Sensex tumbled 285 points on Thursday weighed by heavy selling in Reliance Industries, HDFC Bank and Infosys as January derivatives contracts expired. After swinging over 550 points during the day, the BSE index settled 284.84 points or 0.69% lower at 40,913.82. Nifty closed 93.70 points or 0.77% down at 12,035.80. According to analysts, benchmark indices turned highly volatile ahead of the expiry of January futures and options (F&O) contracts. Further, weakness in other Asian bourses as rising concerns over the impact of coronavirus on the world economy also affected the domestic market, traders said.