Tokyo stocks open mixed Friday as buying tracking overnight gains on Wall Street was offset by selling of export-related issues on a stronger yen against the U.S. dollar. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average rose 122.99 points, or 0.31 percent, from Thursday to 39,721.70. The broader Topix index was down 2.87 points, or 0.11 percent, at 2,715.67. On the top-tier Prime Market, gainers were led by chemical and construction issues, while land transportation and transportation equipment issues led decliners. At 9 a.m., the dollar fetched 147.85-88 yen compared wit…