During the first 20 years of the expansion, Harvard would build 4 million to 5 million square feet of buildings and create at least 5,000 jobs, university officials said. Construction in Allston could begin this summer when Harvard hopes to break ground on a 500,000-square-foot (46,450-square-metre) science complex that will house the school’s stem-cell researchers and other institutes. The science complex, university officials said, would be the nucleus for new interdisciplinary research and is expected to go a long way toward boosting Boston’s economy by encouraging partnerships with biotechnology firms that may displace the region’s long-fading manufacturing base.