Falling for the sixth straight session, the Sensex at the Bombay Stock Exchange tumbled 491 points, or 2.9 per cent– to close at 16,290, today, ahead of the expiry of the monthly contracts in the derivatives segment, and amid weak Asian markets. This is the sharpest fall for the Sensex in 21 weeks. The Nifty at the National Stock Exchange plunged 155 points, or 3.1 percent, to 4,853.Other Asian markets also continued their slide for the eighth straight day, with key stock indices in Japan, Hong Kong, Singapore, South Korea, and China losing between 0.5 percent and 1 percent.