mRNA and TIM mRNA. PER and TIM proteins are synthesized in the cytoplasm, and exhibit a smooth increase in levels over the course of the day. Their RNA levels peak early in the evening and protein levels peak around daybreak. But their proteins levels are maintained at constantly low level until dusk, because during daylight also activates the doubletime (dbt) gene. DBT protein induces post-translational modifications, that is phosphorylation and turnover of monomeric PER proteins. As PER is translated in the cytoplasm, it is actively phosphorylated by DBT (casein kinase 1ε) and casein kinase 2 (synthesised by And and Tik) as a prelude to premature degradation. The actual degradation is through the ubiquitin-proteasome pathway, and is carried out by a ubiquitin ligase called Slimb (supernumery limbs). At the same time, TIM is itself phosphorylated by shaggy, whose activity declines after sunset. DBT gradually disappears, and withdrawal of DBT promotes PER molecules to get stabilized by physical association with TIM. Hence, maximum production of PER and TIM occurs at dusk. At the same time, CLK/CYC also directly activates vri and Pdp1 (the gene for PAR domain protein 1). VRI accumulates first, 3-6 hour earlier, and start to repress Clk; but the incoming of PDP1 creates a competition by